"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c

:link(lws,http://lammps.sandia.gov)
:link(ld,Manual.html)
:link(lc,Section_commands.html#comm)

:line

fix gcmc command :h3

[Syntax:]

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

ID, group-ID are documented in "fix"_fix.html command :ulb,l
gcmc = style name of this fix command :l
N = invoke this fix every N steps :l
X = number of exchanges to attempt every N steps :l
M = number of MC displacements to attempt every N steps :l
type = atom type of exchanged particles :l
seed = random # seed (positive integer) :l
T = temperature of the ideal gas reservoir (temperature units) :l
mu = chemical potential of the ideal gas reservoir (energy units) :l
displace = maximum Monte Carlo displacement distance (length units) :l
zero or more keyword/value pairs may be appended to args :l
keyword = {molecule}
  {molecule} value = {no} or {yes} :pre
:ule

[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 :pre

[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)"_#Frenkel. If used with the "fix nvt"_fix_nh.html 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"_fix_nvt.html, resulting in a hybrid GCMC+MD simulation.

If used with "fix_nvt"_fix_nvt.html, the temperature of the imaginary
reservoir, T, should be set to be equivalent to the target temperature
used in "fix_nvt"_fix_nvt.html. 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"_neighbor.html 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"_compute_modify.html 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 :pre

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"_restart.html.  This includes information about the random
number generator seed, the next timestep for MC exchanges, etc.  See
the "read_restart"_read_restart.html 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"_fix_modify.html options are relevant to this
fix.

This fix computes a global vector of length 6, which can be accessed
by various "output commands"_Section_howto.html#howto_15.  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 :ul

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"_run.html command.  This fix is not invoked during "energy
minimization"_minimize.html.

[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_start.html#start_3 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"_pair_style.html command, are included in this
calculation.  Long-range interactions due to a
"kspace_style"_kspace_style.html 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"_pair_tersoff.html and "Stillinger-Weber"_pair_sw.html cannot
be used.  "EAM"_pair_eam.html potentials for metals only include the
pair potential portion of the EAM interaction, not the embedding term.

[Related commands:]

"fix_nvt"_fix_nvt.html, "neighbor"_neighbor.html, 
"fix_deposit"_fix_deposit.html, "fix_evaporate"_fix_evaporate.html

[Default:]

The option defaults are molecule = no.

:line

:link(Frenkel)
[(Frenkel)] Frenkel and Smit, Understanding Molecular Simulation, 
Academic Press, London, 2002.
