3 Mixed Quantum-Classical simulation techniques
4 ---------------------------------------------
6 In a molecular mechanics (MM) force field, the influence of electrons is
7 expressed by empirical parameters that are assigned on the basis of
8 experimental data, or on the basis of results from high-level quantum
9 chemistry calculations. These are valid for the ground state of a given
10 covalent structure, and the MM approximation is usually sufficiently
11 accurate for ground-state processes in which the overall connectivity
12 between the atoms in the system remains unchanged. However, for
13 processes in which the connectivity does change, such as chemical
14 reactions, or processes that involve multiple electronic states, such as
15 photochemical conversions, electrons can no longer be ignored, and a
16 quantum mechanical description is required for at least those parts of
17 the system in which the reaction takes place.
19 One approach to the simulation of chemical reactions in solution, or in
20 enzymes, is to use a combination of quantum mechanics (QM) and molecular
21 mechanics (MM). The reacting parts of the system are treated quantum
22 mechanically, with the remainder being modeled using the force field.
23 The current version of |Gromacs| provides interfaces to several popular
24 Quantum Chemistry packages (MOPAC :ref:`150 <refmopac>`,
25 GAMESS-UK \ :ref:`151 <refgamess-uk>`, Gaussian \ :ref:`152 <refg03>` and
26 CPMD \ :ref:`153 <refCar85a>`).
28 |Gromacs| interactions between the two subsystems are either handled as
29 described by Field et al. :ref:`154 <refField90a>` or within
30 the ONIOM approach by Morokuma and coworkers \ :ref:`155 <refMaseras96a>`,
31 :ref:`156 <refSvensson96a>`.
36 Two approaches for describing the interactions between the QM and MM
37 subsystems are supported in this version:
39 #. **Electronic Embedding** The electrostatic interactions between the
40 electrons of the QM region and the MM atoms and between the QM nuclei
41 and the MM atoms are included in the Hamiltonian for the QM
47 H^{QM}_e-\sum_i^n\sum_J^M\frac{e^2Q_J}{4\pi\epsilon_0r_{iJ}}+\sum_A^N\sum_J^M\frac{e^2Z_AQ_J}{e\pi\epsilon_0R_{AJ}},
49 where :math:`n` and :math:`N` are the number of electrons and nuclei
50 in the QM region, respectively, and :math:`M` is the number of
51 charged MM atoms. The first term on the right hand side is the
52 original electronic Hamiltonian of an isolated QM system. The first
53 of the double sums is the total electrostatic interaction between the
54 QM electrons and the MM atoms. The total electrostatic interaction of
55 the QM nuclei with the MM atoms is given by the second double sum.
56 Bonded interactions between QM and MM atoms are described at the MM
57 level by the appropriate force-field terms. Chemical bonds that
58 connect the two subsystems are capped by a hydrogen atom to complete
59 the valence of the QM region. The force on this atom, which is
60 present in the QM region only, is distributed over the two atoms of
61 the bond. The cap atom is usually referred to as a link atom.
63 #. **ONIOM** In the ONIOM approach, the energy and gradients are first
64 evaluated for the isolated QM subsystem at the desired level of *ab
65 initio* theory. Subsequently, the energy and gradients of the total
66 system, including the QM region, are computed using the molecular
67 mechanics force field and added to the energy and gradients
68 calculated for the isolated QM subsystem. Finally, in order to
69 correct for counting the interactions inside the QM region twice, a
70 molecular mechanics calculation is performed on the isolated QM
71 subsystem and the energy and gradients are subtracted. This leads to
72 the following expression for the total QM/MM energy (and gradients
78 +E_{I+II}^{MM}-E_{I}^{MM},
80 where the subscripts I and II refer to the QM and MM subsystems,
81 respectively. The superscripts indicate at what level of theory the
82 energies are computed. The ONIOM scheme has the advantage that it is
83 not restricted to a two-layer QM/MM description, but can easily
84 handle more than two layers, with each layer described at a different
90 QMMM is currently not supported in GROMACS.