revised documentation, added temper_grem ref to fix_grem

or constant pressure algorithms can be used.

The fix enforces a generalized ensemble in a single replica

only. Typically, different replicas only differ by {lambda}

for simplicity, but this is not necessary. Multi-replica 

runs need to be run outside of LAMMPS. An example of this 

can be found in examples/USER/misc/grem

only. Typically, this ideaology is combined with replica 

exchange with replicas differing by {lambda} only

for simplicity, but this is not required. A multi-replica 

simulation can be run within the LAMMPS environment using the

"grem"_temper_grem.html command. This utilizes LAMMPS partition

mode and requires the number of available processors be 

on the order of the number of desired replicas. A 100-replica

simulation would require at least 100 processors (1 per world

at minimum). If a many replicas are needed on a small number of

processors, multi-replica runs can be run outside of LAMMPS.

An example of this can be found in examples/USER/misc/grem and 

has no limit on the number of replicas per processor. However, 

this is very inefficient and error prone and should be avoided

if possible. 

In general, defining the generalized ensembles is unique for 

every system. When starting a many-replica simulation without 

[Related commands:]

"fix_nh"_fix_nh.html

"thermo_modify"_thermo_modify.html

"grem"_temper_grem.html, "fix_nh"_fix_nh.html, "thermo_modify"_thermo_modify.html

[Default:] none

[Examples:]

grem 100000 100 ${lambda} fxgREM fxnvt 0 58728

grem 40000 100 ${lambda} fxgREM fxnpt 0 32285 $w :pre

grem 100000 1000 ${lambda} fxgREM fxnvt 0 58728

grem 40000 100 ${lambda} fxgREM fxnpt 0 32285 ${walkers} :pre

[Description:]

Run a parallel tempering or replica exchange simulation using multiple

generalized replicas (ensembles) of a system defined by 

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. Two or more replicas must be used. 

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 detail.

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.

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, and assign one

of them to "fix_grem"_fix_grem.html used by each replica, and to the grem

command.

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

then a setting of

variable w world 2 4 0 1 3 :pre

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 $w as the last argument. This functionality

example above with ${walkers} as the last argument. This functionality

is identical to "temper"_temper.html.

:line