<p>Run a parallel tempering or replica exchange simulation using multiple
replicas (ensembles) of a system. Two or more replicas must be used.</p>
<p>Each replica runs on a partition of one or more processors. Processor
partitions are defined at run-time using the -partition command-line
switch; see <aclass="reference internal"href="Section_start.html#start-7"><spanclass="std std-ref">Section 2.7</span></a> of the
manual. Note that if you have MPI installed, you can run a
multi-replica simulation with more replicas (partitions) than you have
physical processors, e.g you can run a 10-replica simulation on one or
two processors. You will simply not get the performance speed-up you
would see with one or more physical processors per replica. See <aclass="reference internal"href="Section_howto.html#howto-5"><spanclass="std std-ref">this section</span></a> of the manual for further
discussion.</p>
<p>Each replica’s temperature is controlled at a different value by a fix
with <em>fix-ID</em> that controls temperature. Most thermostat fix styles
(with and without included time integration) are supported. The command
will print an error message and abort, if the chosen fix is unsupported.
The desired temperature is specified by <em>temp</em>, which is typically a
variable previously set in the input script, so that each partition is
assigned a different temperature. See the <aclass="reference internal"href="variable.html"><spanclass="doc">variable</span></a>
command for more details. For example:</p>
<divclass="highlight-default"><divclass="highlight"><pre><span></span>variable t world 300.0 310.0 320.0 330.0
fix myfix all nvt temp $t $t 100.0
temper 100000 100 $t myfix 3847 58382
</pre></div>
</div>
<p>would define 4 temperatures, and assign one of them to the thermostat
used by each replica, and to the temper command.</p>
<p>As the tempering simulation runs for <em>N</em> timesteps, a temperature swap
between adjacent ensembles will be attempted every <em>M</em> timesteps. If
<em>seed1</em> is 0, then the swap attempts will alternate between odd and
even pairings. If <em>seed1</em> 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 Boltzmann-weighted Metropolis criterion which uses
<em>seed2</em> in the random number generator.</p>
<p>As a tempering run proceeds, multiple log files and screen output
files are created, one per replica. By default these files are named
log.lammps.M and screen.M where M is the replica number from 0 to N-1,
with N = # of replicas. See the <aclass="reference internal"href="Section_start.html#start-7"><spanclass="std std-ref">section on command-line switches</span></a> for info on how to change these
names.</p>
<p>The main screen and log file (log.lammps) will list information about
which temperature is assigned to each replica at each thermodynamic
output timestep. E.g. for a simulation with 16 replicas:</p>
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