"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)

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temper/grem command :h3

[Syntax:]

temper/grem N M lambda fix-ID thermostat-ID seed1 seed2 index :pre

N = total # of timesteps to run
M = attempt a tempering swap every this many steps
lambda = initial lambda for this ensemble
fix-ID = ID of fix_grem
thermostat-ID = ID of the thermostat that controls kinetic temperature
seed1 = random # seed used to decide on adjacent temperature to partner with
seed2 = random # seed for Boltzmann factor in Metropolis swap
index = which temperature (0 to N-1) I am simulating (optional) :ul

[Examples:]

temper/grem 100000 1000 ${lambda} fxgREM fxnvt 0 58728
temper/grem 40000 100 ${lambda} fxgREM fxnpt 0 32285 ${walkers} :pre

[Description:]

Run a parallel tempering or replica exchange simulation in LAMMPS
partition mode using multiple generalized replicas (ensembles) of a
system defined by "fix grem"_fix_grem.html, which stands for the
generalized replica exchange method (gREM) originally developed by
"(Kim)"_#KimStraub.  It uses non-Boltzmann ensembles to sample over first
order phase transitions. The is done by defining replicas with an
enthalpy dependent effective temperature

Two or more replicas must be used.  See the "temper"_temper.html
command for an explanation of how to run replicas on multiple
partitions of one or more processors.

This command is a modification of the "temper"_temper.html command and
has the same dependencies, restraints, and input variables which are
discussed there in greater detail.

Instead of temperature, this command performs replica exchanges in
lambda as per the generalized ensemble enforced by "fix
grem"_fix_grem.html.  The desired lambda is specified by {lambda},
which is typically a variable previously set in the input script, so
that each partition is assigned a different temperature.  See the
"variable"_variable.html command for more details.  For example:

variable lambda world 400 420 440 460
fix fxnvt all nvt temp 300.0 300.0 100.0
fix fxgREM all grem ${lambda} -0.05 -50000 fxnvt
temper 100000 100 ${lambda} fxgREM fxnvt 3847 58382 :pre

would define 4 lambdas with constant kinetic temperature but unique
generalized temperature, and assign one of them to "fix
grem"_fix_grem.html used by each replica, and to the grem command.

As the gREM simulation runs for {N} timesteps, a swap between adjacent
ensembles will be attempted every {M} timesteps.  If {seed1} is 0,
then the swap attempts will alternate between odd and even pairings.
If {seed1} is non-zero then it is used as a seed in a random number
generator to randomly choose an odd or even pairing each time.  Each
attempted swap of temperatures is either accepted or rejected based on
a Metropolis criterion, derived for gREM by "(Kim)"_#Kim, which uses
{seed2} in the random number generator.

File management works identical to the "temper"_temper.html command.
Dump files created by this fix contain continuous trajectories and
require post-processing to obtain per-replica information.

The last argument {index} in the grem command is optional and is used
when restarting a run from a set of restart files (one for each
replica) which had previously swapped to new lambda.  This is done
using a variable. For example if the log file listed the following for
a simulation with 5 replicas:

500000 2 4 0 1 3 :pre

then a setting of

variable walkers world 2 4 0 1 3 :pre

would be used to restart the run with a grem command like the example
above with ${walkers} as the last argument. This functionality is
identical to "temper"_temper.html.

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[Restrictions:]

This command can only be used if LAMMPS was built with the USER-MISC
package.  See the "Making LAMMPS"_Section_start.html#start_3 section
for more info on packages.

This command must be used with "fix grem"_fix_grem.html.

[Related commands:]

"fix grem"_fix_grem.html, "temper"_temper.html, "variable"_variable.html

[Default:] none

:link(KimStraub)
[(Kim)] Kim, Keyes, Straub, J Chem Phys, 132, 224107 (2010).
