lammps/examples/UNITS

This directory has 3 scripts which show how to run the same problem
using the 3 most common units system used in LAMMPS: lj, real, and
metal units.  As stated on the units command doc page:

"Any simulation you perform for one choice of units can be duplicated
with any other unit setting LAMMPS supports. ...  To perform the same
simulation in a different set of units you must change all the
unit-based input parameters in your input script and other input files
(data file, potential files, etc) correctly to the new units.  And you
must correctly convert all output from the new units to the old units
when comparing to the original results.  That is often not simple to
do."

These examples are meant to illustrate how to do this for a simple
Lennard-Jones liquid (argon).  All of the scripts have a set of
variables defined at the top which can be changed as command line
arguments (e.g. -v cutoff 3.0).  All 3 scripts give identical output,
modulo round-offs due to the finite precision of the conversion
factors used, either internally in LAMMPS or in the scripts.  If there
were run for a long time, the trajectories would diverge, but they
would still give statistically identical results.

The LJ script is the simplest; it is similar to the bench/in.lj
script.

The real and metal scripts each have a set of variables at the top
which define scale factors for converting quantities like distance,
energy, pressure from reduced LJ units to real or metal units.  Once
these are defined the rest of the input script is very similar to the
LJ script.  The appropriate scale factor is applied to every input.
Output quantities are printed in both the native real/metal units and
unscaled back to LJ units.  So that you can see the outputs are the
same if you examine the log files.  Comments about this comparison
are at the bottom of the real and metal scripts.

These 3 scripts are provided, because converting from lj reduced units
to physical units (e.g. real or metal) or vice versa is the trickiest
case.  Converting input scripts between 2 sets of physical units
(e.g. reak <--> metal) is much easier.  But you can use the same ideas
as in these scripts; just define a set of scale/unscale factors.

See Allen & Tildesley's Computer Simulation of Liquids, Appendix B for
a nice discussion of reduced units.  It will explain the conversion
formulas used in the real and metal scripts.

Hopefully, if you study these scripts, you should be able to convert
an input script of your own, written in one set of units, to an
identical input script in an alternate set of units.  Where
"identical" means it runs the same simulation in a statistical sense.

You can find the full set of scale factors LAMMPS uses internally for
different unit systems it supports, at the top of the src/update.cpp
file.  A couple of those values are used in the real and metal
scripts.