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### run_style command

Syntax:

    run_style style args

- style = verlet or respa

        verlet args = none
        respa args = N n1 n2 ... keyword values ...
          N = # of levels of rRESPA
          n1, n2, ... = loop factor between rRESPA levels (N-1 values)
          zero or more keyword/value pairings may be appended to the loop factors
          keyword = bond or angle or dihedral or improper or
      	      pair or inner or middle or outer or kspace
            bond value = M
              M = which level (1-N) to compute bond forces in
            angle value = M
              M = which level (1-N) to compute angle forces in
            dihedral value = M
              M = which level (1-N) to compute dihedral forces in
            improper value = M
              M = which level (1-N) to compute improper forces in
            pair value = M
              M = which level (1-N) to compute pair forces in
            inner values = M cut1 cut2
              M = which level (1-N) to compute pair inner forces in
      	cut1 = inner cutoff between pair inner and
      	       pair middle or outer  (distance units)
      	cut2 = outer cutoff between pair inner and
      	       pair middle or outer  (distance units)
            middle values = M cut1 cut2
              M = which level (1-N) to compute pair middle forces in
      	cut1 = inner cutoff between pair middle and pair outer (distance units)
      	cut2 = outer cutoff between pair middle and pair outer (distance units)
            outer value = M
              M = which level (1-N) to compute pair outer forces in
            kspace value = M
              M = which level (1-N) to compute kspace forces in

Examples:

    run_style verlet
    run_style respa 4 2 2 2 bond 1 dihedral 2 pair 3 kspace 4
    run_style respa 4 2 2 2 bond 1 dihedral 2 inner 3 5.0 6.0 outer 4 kspace 4

Description:

Choose the style of time integrator used for molecular dynamics simulations performed by LAMMPS.

The verlet style is a velocity-Verlet integrator.

The respa style implements the rRESPA multi-timescale integrator (Tuckerman) with N hierarchical levels, where level 1 is the innermost loop (shortest timestep) and level N is the outermost loop (largest timestep). The loop factor arguments specify what the looping factor is between levels. N1 specifies the number of iterations of level 1 for a single iteration of level 2, N2 is the iterations of level 2 per iteration of level 3, etc. N-1 looping parameters must be specified.

The timestep command sets the timestep for the outermost rRESPA level. Thus if the example command above for a 4-level rRESPA had an outer timestep of 4.0 fmsec, the inner timestep would be 8x smaller or 0.5 fmsec. All other LAMMPS commands that specify number of timesteps (e.g. neigh_modify parameters, dump every N timesteps, etc) refer to the outermost timesteps.

The rRESPA keywords enable you to specify at what level of the hierarchy various forces will be computed. If not specified, the defaults are that bond forces are computed at level 1 (innermost loop), angle forces are computed where bond forces are, dihedral forces are computed where angle forces are, improper forces are computed where dihedral forces are, pair forces are computed at the outermost level, and kspace forces are computed where pair forces are. The inner, middle, outer forces have no defaults.

The inner and middle keywords take additional arguments for cutoffs that are used by the pairwise force computations. If the 2 cutoffs for inner are 5.0 and 6.0, this means that all pairs up to 6.0 apart are computed by the inner force. Those between 5.0 and 6.0 have their force go ramped to 0.0 so the overlap with the next regime (middle or outer) is smooth. The next regime (middle or outer) will compute forces for all pairs from 5.0 outward, with those from 5.0 to 6.0 having their value ramped in an inverse manner.

Only some pair potentials support the use of the inner and middle and outer keywords. If not, only the pair keyword can be used with that pair style, meaning all pairwise forces are computed at the same rRESPA level. See the doc pages for individual pair styles for details.

When using rRESPA (or for any MD simulation) care must be taken to choose a timestep size(s) that insures the Hamiltonian for the chosen ensemble is conserved. For the constant NVE ensemble, total energy must be conserved. Unfortunately, it is difficult to know a priori how well energy will be conserved, and a fairly long test simulation (~10 ps) is usually necessary in order to verify that no long-term drift in energy occurs with the trial set of parameters.

With that caveat, a few rules-of-thumb may be useful in selecting respa settings. The following applies mostly to biomolecular simulations using the CHARMM or a similar all-atom force field, but the concepts are adaptable to other problems. Without SHAKE, bonds involving hydrogen atoms exhibit high-frequency vibrations and require a timestep on the order of 0.5 fmsec in order to conserve energy. The relatively inexpensive force computations for the bonds, angles, impropers, and dihedrals can be computed on this innermost 0.5 fmsec step. The outermost timestep cannot be greater than 4.0 fmsec without risking energy drift. Smooth switching of forces between the levels of the rRESPA hierarchy is also necessary to avoid drift, and a 1-2 angstrom "healing distance" (the distance between the outer and inner cutoffs) works reasonably well. We thus recommend the following settings for use of the respa style without SHAKE in biomolecular simulations:

    timestep  4.0
    run_style respa 4 2 2 2 inner 2 4.5 6.0 middle 3 8.0 10.0 outer 4

With these settings, users can expect good energy conservation and roughly a 2.5 fold speedup over the verlet style with a 0.5 fmsec timestep.

If SHAKE is used with the respa style, time reversibility is lost, but substantially longer time steps can be achieved. For biomolecular simulations using the CHARMM or similar all-atom force field, bonds involving hydrogen atoms exhibit high frequency vibrations and require a time step on the order of 0.5 fmsec in order to conserve energy. These high frequency modes also limit the outer time step sizes since the modes are coupled. It is therefore desirable to use SHAKE with respa in order to freeze out these high frequency motions and increase the size of the time steps in the respa hierarchy. The following settings can be used for biomolecular simulations with SHAKE and rRESPA:

    fix             2 all shake 0.000001 500 0 m 1.0 a 1
    timestep        4.0
    run_style	respa 2 2 inner 1 4.0 5.0 outer 2

With these settings, users can expect good energy conservation and roughly a 1.5 fold speedup over the verlet style with SHAKE and a 2.0 fmsec timestep.

For non-biomolecular simulations, the respa style can be advantageous if there is a clear separation of time scales - fast and slow modes in the simulation. Even a LJ system can benefit from rRESPA if the interactions are divided by the inner, middle and outer keywords. A 2-fold or more speedup can be obtained while maintaining good energy conservation. In real units, for a pure LJ fluid at liquid density, with a sigma of 3.0 angstroms, and epsilon of 0.1 Kcal/mol, the following settings seem to work well:

    timestep  36.0
    run_style respa 3 3 4 inner 1 3.0 4.0 middle 2 6.0 7.0 outer 3

Restrictions: none

Whenever using rRESPA, the user should experiment with trade-offs in speed and accuracy for their system, and verify that they are conserving energy to adequate precision.

Related commands:

timestep, run

Default:

    run_style verlet

(Tuckerman) Tuckerman, Berne and Martyna, J Chem Phys, 97, p 1990 (1992).