I would like to ask if there is any particular reason why pair styles:

lj/charmm/coul/charmm and ../long

cannot be modified via:

pair_modify mix geometric

I need to use geometric mean to obtain my VdW parameters, so I have
modified pair_lj_charmm_coul_charmm.cpp and pair_lj_charmm_coul_long.cpp
to contain:

I don't recall details, but I think arithmetic mixing
was imposed b/c that is what the CHARMM force
field does and this particular pair potential was added
to provide an exact match to a particular CHARMM ff.

Paul - do you recall? Is there a good reason not to
allow geometric mixing for both the primary
and 1-4 sigma/epsilon? Note the default mix rule
is geometric, so maybe this was just meant to
override the default for the CHARMM ff?

Yes, my recollection is that arithmetic mixing was imposed to match CHARMM. Seems to me that arithmetic should be the default when using the CHARMM pair styles, but geometric should be allowed.

Just posted a 21Dec10 patch that now allows
different mixing options for the CHARMM pair styles.
Arithmetic is still the default for these, whereas geometric
is the default for most other pair styles. So you'll need
to use pair_modify mix to change to geometric.