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### fix gcmc command

Syntax:

    fix ID group-ID gcmc N X M type seed T mu displace keyword values ...

- ID, group-ID are documented in fix command
- gcmc = style name of this fix command
- N = invoke this fix every N steps
- X = number of exchanges to attempt every N steps
- M = number of MC displacements to attempt every N steps
- type = atom type of exchanged particles
- seed = random # seed (positive integer)
- T = temperature of the ideal gas reservoir (temperature units)
- mu = chemical potential of the ideal gas reservoir (energy units)
- displace = maximum Monte Carlo displacement distance (length units)
- zero or more keyword/value pairs may be appended to args

      keyword = molecule
        molecule value = no or yes

Examples:

    fix 2 all gcmc 10 1000 1000 2 29494 298.0 -0.5 0.01
    fix 3 all gcmc 10 100 100 1 3456543 3.0 -2.5 0.1 molecule yes

Description:

This fix performs grand canonical Monte Carlo (GCMC) exchanges of particles of the given type with an imaginary ideal gas reservoir at the specified T and chemical potential (mu) as discussed in (Frenkel). If used with the fix nvt command, simulations in the grand canonical enemble (muVT, constant chemical potential, constant volume, and constant temperature) can be performed. Specific uses include computing isotherms in microporous materials, or computing vapor-liquid coexistence curves.

Perform up to X exchanges of particles of the given type between the simulation domain and the imaginary reservoir every N timesteps. Also perform M Monte Carlo displacements of particles of the given type within the simulation domain. M should typically be chosen to be approximately equal to the expected number of particles of the given type within the domain, which will result in roughly one MC translation per particle per MC cycle.

This fix cannot be used to perform MC displacements of particles other than the exchanged type. All particles in the simulation domain can be moved using regular time integration displacements, e.g. via fix_nvt, resulting in a hybrid GCMC+MD simulation.

If used with fix_nvt, the temperature of the imaginary reservoir, 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 domain.

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 displacements per timestep that a particle can move beyond the neighbor list skin distance. See the neighbor command for details.

When a particle is to be inserted, its coordinates are chosen as a random position within the current simulation domain, and its velocity is randomly chosen from the specified temperature distribution given by T.

Exchanged particles have the specified atom type and are assigned to two groups: the default group "all" and the group specified in the fix gcmc command (which can also be "all").

If the setting for the molecule keyword is no, then only single atoms are exchanged. In this case, you should ensure you do not delete only a portion of a molecule (only some of its atoms), or LAMMPS will soon generate an error when it tries to find those atoms. LAMMPS will warn you if any of the atoms eligible for deletion have a non-zero molecule ID, but does not check for this at the time of deletion.

If the setting for the molecule keyword is yes, entire molecules are exchanged. This feature is not yet supported.

Use of this fix typically will cause the number of atoms to fluctuate, therefore, you will want to use the compute_modify command to insure that the current number of atoms is used as a normalizing factor each time temperature is computed. Here is the necessary command:

    compute_modify thermo_temp dynamic yes

If LJ units are used, note that a value of 0.18292026 is used by this fix as the reduced value for Planck's constant. This value was derived from LJ paramters for argon, where h* = h/sqrt(sigma^2 * epsilon * mass), sigma = 3.429 angstroms, epsilon/k = 121.85 K, and mass = 39.948 amu.

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

This fix writes the state of the deposition to binary restart files. This includes information about the random number generator seed, the next timestep for MC exchanges, 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.

None of the fix_modify options are relevant to this fix.

This fix computes a global vector of length 6, which can be accessed by various output commands. The vector values are the following global cummulative quantities:

- 1 = displacement attempts
- 2 = displacement successes
- 3 = deletion attempts
- 4 = deletion successes
- 5 = insertion attempts
- 6 = insertion successes

The vector values calculated by this fix are "extensive".

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 Making LAMMPS section for more info.

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

You cannot currently exchange charged particles or molecules with a net charge.

Only pairwise interactions, as defined by the pair_style command, are included in this calculation. Long-range interactions due to a kspace_style command are not included. Not all pair potentials can be evaluated in a pairwise mode as required by this fix. For example, 3-body potentials, such as Tersoff and Stillinger-Weber cannot be used. EAM potentials for metals only include the pair potential portion of the EAM interaction, not the embedding term.

Related commands:

fix_nvt, neighbor, fix_deposit, fix_evaporate

Default:

The option defaults are molecule = no.

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