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fix gemc command

Syntax

fix ID group-ID gemc N M X V T displace maxvol seed

  • ID, group-ID are documented in fix command

  • gemc = style name of this fix command

  • N = invoke this fix every N steps

  • M = average number of MC moves to attempt every N steps

  • X = average number of GEMC exchanges to attempt every N steps

  • V = average number of GEMC volume changes to attempt every N steps

  • T = temperature of the Gibbs ensemble (temperature units)

  • displace = maximum Monte Carlo translation distance (length units)

  • maxdlogvolratio = maximum change in ln(vol1/vol2) (unitless)

  • seed = random # seed (positive integer)

  • zero keyword/value pairs may be appended to args

Examples

fix 2 all gemc 100 10 20 30 1.1 0.5 100.0 29494

Description

Added in version TBD.

This fix performs Gibbs ensemble Monte Carlo (GEMC) exchanges of atoms and volume between two simulation cells at specified T. It also attempts Monte Carlo (MC) moves (atom translations) within the simulation cell. This is usually used to establish thermodynamic equilibrium between bulk vapor and liquid phases, as discussed in (Frenkel). If used with the fix nvt command, hybrid MD/MC simulations in the Gibbs ensemble (equal pressure, equal chemical potential, constant total volume, and constant temperature) can be performed. Specific uses include computing vapor-liquid coexistence curves.

Every N timesteps the fix attempts GEMC atom exchanges, GEMC volume changes, and MC moves of atoms. On those timesteps, the average number of attempted GEMC atom exchanges is X, the average number of volume changes is V, and the average number of attempted MC moves is M.

This fix requires that LAMMPS be run with two partitions that instantiate the two simulation cells. This requires using the -partition command-line switch. For example, on a workstation with 12 cores, -partition 2x6 could be used. For better performance, it is recommended that the two partitions be initialized at two different densities. This allows assigning only one core to the partition running the vapor phase and all the other cores to the partition running the liquid phase. This can be achieved in a single script by using the partition command to initiate the two partitions at different densities. On a workstation with 12 processor cores, the command line option -partition 1 8 can be used to assign 1 core to the first (vapor) partition and 8 cores to the second (liquid) partition.

If used with fix nvt, the temperature of the Gibbs ensemble, T, should be set to be equivalent to the target temperature used in fix nvt. Otherwise, the imaginary reservoir will not be in thermal equilibrium with the simulation cell. Also, it is important that the temperature used by fix nvt is dynamically updated, which can be achieved as follows:

compute mdtemp mdatoms temp
compute_modify mdtemp dynamic/dof yes
fix mdnvt mdatoms nvt temp 300.0 300.0 10.0
fix_modify mdnvt temp mdtemp

Note that neighbor lists are re-built every timestep that this fix is invoked, so you should not set N to be too small. However, periodic rebuilds are necessary in order to avoid dangerous rebuilds and missed interactions. Specifically, avoid performing so many MC translations per timestep that atoms can move beyond the neighbor list skin distance. See the neighbor command for details.

When an atom is inserted in either partition, its coordinates are chosen at a random position within the current simulation cell, and new atom velocities are randomly chosen from the specified temperature distribution given by T.

Some fixes have an associated potential energy. Examples of such fixes include: efield, gravity, addforce, langevin, restrain, temp/berendsen, temp/rescale, and wall fixes. For that energy to be included in the total potential energy of the system (the quantity used when performing GEMC atom exchange, GEMC volume exchange and MC moves), you MUST enable the fix_modify energy option for that fix. The doc pages for individual fix commands specify if this should be done.

Use of this fix typically will cause the number of atoms in each cell to fluctuate, therefore, you will want to use the compute_modify dynamic/dof command to ensure that the current number of atoms is used as a normalizing factor each time temperature is computed. A simple example of this is:

compute_modify thermo_temp dynamic/dof yes

A more complicated example is listed earlier on this page in the context of NVT dynamics.

Note

If the density of the cell is initially very small or zero, and increases to a much larger density after a period of equilibration, then certain quantities that are only calculated once at the start (kspace parameters) may no longer be accurate. The solution is to start a new simulation after the equilibrium density has been reached.

Restart, fix_modify, output, run start/stop, minimize info

This fix writes the state of the fix to binary restart files. This includes information about the random number generator seed, the next timestep for MC exchanges, the number of MC step attempts and successes etc. See the read_restart command for info on how to re-specify a fix in an input script that reads a restart file, so that the operation of the fix continues in an uninterrupted fashion.

Note

For this to work correctly, the timestep must not be changed after reading the restart with reset_timestep. The fix will try to detect it and stop with an error.

None of the fix_modify options are relevant to this fix.

This fix computes a global vector of length 8, which can be accessed by various output commands. The vector values are the following global cumulative quantities:

  1. translation attempts

  2. translation successes

  3. exchange attempts

  4. exchange successes

  5. volume change attempts

  6. volume change successes

The vector values calculated by this fix are “intensive”.

No parameter of this fix can be used with the start/stop keywords of the run command. This fix is not invoked during energy minimization.

Restrictions

This fix is part of the MC package. It is only enabled if LAMMPS was built with that package. See the Build package doc page for more info.

Do not set neigh_modify once yes or else this fix will never be called. Reneighboring is required.

Fix gemc currently only supports MC moves and exchanges on individual atoms.

Use of multiple fix gemc commands in the same input script can be problematic.

Defaults

None

(Frenkel) Frenkel and Smit, Understanding Molecular Simulation, Academic Press, London, 2002.