Pair_modify special setting for pair hybrid

LAMMPS LAMMPS General Discussion
1

Dear LAMMPS users,
I’m getting the following error:
ERROR: Pair_modify special setting for pair hybrid incompatible with global special_bonds setting
The relative lines in my script and some of the pair_coeffs:
pair_style hybrid lj/charmm/coul/charmm 12.0 14 lj/cut/coul/long 12.0
special_bonds charmm
pair_modify pair lj/cut/coul/long special 0.0 0.0 0.5

pair_coeff 1 1 …
pair_coeff 1 2 …
pair_coeff 1 3 …
…
dihedral_style charmm opls
dihedral_coeff 1 charmm …
dihedral_coeff 2 opls …
…

I’m mixing two different force fields, and they differ in the weighting of the 1-4 interactions. I want to define the weighting of the 1-4 interactions factor of lj/charmm/coul/charmm explicitly by dihedral_style charmm.
If I am using the dihedral_style charmm, I should use special bonds charmm. Then I can not modify the weight of the oplsa on lj/coul 0.0 0.0 0.5 ( as required for oplsa) using pair modify. However, what I am trying to do is to override 0.0, which violates the requirements for overriding special_bonds settings in hybrid styles.
How can I combine CHARMM with OPLS?

Can anybody help me?
Thank you in advance.

2

This is incorrect. You MUST NOT combine a cutoff Coulomb with a long-range Coulomb.

There is a note in the pair_modify documentation about this. Please also note that the special_bonds documentation explains that special_bonds charmm is just an alias for special_bonds lj/coul 0.0 0.0 0.0 which is indeed incompatible with the requirement for the pair_modify override.

You should not. Those two force fields have very different parameterization strategies and are based on different water potential interactions and thus they are not compatible with each other. You should use either all CHARMM or all OPLS as your force field.

This is incorrect as well.

3

thanks so much.