# Multi-replica simulations

Several commands in LAMMPS run multi-replica simulations, meaning that
multiple instances (replicas) of your simulation are run simultaneously,
with small amounts of data exchanged between replicas periodically.

These are the relevant commands:

-   [hyper](hyper) for bond boost hyperdynamics (HD)
-   [neb](neb) for nudged elastic band calculations (NEB)
-   [neb_spin](neb_spin) for magnetic nudged elastic band calculations
-   [prd](prd) for parallel replica dynamics (PRD)
-   [tad](tad) for temperature accelerated dynamics (TAD)
-   [temper](temper) for parallel tempering with fixed volume
-   [temper/npt](temper_npt) for parallel tempering extended for NPT
-   [temper/grem](temper_grem) for parallel tempering with generalized
    replica exchange (gREM)
-   [fix pimd](fix_pimd) for path-integral molecular dynamics (PIMD)

NEB is a method for finding transition states and barrier potential
energies. HD, PRD, and TAD are methods for performing accelerated
dynamics to find and perform infrequent events. Parallel tempering or
replica exchange runs different replicas at a series of temperature to
facilitate rare-event sampling. PIMD runs different replicas whose
individual particles in different replicas are coupled together by
springs to model a system of ring-polymers which can represent the
quantum nature of atom cores.

These commands can only be used if LAMMPS was built with the REPLICA
package. See the [Build package](Build_package) page for more info.

In all these cases, you must run with one or more processors per
replica. The processors assigned to each replica are determined at
run-time by using the [-partition command-line switch](Run_options) to
launch LAMMPS on multiple partitions, which in this context are the same
as replicas. E.g. these commands:

``` bash
mpirun -np 16 lmp_linux -partition 8x2 -in in.temper
mpirun -np 8 lmp_linux -partition 8x1 -in in.neb
```

would each run 8 replicas, on either 16 or 8 processors. Note the use of
the [-in command-line switch](Run_options) to specify the input script
which is required when running in multi-replica mode.

Also note that with MPI installed on a machine (e.g. your desktop), you
can run on more (virtual) processors than you have physical processors.
Thus, the above commands could be run on a single-processor (or
few-processor) desktop so that you can run a multi-replica simulation on
more replicas than you have physical processors. This is useful for
testing and debugging, since with most modern processors and MPI
libraries, the efficiency of a calculation can severely diminish when
oversubscribing processors.