Reimplement constant acceleration groups
[alexxy/gromacs.git] / src / gromacs / modularsimulator / modularsimulator.cpp
index d4bf96184140130af893055dc71af801c18355e4..03e64cc6e36c6fdb4c95fd04625eab68fed98a4a 100644 (file)
@@ -1,7 +1,7 @@
 /*
  * This file is part of the GROMACS molecular simulation package.
  *
- * Copyright (c) 2019,2020, by the GROMACS development team, led by
+ * Copyright (c) 2019,2020,2021, by the GROMACS development team, led by
  * Mark Abraham, David van der Spoel, Berk Hess, and Erik Lindahl,
  * and including many others, as listed in the AUTHORS file in the
  * top-level source directory and at http://www.gromacs.org.
 #include "gromacs/ewald/pme.h"
 #include "gromacs/ewald/pme_load_balancing.h"
 #include "gromacs/ewald/pme_pp.h"
-#include "gromacs/gmxlib/network.h"
+#include "gromacs/fileio/checkpoint.h"
 #include "gromacs/gmxlib/nrnb.h"
-#include "gromacs/math/vec.h"
+#include "gromacs/listed_forces/listed_forces.h"
 #include "gromacs/mdlib/checkpointhandler.h"
 #include "gromacs/mdlib/constr.h"
+#include "gromacs/mdlib/coupling.h"
 #include "gromacs/mdlib/energyoutput.h"
-#include "gromacs/mdlib/forcerec.h"
 #include "gromacs/mdlib/mdatoms.h"
 #include "gromacs/mdlib/resethandler.h"
-#include "gromacs/mdlib/stat.h"
-#include "gromacs/mdlib/update.h"
 #include "gromacs/mdrun/replicaexchange.h"
 #include "gromacs/mdrun/shellfc.h"
 #include "gromacs/mdrunutility/handlerestart.h"
 #include "gromacs/mdtypes/mdatom.h"
 #include "gromacs/mdtypes/mdrunoptions.h"
 #include "gromacs/mdtypes/observableshistory.h"
-#include "gromacs/mdtypes/state.h"
 #include "gromacs/nbnxm/nbnxm.h"
-#include "gromacs/timing/walltime_accounting.h"
 #include "gromacs/topology/mtop_util.h"
 #include "gromacs/topology/topology.h"
-#include "gromacs/utility/cstringutil.h"
+#include "gromacs/trajectory/trajectoryframe.h"
 #include "gromacs/utility/fatalerror.h"
+#include "gromacs/utility/int64_to_int.h"
 
-#include "compositesimulatorelement.h"
+#include "andersentemperaturecoupling.h"
 #include "computeglobalselement.h"
 #include "constraintelement.h"
-#include "energyelement.h"
+#include "expandedensembleelement.h"
+#include "firstorderpressurecoupling.h"
 #include "forceelement.h"
-#include "freeenergyperturbationelement.h"
+#include "mttk.h"
+#include "nosehooverchains.h"
 #include "parrinellorahmanbarostat.h"
-#include "propagator.h"
-#include "signallers.h"
+#include "pullelement.h"
+#include "simulatoralgorithm.h"
 #include "statepropagatordata.h"
-#include "trajectoryelement.h"
-#include "vrescalethermostat.h"
+#include "velocityscalingtemperaturecoupling.h"
 
 namespace gmx
 {
 void ModularSimulator::run()
 {
-    GMX_LOG(mdlog.info).asParagraph().appendText("Using the modular simulator.");
-    constructElementsAndSignallers();
-    simulatorSetup();
-    for (auto& signaller : signallerCallList_)
-    {
-        signaller->signallerSetup();
-    }
-    if (domDecHelper_)
-    {
-        domDecHelper_->setup();
-    }
+    GMX_LOG(legacySimulatorData_->mdlog.info)
+            .asParagraph()
+            .appendText("Using the modular simulator.");
 
-    for (auto& element : elementsOwnershipList_)
-    {
-        element->elementSetup();
-    }
-    if (pmeLoadBalanceHelper_)
+    ModularSimulatorAlgorithmBuilder algorithmBuilder(compat::make_not_null(legacySimulatorData_),
+                                                      std::move(checkpointDataHolder_));
+    addIntegrationElements(&algorithmBuilder);
+    auto algorithm = algorithmBuilder.build();
+
+    while (const auto* task = algorithm.getNextTask())
     {
-        // State must have been initialized so pmeLoadBalanceHelper_ gets a valid box
-        pmeLoadBalanceHelper_->setup();
+        // execute task
+        (*task)();
     }
+}
 
-    while (step_ <= signalHelper_->lastStep_)
+void ModularSimulator::addIntegrationElements(ModularSimulatorAlgorithmBuilder* builder)
+{
+    const bool isTrotter = inputrecNvtTrotter(legacySimulatorData_->inputrec)
+                           || inputrecNptTrotter(legacySimulatorData_->inputrec)
+                           || inputrecNphTrotter(legacySimulatorData_->inputrec);
+    if (legacySimulatorData_->inputrec->eI == IntegrationAlgorithm::MD)
     {
-        populateTaskQueue();
-
-        while (!taskQueue_.empty())
+        // The leap frog integration algorithm
+        builder->add<ForceElement>();
+        builder->add<StatePropagatorData::Element>();
+        if (legacySimulatorData_->inputrec->etc == TemperatureCoupling::VRescale
+            || legacySimulatorData_->inputrec->etc == TemperatureCoupling::Berendsen
+            || legacySimulatorData_->inputrec->etc == TemperatureCoupling::NoseHoover)
         {
-            auto task = std::move(taskQueue_.front());
-            taskQueue_.pop();
-            // run function
-            (*task)();
+            builder->add<VelocityScalingTemperatureCoupling>(Offset(-1),
+                                                             UseFullStepKE::No,
+                                                             ReportPreviousStepConservedEnergy::No,
+                                                             PropagatorTag("LeapFrogPropagator"));
+        }
+        builder->add<Propagator<IntegrationStage::LeapFrog>>(
+                PropagatorTag("LeapFrogPropagator"), TimeStep(legacySimulatorData_->inputrec->delta_t));
+        if (legacySimulatorData_->constr)
+        {
+            builder->add<ConstraintsElement<ConstraintVariable::Positions>>();
         }
-    }
 
-    for (auto& element : elementsOwnershipList_)
-    {
-        element->elementTeardown();
-    }
-    if (pmeLoadBalanceHelper_)
-    {
-        pmeLoadBalanceHelper_->teardown();
-    }
-    simulatorTeardown();
-}
+        if (legacySimulatorData_->inputrec->bPull)
+        {
+            builder->add<PullElement>();
+        }
 
-void ModularSimulator::simulatorSetup()
-{
-    if (!mdrunOptions.writeConfout)
-    {
-        // This is on by default, and the main known use case for
-        // turning it off is for convenience in benchmarking, which is
-        // something that should not show up in the general user
-        // interface.
-        GMX_LOG(mdlog.info)
-                .asParagraph()
-                .appendText(
-                        "The -noconfout functionality is deprecated, and "
-                        "may be removed in a future version.");
+        builder->add<ComputeGlobalsElement<ComputeGlobalsAlgorithm::LeapFrog>>();
+        if (legacySimulatorData_->inputrec->epc == PressureCoupling::ParrinelloRahman)
+        {
+            builder->add<ParrinelloRahmanBarostat>(Offset(-1), PropagatorTag("LeapFrogPropagator"));
+        }
+        else if (legacySimulatorData_->inputrec->epc == PressureCoupling::Berendsen
+                 || legacySimulatorData_->inputrec->epc == PressureCoupling::CRescale)
+        {
+            builder->add<FirstOrderPressureCoupling>(0, ReportPreviousStepConservedEnergy::No);
+        }
     }
-
-    if (MASTER(cr))
+    else if (legacySimulatorData_->inputrec->eI == IntegrationAlgorithm::VV && !isTrotter)
     {
-        char        sbuf[STEPSTRSIZE], sbuf2[STEPSTRSIZE];
-        std::string timeString;
-        fprintf(stderr, "starting mdrun '%s'\n", *(top_global->name));
-        if (inputrec->nsteps >= 0)
+        // The velocity verlet integration algorithm
+        builder->add<ForceElement>();
+        builder->add<Propagator<IntegrationStage::VelocitiesOnly>>(
+                PropagatorTag("VelocityHalfStep"), TimeStep(0.5 * legacySimulatorData_->inputrec->delta_t));
+        if (legacySimulatorData_->constr)
         {
-            timeString = formatString("%8.1f", static_cast<double>(inputrec->init_step + inputrec->nsteps)
-                                                       * inputrec->delta_t);
+            builder->add<ConstraintsElement<ConstraintVariable::Velocities>>();
         }
-        else
+        builder->add<ComputeGlobalsElement<ComputeGlobalsAlgorithm::VelocityVerlet>>();
+        // Here, we have x / v / f at the full time step
+        builder->add<StatePropagatorData::Element>();
+        if (legacySimulatorData_->inputrec->bExpanded)
         {
-            timeString = "infinite";
+            builder->add<ExpandedEnsembleElement>();
         }
-        if (inputrec->init_step > 0)
+        if (legacySimulatorData_->inputrec->etc == TemperatureCoupling::VRescale
+            || legacySimulatorData_->inputrec->etc == TemperatureCoupling::Berendsen)
         {
-            fprintf(stderr, "%s steps, %s ps (continuing from step %s, %8.1f ps).\n",
-                    gmx_step_str(inputrec->init_step + inputrec->nsteps, sbuf), timeString.c_str(),
-                    gmx_step_str(inputrec->init_step, sbuf2), inputrec->init_step * inputrec->delta_t);
+            builder->add<VelocityScalingTemperatureCoupling>(
+                    Offset(0),
+                    UseFullStepKE::Yes,
+                    ReportPreviousStepConservedEnergy::Yes,
+                    PropagatorTag("VelocityHalfAndPositionFullStep"));
         }
-        else
+        else if (ETC_ANDERSEN(legacySimulatorData_->inputrec->etc))
         {
-            fprintf(stderr, "%s steps, %s ps.\n", gmx_step_str(inputrec->nsteps, sbuf),
-                    timeString.c_str());
+            builder->add<AndersenTemperatureCoupling>();
         }
-        fprintf(fplog, "\n");
-    }
-
-    walltime_accounting_start_time(walltime_accounting);
-    wallcycle_start(wcycle, ewcRUN);
-    print_start(fplog, cr, walltime_accounting, "mdrun");
-
-    step_ = inputrec->init_step;
-}
-
-void ModularSimulator::preStep(Step step, Time gmx_unused time, bool isNeighborSearchingStep)
-{
-    if (stopHandler_->stoppingAfterCurrentStep(isNeighborSearchingStep) && step != signalHelper_->lastStep_)
-    {
-        /*
-         * Stop handler wants to stop after the current step, which was
-         * not known when building the current task queue. This happens
-         * e.g. when a stop is signalled by OS. We therefore want to purge
-         * the task queue now, and re-schedule this step as last step.
-         */
-        // clear task queue
-        std::queue<SimulatorRunFunctionPtr>().swap(taskQueue_);
-        // rewind step
-        step_ = step;
-        return;
-    }
-
-    resetHandler_->setSignal(walltime_accounting);
-    // This is a hack to avoid having to rewrite StopHandler to be a NeighborSearchSignaller
-    // and accept the step as input. Eventually, we want to do that, but currently this would
-    // require introducing NeighborSearchSignaller in the legacy do_md or a lot of code
-    // duplication.
-    stophandlerIsNSStep_    = isNeighborSearchingStep;
-    stophandlerCurrentStep_ = step;
-    stopHandler_->setSignal();
-
-    wallcycle_start(wcycle, ewcSTEP);
-}
-
-void ModularSimulator::postStep(Step step, Time gmx_unused time)
-{
-    // Output stuff
-    if (MASTER(cr))
-    {
-        if (do_per_step(step, inputrec->nstlog))
+        builder->add<Propagator<IntegrationStage::VelocityVerletPositionsAndVelocities>>(
+                PropagatorTag("VelocityHalfAndPositionFullStep"),
+                TimeStep(legacySimulatorData_->inputrec->delta_t));
+        if (legacySimulatorData_->constr)
         {
-            if (fflush(fplog) != 0)
-            {
-                gmx_fatal(FARGS, "Cannot flush logfile - maybe you are out of disk space?");
-            }
+            builder->add<ConstraintsElement<ConstraintVariable::Positions>>();
         }
-    }
-    const bool do_verbose = mdrunOptions.verbose
-                            && (step % mdrunOptions.verboseStepPrintInterval == 0
-                                || step == inputrec->init_step || step == signalHelper_->lastStep_);
-    // Print the remaining wall clock time for the run
-    if (MASTER(cr) && (do_verbose || gmx_got_usr_signal())
-        && !(pmeLoadBalanceHelper_ && pmeLoadBalanceHelper_->pmePrinting()))
-    {
-        print_time(stderr, walltime_accounting, step, inputrec, cr);
-    }
 
-    double cycles = wallcycle_stop(wcycle, ewcSTEP);
-    if (DOMAINDECOMP(cr) && wcycle)
-    {
-        dd_cycles_add(cr->dd, static_cast<float>(cycles), ddCyclStep);
-    }
-
-    resetHandler_->resetCounters(
-            step, step - inputrec->init_step, mdlog, fplog, cr, fr->nbv.get(), nrnb, fr->pmedata,
-            pmeLoadBalanceHelper_ ? pmeLoadBalanceHelper_->loadBalancingObject() : nullptr, wcycle,
-            walltime_accounting);
-}
-
-void ModularSimulator::simulatorTeardown()
-{
-
-    // Stop measuring walltime
-    walltime_accounting_end_time(walltime_accounting);
-
-    if (!thisRankHasDuty(cr, DUTY_PME))
-    {
-        /* Tell the PME only node to finish */
-        gmx_pme_send_finish(cr);
-    }
-
-    walltime_accounting_set_nsteps_done(walltime_accounting, step_ - inputrec->init_step);
-}
-
-void ModularSimulator::populateTaskQueue()
-{
-    auto registerRunFunction = std::make_unique<RegisterRunFunction>(
-            [this](SimulatorRunFunctionPtr ptr) { taskQueue_.push(std::move(ptr)); });
-
-    Time startTime = inputrec->init_t;
-    Time timeStep  = inputrec->delta_t;
-    Time time      = startTime + step_ * timeStep;
-
-    // Run an initial call to the signallers
-    for (auto& signaller : signallerCallList_)
-    {
-        signaller->signal(step_, time);
-    }
-
-    if (checkpointHelper_)
-    {
-        checkpointHelper_->run(step_, time);
-    }
-
-    if (pmeLoadBalanceHelper_)
-    {
-        pmeLoadBalanceHelper_->run(step_, time);
-    }
-    if (domDecHelper_)
-    {
-        domDecHelper_->run(step_, time);
-    }
-
-    do
-    {
-        // local variables for lambda capturing
-        const int  step     = step_;
-        const bool isNSStep = step == signalHelper_->nextNSStep_;
-
-        // register pre-step
-        (*registerRunFunction)(std::make_unique<SimulatorRunFunction>(
-                [this, step, time, isNSStep]() { preStep(step, time, isNSStep); }));
-        // register elements for step
-        for (auto& element : elementCallList_)
+        if (legacySimulatorData_->inputrec->bPull)
         {
-            element->scheduleTask(step_, time, registerRunFunction);
+            builder->add<PullElement>();
         }
-        // register post-step
-        (*registerRunFunction)(
-                std::make_unique<SimulatorRunFunction>([this, step, time]() { postStep(step, time); }));
 
-        // prepare next step
-        step_++;
-        time = startTime + step_ * timeStep;
-        for (auto& signaller : signallerCallList_)
+        builder->add<ComputeGlobalsElement<ComputeGlobalsAlgorithm::VelocityVerlet>>();
+        if (legacySimulatorData_->inputrec->epc == PressureCoupling::ParrinelloRahman)
         {
-            signaller->signal(step_, time);
+            builder->add<ParrinelloRahmanBarostat>(Offset(-1), PropagatorTag("VelocityHalfStep"));
+        }
+        else if (legacySimulatorData_->inputrec->epc == PressureCoupling::Berendsen
+                 || legacySimulatorData_->inputrec->epc == PressureCoupling::CRescale)
+        {
+            builder->add<FirstOrderPressureCoupling>(0, ReportPreviousStepConservedEnergy::Yes);
         }
-    } while (step_ != signalHelper_->nextNSStep_ && step_ <= signalHelper_->lastStep_);
-}
-
-void ModularSimulator::constructElementsAndSignallers()
-{
-    /* When restarting from a checkpoint, it can be appropriate to
-     * initialize ekind from quantities in the checkpoint. Otherwise,
-     * compute_globals must initialize ekind before the simulation
-     * starts/restarts. However, only the master rank knows what was
-     * found in the checkpoint file, so we have to communicate in
-     * order to coordinate the restart.
-     *
-     * TODO (modular) This should become obsolete when checkpoint reading
-     *      happens within the modular simulator framework: The energy
-     *      element should read its data from the checkpoint file pointer,
-     *      and signal to the compute globals element if it needs anything
-     *      reduced.
-     *
-     * TODO (legacy) Consider removing this communication if/when checkpoint
-     *      reading directly follows .tpr reading, because all ranks can
-     *      agree on hasReadEkinState at that time.
-     */
-    bool hasReadEkinState = MASTER(cr) ? state_global->ekinstate.hasReadEkinState : false;
-    if (PAR(cr))
-    {
-        gmx_bcast(sizeof(hasReadEkinState), &hasReadEkinState, cr->mpi_comm_mygroup);
-    }
-    if (hasReadEkinState)
-    {
-        restore_ekinstate_from_state(cr, ekind, &state_global->ekinstate);
-    }
-
-    /*
-     * Build data structures
-     */
-    topologyHolder_ =
-            std::make_unique<TopologyHolder>(*top_global, cr, inputrec, fr, mdAtoms, constr, vsite);
-
-    std::unique_ptr<FreeEnergyPerturbationElement> freeEnergyPerturbationElement    = nullptr;
-    FreeEnergyPerturbationElement*                 freeEnergyPerturbationElementPtr = nullptr;
-    if (inputrec->efep != efepNO)
-    {
-        freeEnergyPerturbationElement =
-                std::make_unique<FreeEnergyPerturbationElement>(fplog, inputrec, mdAtoms);
-        freeEnergyPerturbationElementPtr = freeEnergyPerturbationElement.get();
-    }
-
-    auto statePropagatorData = std::make_unique<StatePropagatorData>(
-            top_global->natoms, fplog, cr, state_global, inputrec->nstxout, inputrec->nstvout,
-            inputrec->nstfout, inputrec->nstxout_compressed, fr->nbv->useGpu(),
-            freeEnergyPerturbationElementPtr, topologyHolder_.get(), fr->bMolPBC,
-            mdrunOptions.writeConfout, opt2fn("-c", nfile, fnm), inputrec, mdAtoms->mdatoms());
-    auto statePropagatorDataPtr = compat::make_not_null(statePropagatorData.get());
-
-    auto energyElement = std::make_unique<EnergyElement>(
-            statePropagatorDataPtr, freeEnergyPerturbationElementPtr, top_global, inputrec, mdAtoms,
-            enerd, ekind, constr, fplog, fcd, mdModulesNotifier, MASTER(cr), observablesHistory,
-            startingBehavior);
-    auto energyElementPtr = compat::make_not_null(energyElement.get());
-
-    /*
-     * Build stop handler
-     */
-    const bool simulationsShareState = false;
-    stopHandler_                     = stopHandlerBuilder->getStopHandlerMD(
-            compat::not_null<SimulationSignal*>(&signals_[eglsSTOPCOND]), simulationsShareState,
-            MASTER(cr), inputrec->nstlist, mdrunOptions.reproducible, nstglobalcomm_,
-            mdrunOptions.maximumHoursToRun, inputrec->nstlist == 0, fplog, stophandlerCurrentStep_,
-            stophandlerIsNSStep_, walltime_accounting);
-
-    /*
-     * Create simulator builders
-     */
-    SignallerBuilder<NeighborSearchSignaller> neighborSearchSignallerBuilder;
-    SignallerBuilder<LastStepSignaller>       lastStepSignallerBuilder;
-    SignallerBuilder<LoggingSignaller>        loggingSignallerBuilder;
-    SignallerBuilder<EnergySignaller>         energySignallerBuilder;
-    TrajectoryElementBuilder                  trajectoryElementBuilder;
-
-    /*
-     * Register data structures to signallers
-     */
-    trajectoryElementBuilder.registerWriterClient(statePropagatorDataPtr);
-    trajectoryElementBuilder.registerSignallerClient(statePropagatorDataPtr);
-    lastStepSignallerBuilder.registerSignallerClient(statePropagatorDataPtr);
-
-    trajectoryElementBuilder.registerWriterClient(energyElementPtr);
-    trajectoryElementBuilder.registerSignallerClient(energyElementPtr);
-    energySignallerBuilder.registerSignallerClient(energyElementPtr);
-
-    // Register the simulator itself to the neighbor search / last step signaller
-    neighborSearchSignallerBuilder.registerSignallerClient(compat::make_not_null(signalHelper_.get()));
-    lastStepSignallerBuilder.registerSignallerClient(compat::make_not_null(signalHelper_.get()));
-
-    /*
-     * Build integrator - this takes care of force calculation, propagation,
-     * constraining, and of the place the statePropagatorData and the energy element
-     * have a full timestep state.
-     */
-    // TODO: Make a CheckpointHelperBuilder
-    std::vector<ICheckpointHelperClient*> checkpointClients = { statePropagatorDataPtr, energyElementPtr,
-                                                                freeEnergyPerturbationElementPtr };
-    CheckBondedInteractionsCallbackPtr checkBondedInteractionsCallback = nullptr;
-    auto                               integrator =
-            buildIntegrator(&neighborSearchSignallerBuilder, &energySignallerBuilder,
-                            &loggingSignallerBuilder, &trajectoryElementBuilder, &checkpointClients,
-                            &checkBondedInteractionsCallback, statePropagatorDataPtr,
-                            energyElementPtr, freeEnergyPerturbationElementPtr, hasReadEkinState);
-
-    /*
-     * Build infrastructure elements
-     */
-
-    if (PmeLoadBalanceHelper::doPmeLoadBalancing(mdrunOptions, inputrec, fr))
-    {
-        pmeLoadBalanceHelper_ = std::make_unique<PmeLoadBalanceHelper>(
-                mdrunOptions.verbose, statePropagatorDataPtr, fplog, cr, mdlog, inputrec, wcycle, fr);
-        neighborSearchSignallerBuilder.registerSignallerClient(
-                compat::make_not_null(pmeLoadBalanceHelper_.get()));
-    }
-
-    if (DOMAINDECOMP(cr))
-    {
-        GMX_ASSERT(checkBondedInteractionsCallback,
-                   "Domain decomposition needs a callback for check the number of bonded "
-                   "interactions.");
-        domDecHelper_ = std::make_unique<DomDecHelper>(
-                mdrunOptions.verbose, mdrunOptions.verboseStepPrintInterval, statePropagatorDataPtr,
-                topologyHolder_.get(), std::move(checkBondedInteractionsCallback), nstglobalcomm_, fplog,
-                cr, mdlog, constr, inputrec, mdAtoms, nrnb, wcycle, fr, vsite, imdSession, pull_work);
-        neighborSearchSignallerBuilder.registerSignallerClient(compat::make_not_null(domDecHelper_.get()));
-    }
-
-    const bool simulationsShareResetCounters = false;
-    resetHandler_                            = std::make_unique<ResetHandler>(
-            compat::make_not_null<SimulationSignal*>(&signals_[eglsRESETCOUNTERS]),
-            simulationsShareResetCounters, inputrec->nsteps, MASTER(cr),
-            mdrunOptions.timingOptions.resetHalfway, mdrunOptions.maximumHoursToRun, mdlog, wcycle,
-            walltime_accounting);
-
-    /*
-     * Build signaller list
-     *
-     * Note that as signallers depend on each others, the order of calling the signallers
-     * matters. It is the responsibility of this builder to ensure that the order is
-     * maintained.
-     */
-    auto energySignaller = energySignallerBuilder.build(
-            inputrec->nstcalcenergy, inputrec->fepvals->nstdhdl, inputrec->nstpcouple);
-    trajectoryElementBuilder.registerSignallerClient(compat::make_not_null(energySignaller.get()));
-    loggingSignallerBuilder.registerSignallerClient(compat::make_not_null(energySignaller.get()));
-    auto trajectoryElement = trajectoryElementBuilder.build(
-            fplog, nfile, fnm, mdrunOptions, cr, outputProvider, mdModulesNotifier, inputrec,
-            top_global, oenv, wcycle, startingBehavior, simulationsShareState);
-    loggingSignallerBuilder.registerSignallerClient(compat::make_not_null(trajectoryElement.get()));
-
-    // Add checkpoint helper here since we need a pointer to the trajectory element and
-    // need to register it with the lastStepSignallerBuilder
-    auto checkpointHandler = std::make_unique<CheckpointHandler>(
-            compat::make_not_null<SimulationSignal*>(&signals_[eglsCHKPT]), simulationsShareState,
-            inputrec->nstlist == 0, MASTER(cr), mdrunOptions.writeConfout,
-            mdrunOptions.checkpointOptions.period);
-    checkpointHelper_ = std::make_unique<CheckpointHelper>(
-            std::move(checkpointClients), std::move(checkpointHandler), inputrec->init_step,
-            trajectoryElement.get(), top_global->natoms, fplog, cr, observablesHistory,
-            walltime_accounting, state_global, mdrunOptions.writeConfout);
-    lastStepSignallerBuilder.registerSignallerClient(compat::make_not_null(checkpointHelper_.get()));
-
-    lastStepSignallerBuilder.registerSignallerClient(compat::make_not_null(trajectoryElement.get()));
-    auto loggingSignaller =
-            loggingSignallerBuilder.build(inputrec->nstlog, inputrec->init_step, inputrec->init_t);
-    lastStepSignallerBuilder.registerSignallerClient(compat::make_not_null(loggingSignaller.get()));
-    auto lastStepSignaller =
-            lastStepSignallerBuilder.build(inputrec->nsteps, inputrec->init_step, stopHandler_.get());
-    neighborSearchSignallerBuilder.registerSignallerClient(compat::make_not_null(lastStepSignaller.get()));
-    auto neighborSearchSignaller = neighborSearchSignallerBuilder.build(
-            inputrec->nstlist, inputrec->init_step, inputrec->init_t);
-
-    addToCallListAndMove(std::move(neighborSearchSignaller), signallerCallList_, signallersOwnershipList_);
-    addToCallListAndMove(std::move(lastStepSignaller), signallerCallList_, signallersOwnershipList_);
-    addToCallListAndMove(std::move(loggingSignaller), signallerCallList_, signallersOwnershipList_);
-    addToCallList(trajectoryElement, signallerCallList_);
-    addToCallListAndMove(std::move(energySignaller), signallerCallList_, signallersOwnershipList_);
-
-    /*
-     * Build the element list
-     *
-     * This is the actual sequence of (non-infrastructure) elements to be run.
-     * For NVE, the trajectory element is used outside of the integrator
-     * (composite) element, as well as the checkpoint helper. The checkpoint
-     * helper should be on top of the loop, and is only part of the simulator
-     * call list to be able to react to the last step being signalled.
-     */
-    addToCallList(checkpointHelper_, elementCallList_);
-    if (freeEnergyPerturbationElement)
-    {
-        addToCallListAndMove(std::move(freeEnergyPerturbationElement), elementCallList_,
-                             elementsOwnershipList_);
     }
-    addToCallListAndMove(std::move(integrator), elementCallList_, elementsOwnershipList_);
-    addToCallListAndMove(std::move(trajectoryElement), elementCallList_, elementsOwnershipList_);
-    // for vv, we need to setup statePropagatorData after the compute
-    // globals so that we reset the right velocities
-    // TODO: Avoid this by getting rid of the need of resetting velocities in vv
-    elementsOwnershipList_.emplace_back(std::move(statePropagatorData));
-    elementsOwnershipList_.emplace_back(std::move(energyElement));
-}
-
-std::unique_ptr<ISimulatorElement>
-ModularSimulator::buildForces(SignallerBuilder<NeighborSearchSignaller>* neighborSearchSignallerBuilder,
-                              SignallerBuilder<EnergySignaller>*         energySignallerBuilder,
-                              StatePropagatorData*                       statePropagatorDataPtr,
-                              EnergyElement*                             energyElementPtr,
-                              FreeEnergyPerturbationElement* freeEnergyPerturbationElement)
-{
-    const bool isVerbose    = mdrunOptions.verbose;
-    const bool isDynamicBox = inputrecDynamicBox(inputrec);
-
-    auto forceElement = std::make_unique<ForceElement>(
-            statePropagatorDataPtr, energyElementPtr, freeEnergyPerturbationElement, isVerbose,
-            isDynamicBox, fplog, cr, inputrec, mdAtoms, nrnb, fr, fcd, wcycle, runScheduleWork, vsite,
-            imdSession, pull_work, constr, &topologyHolder_->globalTopology(), enforcedRotation);
-    topologyHolder_->registerClient(forceElement.get());
-    neighborSearchSignallerBuilder->registerSignallerClient(compat::make_not_null(forceElement.get()));
-    energySignallerBuilder->registerSignallerClient(compat::make_not_null(forceElement.get()));
-
-    // std::move *should* not be needed with c++-14, but clang-3.6 still requires it
-    return std::move(forceElement);
-}
-
-std::unique_ptr<ISimulatorElement> ModularSimulator::buildIntegrator(
-        SignallerBuilder<NeighborSearchSignaller>* neighborSearchSignallerBuilder,
-        SignallerBuilder<EnergySignaller>*         energySignallerBuilder,
-        SignallerBuilder<LoggingSignaller>*        loggingSignallerBuilder,
-        TrajectoryElementBuilder*                  trajectoryElementBuilder,
-        std::vector<ICheckpointHelperClient*>*     checkpointClients,
-        CheckBondedInteractionsCallbackPtr*        checkBondedInteractionsCallback,
-        compat::not_null<StatePropagatorData*>     statePropagatorDataPtr,
-        compat::not_null<EnergyElement*>           energyElementPtr,
-        FreeEnergyPerturbationElement*             freeEnergyPerturbationElementPtr,
-        bool                                       hasReadEkinState)
-{
-    auto forceElement =
-            buildForces(neighborSearchSignallerBuilder, energySignallerBuilder,
-                        statePropagatorDataPtr, energyElementPtr, freeEnergyPerturbationElementPtr);
-
-    // list of elements owned by the simulator composite object
-    std::vector<std::unique_ptr<ISimulatorElement>> elementsOwnershipList;
-    // call list of the simulator composite object
-    std::vector<compat::not_null<ISimulatorElement*>> elementCallList;
-
-    std::function<void()> needToCheckNumberOfBondedInteractions;
-    if (inputrec->eI == eiMD)
+    else if (legacySimulatorData_->inputrec->eI == IntegrationAlgorithm::VV && isTrotter)
     {
-        auto computeGlobalsElement =
-                std::make_unique<ComputeGlobalsElement<ComputeGlobalsAlgorithm::LeapFrog>>(
-                        statePropagatorDataPtr, energyElementPtr, freeEnergyPerturbationElementPtr,
-                        &signals_, nstglobalcomm_, fplog, mdlog, cr, inputrec, mdAtoms, nrnb,
-                        wcycle, fr, &topologyHolder_->globalTopology(), constr, hasReadEkinState);
-        topologyHolder_->registerClient(computeGlobalsElement.get());
-        energySignallerBuilder->registerSignallerClient(compat::make_not_null(computeGlobalsElement.get()));
-        trajectoryElementBuilder->registerSignallerClient(
-                compat::make_not_null(computeGlobalsElement.get()));
-
-        *checkBondedInteractionsCallback =
-                computeGlobalsElement->getCheckNumberOfBondedInteractionsCallback();
-
-        auto propagator = std::make_unique<Propagator<IntegrationStep::LeapFrog>>(
-                inputrec->delta_t, statePropagatorDataPtr, mdAtoms, wcycle);
-
-        addToCallListAndMove(std::move(forceElement), elementCallList, elementsOwnershipList);
-        addToCallList(statePropagatorDataPtr, elementCallList); // we have a full microstate at time t here!
-        if (inputrec->etc == etcVRESCALE)
+        // For a new simulation, avoid the first Trotter half step
+        const auto scheduleTrotterFirstHalfOnInitStep =
+                ((legacySimulatorData_->startingBehavior == StartingBehavior::NewSimulation)
+                         ? ScheduleOnInitStep::No
+                         : ScheduleOnInitStep::Yes);
+        // Define the tags and offsets for MTTK pressure scaling
+        const MttkPropagatorConnectionDetails mttkPropagatorConnectionDetails = {
+            PropagatorTag("ScaleMTTKXPre"),  PropagatorTag("ScaleMTTKXPost"),  Offset(0),
+            PropagatorTag("ScaleMTTKVPre1"), PropagatorTag("ScaleMTTKVPost1"), Offset(1),
+            PropagatorTag("ScaleMTTKVPre2"), PropagatorTag("ScaleMTTKVPost2"), Offset(0)
+        };
+
+        builder->add<ForceElement>();
+        // Propagate velocities from t-dt/2 to t
+        if (legacySimulatorData_->inputrec->epc == PressureCoupling::Mttk)
         {
-            // TODO: With increased complexity of the propagator, this will need further development,
-            //       e.g. using propagators templated for velocity propagation policies and a builder
-            propagator->setNumVelocityScalingVariables(inputrec->opts.ngtc);
-            auto thermostat = std::make_unique<VRescaleThermostat>(
-                    inputrec->nsttcouple, -1, false, inputrec->ld_seed, inputrec->opts.ngtc,
-                    inputrec->delta_t * inputrec->nsttcouple, inputrec->opts.ref_t, inputrec->opts.tau_t,
-                    inputrec->opts.nrdf, energyElementPtr, propagator->viewOnVelocityScaling(),
-                    propagator->velocityScalingCallback(), state_global, cr, inputrec->bContinuation);
-            checkpointClients->emplace_back(thermostat.get());
-            energyElementPtr->setVRescaleThermostat(thermostat.get());
-            addToCallListAndMove(std::move(thermostat), elementCallList, elementsOwnershipList);
+            builder->add<Propagator<IntegrationStage::ScaleVelocities>>(
+                    PropagatorTag("ScaleMTTKVPre1"));
         }
-
-        std::unique_ptr<ParrinelloRahmanBarostat> prBarostat = nullptr;
-        if (inputrec->epc == epcPARRINELLORAHMAN)
+        builder->add<Propagator<IntegrationStage::VelocitiesOnly>>(
+                PropagatorTag("VelocityHalfStep1"),
+                TimeStep(0.5 * legacySimulatorData_->inputrec->delta_t));
+        if (legacySimulatorData_->inputrec->epc == PressureCoupling::Mttk)
         {
-            // Building the PR barostat here since it needs access to the propagator
-            // and we want to be able to move the propagator object
-            prBarostat = std::make_unique<ParrinelloRahmanBarostat>(
-                    inputrec->nstpcouple, -1, inputrec->delta_t * inputrec->nstpcouple,
-                    inputrec->init_step, propagator->viewOnPRScalingMatrix(),
-                    propagator->prScalingCallback(), statePropagatorDataPtr, energyElementPtr,
-                    fplog, inputrec, mdAtoms, state_global, cr, inputrec->bContinuation);
-            energyElementPtr->setParrinelloRahamnBarostat(prBarostat.get());
-            checkpointClients->emplace_back(prBarostat.get());
+            builder->add<Propagator<IntegrationStage::ScaleVelocities>>(
+                    PropagatorTag("ScaleMTTKVPost1"));
         }
-        addToCallListAndMove(std::move(propagator), elementCallList, elementsOwnershipList);
-        if (constr)
+        if (legacySimulatorData_->constr)
         {
-            auto constraintElement = std::make_unique<ConstraintsElement<ConstraintVariable::Positions>>(
-                    constr, statePropagatorDataPtr, energyElementPtr, freeEnergyPerturbationElementPtr,
-                    MASTER(cr), fplog, inputrec, mdAtoms->mdatoms());
-            auto constraintElementPtr = compat::make_not_null(constraintElement.get());
-            energySignallerBuilder->registerSignallerClient(constraintElementPtr);
-            trajectoryElementBuilder->registerSignallerClient(constraintElementPtr);
-            loggingSignallerBuilder->registerSignallerClient(constraintElementPtr);
-
-            addToCallListAndMove(std::move(constraintElement), elementCallList, elementsOwnershipList);
+            builder->add<ConstraintsElement<ConstraintVariable::Velocities>>();
         }
+        builder->add<ComputeGlobalsElement<ComputeGlobalsAlgorithm::VelocityVerlet>>();
 
-        addToCallListAndMove(std::move(computeGlobalsElement), elementCallList, elementsOwnershipList);
-        addToCallList(energyElementPtr, elementCallList); // we have the energies at time t here!
-        if (prBarostat)
+        // Propagate extended system variables from t-dt/2 to t
+        if (legacySimulatorData_->inputrec->epc == PressureCoupling::Mttk)
         {
-            addToCallListAndMove(std::move(prBarostat), elementCallList, elementsOwnershipList);
+            builder->add<MttkElement>(
+                    Offset(-1), scheduleTrotterFirstHalfOnInitStep, mttkPropagatorConnectionDetails);
+        }
+        if (legacySimulatorData_->inputrec->etc == TemperatureCoupling::NoseHoover)
+        {
+            builder->add<NoseHooverChainsElement>(NhcUsage::System,
+                                                  Offset(-1),
+                                                  UseFullStepKE::Yes,
+                                                  scheduleTrotterFirstHalfOnInitStep,
+                                                  PropagatorTag("ScaleNHC"));
+            builder->add<Propagator<IntegrationStage::ScaleVelocities>>(PropagatorTag("ScaleNHC"));
+        }
+        if (legacySimulatorData_->inputrec->epc == PressureCoupling::Mttk)
+        {
+            builder->add<NoseHooverChainsElement>(NhcUsage::Barostat,
+                                                  Offset(-1),
+                                                  UseFullStepKE::Yes,
+                                                  scheduleTrotterFirstHalfOnInitStep,
+                                                  mttkPropagatorConnectionDetails);
+        }
+        // We have a full state at time t here
+        builder->add<StatePropagatorData::Element>();
+        if (legacySimulatorData_->inputrec->bExpanded)
+        {
+            builder->add<ExpandedEnsembleElement>();
         }
-    }
-    else if (inputrec->eI == eiVV)
-    {
-        auto computeGlobalsElement =
-                std::make_unique<ComputeGlobalsElement<ComputeGlobalsAlgorithm::VelocityVerlet>>(
-                        statePropagatorDataPtr, energyElementPtr, freeEnergyPerturbationElementPtr,
-                        &signals_, nstglobalcomm_, fplog, mdlog, cr, inputrec, mdAtoms, nrnb,
-                        wcycle, fr, &topologyHolder_->globalTopology(), constr, hasReadEkinState);
-        topologyHolder_->registerClient(computeGlobalsElement.get());
-        energySignallerBuilder->registerSignallerClient(compat::make_not_null(computeGlobalsElement.get()));
-        trajectoryElementBuilder->registerSignallerClient(
-                compat::make_not_null(computeGlobalsElement.get()));
-
-        *checkBondedInteractionsCallback =
-                computeGlobalsElement->getCheckNumberOfBondedInteractionsCallback();
-
-        auto propagatorVelocities = std::make_unique<Propagator<IntegrationStep::VelocitiesOnly>>(
-                inputrec->delta_t * 0.5, statePropagatorDataPtr, mdAtoms, wcycle);
-        auto propagatorVelocitiesAndPositions =
-                std::make_unique<Propagator<IntegrationStep::VelocityVerletPositionsAndVelocities>>(
-                        inputrec->delta_t, statePropagatorDataPtr, mdAtoms, wcycle);
-
-        addToCallListAndMove(std::move(forceElement), elementCallList, elementsOwnershipList);
 
-        std::unique_ptr<ParrinelloRahmanBarostat> prBarostat = nullptr;
-        if (inputrec->epc == epcPARRINELLORAHMAN)
+        // Propagate extended system variables from t to t+dt/2
+        if (legacySimulatorData_->inputrec->epc == PressureCoupling::Mttk)
+        {
+            builder->add<NoseHooverChainsElement>(NhcUsage::Barostat,
+                                                  Offset(0),
+                                                  UseFullStepKE::Yes,
+                                                  ScheduleOnInitStep::Yes,
+                                                  mttkPropagatorConnectionDetails);
+        }
+        if (legacySimulatorData_->inputrec->etc == TemperatureCoupling::NoseHoover)
         {
-            // Building the PR barostat here since it needs access to the propagator
-            // and we want to be able to move the propagator object
-            prBarostat = std::make_unique<ParrinelloRahmanBarostat>(
-                    inputrec->nstpcouple, -1, inputrec->delta_t * inputrec->nstpcouple,
-                    inputrec->init_step, propagatorVelocities->viewOnPRScalingMatrix(),
-                    propagatorVelocities->prScalingCallback(), statePropagatorDataPtr, energyElementPtr,
-                    fplog, inputrec, mdAtoms, state_global, cr, inputrec->bContinuation);
-            energyElementPtr->setParrinelloRahamnBarostat(prBarostat.get());
-            checkpointClients->emplace_back(prBarostat.get());
+            builder->add<NoseHooverChainsElement>(NhcUsage::System,
+                                                  Offset(0),
+                                                  UseFullStepKE::Yes,
+                                                  ScheduleOnInitStep::Yes,
+                                                  PropagatorTag("VelocityHalfStep2"));
         }
-        addToCallListAndMove(std::move(propagatorVelocities), elementCallList, elementsOwnershipList);
-        if (constr)
+        if (legacySimulatorData_->inputrec->epc == PressureCoupling::Mttk)
         {
-            auto constraintElement = std::make_unique<ConstraintsElement<ConstraintVariable::Velocities>>(
-                    constr, statePropagatorDataPtr, energyElementPtr, freeEnergyPerturbationElementPtr,
-                    MASTER(cr), fplog, inputrec, mdAtoms->mdatoms());
-            energySignallerBuilder->registerSignallerClient(compat::make_not_null(constraintElement.get()));
-            trajectoryElementBuilder->registerSignallerClient(
-                    compat::make_not_null(constraintElement.get()));
-            loggingSignallerBuilder->registerSignallerClient(
-                    compat::make_not_null(constraintElement.get()));
+            builder->add<MttkElement>(Offset(0), ScheduleOnInitStep::Yes, mttkPropagatorConnectionDetails);
+            builder->add<Propagator<IntegrationStage::ScaleVelocities>>(
+                    PropagatorTag("ScaleMTTKVPre2"));
+        }
 
-            addToCallListAndMove(std::move(constraintElement), elementCallList, elementsOwnershipList);
+        // Propagate velocities from t to t+dt/2
+        builder->add<Propagator<IntegrationStage::VelocitiesOnly>>(
+                PropagatorTag("VelocityHalfStep2"),
+                TimeStep(0.5 * legacySimulatorData_->inputrec->delta_t));
+        if (legacySimulatorData_->inputrec->epc == PressureCoupling::Mttk)
+        {
+            builder->add<Propagator<IntegrationStage::ScaleVelocities>>(
+                    PropagatorTag("ScaleMTTKVPost2"));
+            builder->add<Propagator<IntegrationStage::ScalePositions>>(
+                    PropagatorTag("ScaleMTTKXPre"));
         }
-        addToCallList(compat::make_not_null(computeGlobalsElement.get()), elementCallList);
-        addToCallList(statePropagatorDataPtr, elementCallList); // we have a full microstate at time t here!
-        if (inputrec->etc == etcVRESCALE)
+        // Propagate positions from t to t+dt
+        builder->add<Propagator<IntegrationStage::PositionsOnly>>(
+                PropagatorTag("PositionFullStep"), TimeStep(legacySimulatorData_->inputrec->delta_t));
+        if (legacySimulatorData_->inputrec->epc == PressureCoupling::Mttk)
         {
-            // TODO: With increased complexity of the propagator, this will need further development,
-            //       e.g. using propagators templated for velocity propagation policies and a builder
-            propagatorVelocitiesAndPositions->setNumVelocityScalingVariables(inputrec->opts.ngtc);
-            auto thermostat = std::make_unique<VRescaleThermostat>(
-                    inputrec->nsttcouple, 0, true, inputrec->ld_seed, inputrec->opts.ngtc,
-                    inputrec->delta_t * inputrec->nsttcouple, inputrec->opts.ref_t,
-                    inputrec->opts.tau_t, inputrec->opts.nrdf, energyElementPtr,
-                    propagatorVelocitiesAndPositions->viewOnVelocityScaling(),
-                    propagatorVelocitiesAndPositions->velocityScalingCallback(), state_global, cr,
-                    inputrec->bContinuation);
-            checkpointClients->emplace_back(thermostat.get());
-            energyElementPtr->setVRescaleThermostat(thermostat.get());
-            addToCallListAndMove(std::move(thermostat), elementCallList, elementsOwnershipList);
+            builder->add<Propagator<IntegrationStage::ScalePositions>>(
+                    PropagatorTag("ScaleMTTKXPost"));
         }
-        addToCallListAndMove(std::move(propagatorVelocitiesAndPositions), elementCallList,
-                             elementsOwnershipList);
-        if (constr)
+        if (legacySimulatorData_->constr)
         {
-            auto constraintElement = std::make_unique<ConstraintsElement<ConstraintVariable::Positions>>(
-                    constr, statePropagatorDataPtr, energyElementPtr, freeEnergyPerturbationElementPtr,
-                    MASTER(cr), fplog, inputrec, mdAtoms->mdatoms());
-            energySignallerBuilder->registerSignallerClient(compat::make_not_null(constraintElement.get()));
-            trajectoryElementBuilder->registerSignallerClient(
-                    compat::make_not_null(constraintElement.get()));
-            loggingSignallerBuilder->registerSignallerClient(
-                    compat::make_not_null(constraintElement.get()));
+            builder->add<ConstraintsElement<ConstraintVariable::Positions>>();
+        }
+
+        if (legacySimulatorData_->inputrec->bPull)
+        {
+            builder->add<PullElement>();
+        }
+
+        builder->add<ComputeGlobalsElement<ComputeGlobalsAlgorithm::VelocityVerlet>>();
 
-            addToCallListAndMove(std::move(constraintElement), elementCallList, elementsOwnershipList);
+        // Propagate box from t to t+dt
+        if (legacySimulatorData_->inputrec->epc == PressureCoupling::Mttk)
+        {
+            builder->add<MttkBoxScaling>(mttkPropagatorConnectionDetails);
         }
-        addToCallListAndMove(std::move(computeGlobalsElement), elementCallList, elementsOwnershipList);
-        addToCallList(energyElementPtr, elementCallList); // we have the energies at time t here!
-        if (prBarostat)
+        else if (legacySimulatorData_->inputrec->epc == PressureCoupling::CRescale)
         {
-            addToCallListAndMove(std::move(prBarostat), elementCallList, elementsOwnershipList);
+            // Legacy implementation allows combination of C-Rescale with Trotter Nose-Hoover
+            builder->add<FirstOrderPressureCoupling>(0, ReportPreviousStepConservedEnergy::Yes);
         }
     }
     else
     {
         gmx_fatal(FARGS, "Integrator not implemented for the modular simulator.");
     }
-
-    auto integrator = std::make_unique<CompositeSimulatorElement>(std::move(elementCallList),
-                                                                  std::move(elementsOwnershipList));
-    // std::move *should* not be needed with c++-14, but clang-3.6 still requires it
-    return std::move(integrator);
+    builder->add<EnergyData::Element>();
 }
 
 bool ModularSimulator::isInputCompatible(bool                             exitOnFailure,
@@ -764,8 +361,6 @@ bool ModularSimulator::isInputCompatible(bool                             exitOn
         return condition;
     };
 
-    bool isInputCompatible = true;
-
     // GMX_USE_MODULAR_SIMULATOR allows to use modular simulator also for non-standard uses,
     // such as the leap-frog integrator
     const auto modularSimulatorExplicitlyTurnedOn = (getenv("GMX_USE_MODULAR_SIMULATOR") != nullptr);
@@ -781,70 +376,46 @@ bool ModularSimulator::isInputCompatible(bool                             exitOn
             "or unset both to recover default behavior.");
 
     GMX_RELEASE_ASSERT(
-            !(modularSimulatorExplicitlyTurnedOff && inputrec->eI == eiVV
-              && inputrec->epc == epcPARRINELLORAHMAN),
+            !(modularSimulatorExplicitlyTurnedOff && inputrec->eI == IntegrationAlgorithm::VV
+              && inputrec->epc == PressureCoupling::ParrinelloRahman),
             "Cannot use a Parrinello-Rahman barostat with md-vv and "
             "GMX_DISABLE_MODULAR_SIMULATOR=ON, "
             "as the Parrinello-Rahman barostat is not implemented in the legacy simulator. Unset "
             "GMX_DISABLE_MODULAR_SIMULATOR or use a different pressure control algorithm.");
 
-    isInputCompatible =
-            isInputCompatible
-            && conditionalAssert(
-                       inputrec->eI == eiMD || inputrec->eI == eiVV,
-                       "Only integrators md and md-vv are supported by the modular simulator.");
+    bool isInputCompatible = conditionalAssert(
+            inputrec->eI == IntegrationAlgorithm::MD || inputrec->eI == IntegrationAlgorithm::VV,
+            "Only integrators md and md-vv are supported by the modular simulator.");
     isInputCompatible = isInputCompatible
-                        && conditionalAssert(inputrec->eI != eiMD || modularSimulatorExplicitlyTurnedOn,
+                        && conditionalAssert(inputrec->eI != IntegrationAlgorithm::MD
+                                                     || modularSimulatorExplicitlyTurnedOn,
                                              "Set GMX_USE_MODULAR_SIMULATOR=ON to use the modular "
                                              "simulator with integrator md.");
-    isInputCompatible =
-            isInputCompatible
-            && conditionalAssert(!doRerun, "Rerun is not supported by the modular simulator.");
-    isInputCompatible =
-            isInputCompatible
-            && conditionalAssert(
-                       inputrec->etc == etcNO || inputrec->etc == etcVRESCALE,
-                       "Only v-rescale thermostat is supported by the modular simulator.");
     isInputCompatible =
             isInputCompatible
             && conditionalAssert(
-                       inputrec->epc == epcNO || inputrec->epc == epcPARRINELLORAHMAN,
-                       "Only Parrinello-Rahman barostat is supported by the modular simulator.");
+                    !inputrec->useMts,
+                    "Multiple time stepping is not supported by the modular simulator.");
     isInputCompatible =
             isInputCompatible
-            && conditionalAssert(
-                       !(inputrecNptTrotter(inputrec) || inputrecNphTrotter(inputrec)
-                         || inputrecNvtTrotter(inputrec)),
-                       "Legacy Trotter decomposition is not supported by the modular simulator.");
-    isInputCompatible = isInputCompatible
-                        && conditionalAssert(inputrec->efep == efepNO || inputrec->efep == efepYES
-                                                     || inputrec->efep == efepSLOWGROWTH,
-                                             "Expanded ensemble free energy calculation is not "
-                                             "supported by the modular simulator.");
-    isInputCompatible = isInputCompatible
-                        && conditionalAssert(!inputrec->bPull,
-                                             "Pulling is not supported by the modular simulator.");
+            && conditionalAssert(!doRerun, "Rerun is not supported by the modular simulator.");
     isInputCompatible =
             isInputCompatible
-            && conditionalAssert(inputrec->opts.ngacc == 1 && inputrec->opts.acc[0][XX] == 0.0
-                                         && inputrec->opts.acc[0][YY] == 0.0
-                                         && inputrec->opts.acc[0][ZZ] == 0.0 && inputrec->cos_accel == 0.0,
+            && conditionalAssert(!inputrec->useConstantAcceleration && inputrec->cos_accel == 0.0,
                                  "Acceleration is not supported by the modular simulator.");
     isInputCompatible =
             isInputCompatible
-            && conditionalAssert(inputrec->opts.ngfrz == 1 && inputrec->opts.nFreeze[0][XX] == 0
-                                         && inputrec->opts.nFreeze[0][YY] == 0
-                                         && inputrec->opts.nFreeze[0][ZZ] == 0,
+            && conditionalAssert(!inputrecFrozenAtoms(inputrec),
                                  "Freeze groups are not supported by the modular simulator.");
     isInputCompatible =
             isInputCompatible
             && conditionalAssert(
-                       inputrec->deform[XX][XX] == 0.0 && inputrec->deform[XX][YY] == 0.0
-                               && inputrec->deform[XX][ZZ] == 0.0 && inputrec->deform[YY][XX] == 0.0
-                               && inputrec->deform[YY][YY] == 0.0 && inputrec->deform[YY][ZZ] == 0.0
-                               && inputrec->deform[ZZ][XX] == 0.0 && inputrec->deform[ZZ][YY] == 0.0
-                               && inputrec->deform[ZZ][ZZ] == 0.0,
-                       "Deformation is not supported by the modular simulator.");
+                    inputrec->deform[XX][XX] == 0.0 && inputrec->deform[XX][YY] == 0.0
+                            && inputrec->deform[XX][ZZ] == 0.0 && inputrec->deform[YY][XX] == 0.0
+                            && inputrec->deform[YY][YY] == 0.0 && inputrec->deform[YY][ZZ] == 0.0
+                            && inputrec->deform[ZZ][XX] == 0.0 && inputrec->deform[ZZ][YY] == 0.0
+                            && inputrec->deform[ZZ][ZZ] == 0.0,
+                    "Deformation is not supported by the modular simulator.");
     isInputCompatible =
             isInputCompatible
             && conditionalAssert(gmx_mtop_interaction_count(globalTopology, IF_VSITE) == 0,
@@ -859,8 +430,8 @@ bool ModularSimulator::isInputCompatible(bool                             exitOn
     isInputCompatible =
             isInputCompatible
             && conditionalAssert(
-                       gmx_mtop_ftype_count(globalTopology, F_ORIRES) == 0,
-                       "Orientation restraints are not supported by the modular simulator.");
+                    gmx_mtop_ftype_count(globalTopology, F_ORIRES) == 0,
+                    "Orientation restraints are not supported by the modular simulator.");
     isInputCompatible =
             isInputCompatible
             && conditionalAssert(ms == nullptr,
@@ -873,11 +444,11 @@ bool ModularSimulator::isInputCompatible(bool                             exitOn
     int numEnsembleRestraintSystems;
     if (fcd)
     {
-        numEnsembleRestraintSystems = fcd->disres.nsystems;
+        numEnsembleRestraintSystems = fcd->disres->nsystems;
     }
     else
     {
-        auto distantRestraintEnsembleEnvVar = getenv("GMX_DISRE_ENSEMBLE_SIZE");
+        auto* distantRestraintEnsembleEnvVar = getenv("GMX_DISRE_ENSEMBLE_SIZE");
         numEnsembleRestraintSystems =
                 (ms != nullptr && distantRestraintEnsembleEnvVar != nullptr)
                         ? static_cast<int>(strtol(distantRestraintEnsembleEnvVar, nullptr, 10))
@@ -895,16 +466,12 @@ bool ModularSimulator::isInputCompatible(bool                             exitOn
             isInputCompatible
             && conditionalAssert(!inputrec->bSimTemp,
                                  "Simulated tempering is not supported by the modular simulator.");
-    isInputCompatible = isInputCompatible
-                        && conditionalAssert(!inputrec->bExpanded,
-                                             "Expanded ensemble simulations are not supported by "
-                                             "the modular simulator.");
     isInputCompatible =
             isInputCompatible
             && conditionalAssert(!doEssentialDynamics,
                                  "Essential dynamics is not supported by the modular simulator.");
     isInputCompatible = isInputCompatible
-                        && conditionalAssert(inputrec->eSwapCoords == eswapNO,
+                        && conditionalAssert(inputrec->eSwapCoords == SwapType::No,
                                              "Ion / water position swapping is not supported by "
                                              "the modular simulator.");
     isInputCompatible =
@@ -919,8 +486,8 @@ bool ModularSimulator::isInputCompatible(bool                             exitOn
     isInputCompatible =
             isInputCompatible
             && conditionalAssert(
-                       getenv("GMX_FORCE_UPDATE_DEFAULT_GPU") == nullptr,
-                       "Integration on the GPU is not supported by the modular simulator.");
+                    getenv("GMX_FORCE_UPDATE_DEFAULT_GPU") == nullptr,
+                    "Integration on the GPU is not supported by the modular simulator.");
     // Modular simulator is centered around NS updates
     // TODO: think how to handle nstlist == 0
     isInputCompatible = isInputCompatible
@@ -930,15 +497,38 @@ bool ModularSimulator::isInputCompatible(bool                             exitOn
     isInputCompatible = isInputCompatible
                         && conditionalAssert(!GMX_FAHCORE,
                                              "GMX_FAHCORE not supported by the modular simulator.");
-
+    if (!isInputCompatible
+        && (inputrec->eI == IntegrationAlgorithm::VV && inputrec->epc == PressureCoupling::ParrinelloRahman))
+    {
+        gmx_fatal(FARGS,
+                  "Requested Parrinello-Rahman barostat with md-vv. This combination is only "
+                  "available in the modular simulator. Some other selected options are, however, "
+                  "only available in the legacy simulator. Use a different pressure control "
+                  "algorithm.");
+    }
     return isInputCompatible;
 }
 
+ModularSimulator::ModularSimulator(std::unique_ptr<LegacySimulatorData>      legacySimulatorData,
+                                   std::unique_ptr<ReadCheckpointDataHolder> checkpointDataHolder) :
+    legacySimulatorData_(std::move(legacySimulatorData)),
+    checkpointDataHolder_(std::move(checkpointDataHolder))
+{
+    checkInputForDisabledFunctionality();
+}
+
 void ModularSimulator::checkInputForDisabledFunctionality()
 {
-    isInputCompatible(true, inputrec, doRerun, *top_global, ms, replExParams, fcd,
-                      opt2bSet("-ei", nfile, fnm), membed != nullptr);
-    if (observablesHistory->edsamHistory)
+    isInputCompatible(true,
+                      legacySimulatorData_->inputrec,
+                      legacySimulatorData_->mdrunOptions.rerun,
+                      legacySimulatorData_->top_global,
+                      legacySimulatorData_->ms,
+                      legacySimulatorData_->replExParams,
+                      legacySimulatorData_->fr->fcdata.get(),
+                      opt2bSet("-ei", legacySimulatorData_->nfile, legacySimulatorData_->fnm),
+                      legacySimulatorData_->membed != nullptr);
+    if (legacySimulatorData_->observablesHistory->edsamHistory)
     {
         gmx_fatal(FARGS,
                   "The checkpoint is from a run with essential dynamics sampling, "
@@ -947,15 +537,31 @@ void ModularSimulator::checkInputForDisabledFunctionality()
     }
 }
 
-SignallerCallbackPtr ModularSimulator::SignalHelper::registerLastStepCallback()
+void ModularSimulator::readCheckpointToTrxFrame(t_trxframe*               fr,
+                                                ReadCheckpointDataHolder* readCheckpointDataHolder,
+                                                const CheckpointHeaderContents& checkpointHeaderContents)
 {
-    return std::make_unique<SignallerCallback>(
-            [this](Step step, Time gmx_unused time) { this->lastStep_ = step; });
+    GMX_RELEASE_ASSERT(checkpointHeaderContents.isModularSimulatorCheckpoint,
+                       "ModularSimulator::readCheckpointToTrxFrame can only read checkpoints "
+                       "written by modular simulator.");
+    fr->bStep = true;
+    fr->step = int64_to_int(checkpointHeaderContents.step, "conversion of checkpoint to trajectory");
+    fr->bTime = true;
+    fr->time  = checkpointHeaderContents.t;
+
+    fr->bAtoms = false;
+
+    StatePropagatorData::readCheckpointToTrxFrame(
+            fr, readCheckpointDataHolder->checkpointData(StatePropagatorData::checkpointID()));
+    if (readCheckpointDataHolder->keyExists(FreeEnergyPerturbationData::checkpointID()))
+    {
+        FreeEnergyPerturbationData::readCheckpointToTrxFrame(
+                fr, readCheckpointDataHolder->checkpointData(FreeEnergyPerturbationData::checkpointID()));
+    }
+    else
+    {
+        FreeEnergyPerturbationData::readCheckpointToTrxFrame(fr, std::nullopt);
+    }
 }
 
-SignallerCallbackPtr ModularSimulator::SignalHelper::registerNSCallback()
-{
-    return std::make_unique<SignallerCallback>(
-            [this](Step step, Time gmx_unused time) { this->nextNSStep_ = step; });
-}
 } // namespace gmx