Simeltaneous use of potentials: comb3 & LJ

Hi,

For a system of simulation, I need to include different residues. For my case, literature shows that comb3 can predict the behavior of one kin of residue (some atom types), while LJ (opls) has been shown to be appropriate for the other atom types of the system. I therefore need to apply comb3 and LJ (opls) at the same time by hybrid command.
Considering comb3 to be a manybody potential, is it possible to do that in lammps? What solutions lammps offer for simulation of the cases like this?
Thanks

Bests
X

Hi,

For a system of simulation, I need to include different residues. For my
case, literature shows that comb3 can predict the behavior of one kin of
residue (some atom types), while LJ (opls) has been shown to be appropriate
for the other atom types of the system. I therefore need to apply comb3 and
LJ (opls) at the same time by hybrid command.
Considering comb3 to be a manybody potential, is it possible to do that in
lammps?

technically, yes. whether this will result in a meaningful simulation
is a very different story. it is almost always preferable to use a
potential style that encompasses the entire system.

What solutions lammps offer for simulation of the cases like this?

pair_style hybrid allows mechanical coupling between different types
of potentials, but in your case, there are multiple significant
complications:

- OPLS usually requires using a long range coulomb pair_style and a
kspace solver, but COMB3 already includes this and kspace cannot be
applied on a subset only.

- COMB3 uses charge equilibration and that would need to "see" the
OPLS charges for correct behavior, but those must not be equilibrated
and they have not been parameterized for COMB3. essentially, you have
a problem similar to what people doing QM/MM calculations face and
that is non-trivial to solve and pair_style hybrid doesn't do that at
all.

- you also need parameters for the mixed interactions (between the
COMB3 part and the OPLS part). those *have* to be pairwise additive
potentials.

- for mechanical coupling like pair_style hybrid offers, usually you
need well separated regions for the different potentials, so that the
errors from mechanical coupling are small.

axel.