Different LJ mixing rules for atom types

Dear All

I use various FF in my simulation, each having its own mix rule.

Is it possible to specify different mixing rule in LAMMPS?
I’ve already looked up pair_modify, but seemingly it does not support my case.

Does anybody have such experience?

Many thanks in advance

This is usually done by setting coefficients for each pair explicitly.

Are you sure this is the ONLY option?

Some FF is really complex and inserting interaction parameters explicitly is really tedious!!!

Well, if it can be automated you could write a simple script that generates the coefficient-setting code and include that in your lammps script. That would be less tedious, especially if you need to make some changes every once in a while.

For Lennard-Jones, you can specify a mixing rule like “geometric” or “arithmetic” or “sixthpower”. See http://lammps.sandia.gov/doc/pair_modify.html.

Stan

For Lennard-Jones, you can specify a mixing rule like “geometric” or
“arithmetic” or “sixthpower”. See
http://lammps.sandia.gov/doc/pair_modify.html.

also, LAMMPS has a "write_coeffs" command, so you could set the mixing
rules with pair_modify to different settings and then write out all
mixed coefficients and then assemble your desired set of coefficients.

however, there is a **big fat warning**: different mixing rules
usually mean, that you have parameters obtained from different
parameterization strategies. mixing such parameters is a big problem,
as each set has a different strategy to balance interactions, which
means, that you will have inconsistent total interactions. after all,
it is ambiguous which mixing rule applies to obtaining mixed
parameters from two sets that require a different mixing rule each?

in general, it is much preferable to look up parameter sets that are
consistent. be aware, that careful reviewers will notice the
inconsistency and complain about it and it will taint the merit of
your studies.

axel.