6 The |Gromacs| MD and analysis programs use user-defined *groups* of atoms
7 to perform certain actions on. The maximum number of groups is 256, but
8 each atom can only belong to six different groups, one each of the
11 temperature-coupling group
12 The temperature coupling parameters (reference temperature, time
13 constant, number of degrees of freedom, see :ref:`update`) can be
14 defined for each T-coupling group separately. For example, in a
15 solvated macromolecule the solvent (that tends to generate more
16 heating by force and integration errors) can be coupled with a
17 shorter time constant to a bath than is a macromolecule, or a
18 surface can be kept cooler than an adsorbing molecule. Many
19 different T-coupling groups may be defined. See also center of mass
24 Atoms that belong to a freeze group are kept stationary in the
25 dynamics. This is useful during equilibration, *e.g.* to avoid badly
26 placed solvent molecules giving unreasonable kicks to protein atoms,
27 although the same effect can also be obtained by putting a
28 restraining potential on the atoms that must be protected. The
29 freeze option can be used, if desired, on just one or two
30 coordinates of an atom, thereby freezing the atoms in a plane or on
31 a line. When an atom is partially frozen, constraints will still be
32 able to move it, even in a frozen direction. A fully frozen atom can
33 not be moved by constraints. Many freeze groups can be defined.
34 Frozen coordinates are unaffected by pressure scaling; in some cases
35 this can produce unwanted results, particularly when constraints are
36 also used (in this case you will get very large pressures).
37 Accordingly, it is recommended to avoid combining freeze groups with
38 constraints and pressure coupling. For the sake of equilibration it
39 could suffice to start with freezing in a constant volume
40 simulation, and afterward use position restraints in conjunction
41 with constant pressure.
45 On each atom in an “accelerate group” an acceleration
46 :math:`\mathbf{a}^g` is imposed. This is equivalent to
47 a mass-weighted external force. This feature makes it possible to
48 drive the system into a non-equilibrium state to compute,
49 for example, transport properties.
53 Mutual interactions between all energy-monitor groups are compiled
54 during the simulation. This is done separately for Lennard-Jones and
55 Coulomb terms. In principle up to 256 groups could be defined, but
56 that would lead to 256\ :math:`\times`\ 256 items! Better use this
59 All non-bonded interactions between pairs of energy-monitor groups
60 can be excluded (see details in the User Guide). Pairs of particles
61 from excluded pairs of energy-monitor groups are not put into the
62 pair list. This can result in a significant speedup for simulations
63 where interactions within or between parts of the system are not
68 In |Gromacs|, the center of mass (COM) motion can be removed, for
69 either the complete system or for groups of atoms. The latter is
70 useful, *e.g.* for systems where there is limited friction (*e.g.*
71 gas systems) to prevent center of mass motion to occur. It makes
72 sense to use the same groups for temperature coupling and center of
75 Compressed position output group
77 In order to further reduce the size of the compressed trajectory
78 file (:ref:`xtc` or :ref:`tng`), it is possible to
79 store only a subset of all particles. All x-compression groups that
80 are specified are saved, the rest are not. If no such groups are
81 specified, than all atoms are saved to the compressed trajectory
84 The use of groups in |Gromacs| tools is described in
85 sec. :ref:`usinggroups`.