Adding Coulombic interactions for CG interactions

[mrshirts]

Is your feature request related to a problem? Please describe.
We need to be able to describe CG ionomers.

Describe the solution you'd like
Having an XML that includes the ability to pass charges and hence Coulombic interactions to MD engines via GMSO.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

[StephMcCallum]

Summary from Slack conversation:

• Link to LAMMPS Coulomb interaction
• User's need to be able to specify dielectric constant
• Open question: Is the potential atom or system level?
• Open question: Do we want to include long range interactions?

If flag_lj is set to long, no cutoff is used on the LJ 1/r^6 dispersion term. The long-range portion can be calculated by using the kspace_style ewald/disp or pppm/disp commands. The specified LJ cutoff then determines which portion of the LJ interactions are computed directly by the pair potential versus which part is computed in reciprocal space via the Kspace style. If flag_lj is set to cut, the LJ interactions are simply cutoff, as with pair_style lj/cut.

If flag_coul is set to long, no cutoff is used on the Coulombic interactions. The long-range portion can calculated by using any of several kspace_style command options such as pppm or ewald. Note that if flag_lj is also set to long, then the ewald/disp or pppm/disp Kspace style needs to be used to perform the long-range calculations for both the LJ and Coulombic interactions. If flag_coul is set to off, Coulombic interactions are not computed.

What currently exists in GMSO:

• charges in the XML
• HOOMD pppm Coulomb parser. This force calculation is included when charges on atoms in the XML are non-zero. The parameters for the pppm calculation (resolution and order) are not included in the XML, but are kwargs to the to_hoomd_forcefield function. This expression for pppm does not exist in an GMSO xml (so why do we need expressions?)

What needs to be added:
HOOMD: Nothing, CG ionomers can be ran with current implementation. I think it would be nice to add pppm kwargs to the XML so that the user is explicitly adding the Coulombic interactions and to have all forcefield parameters in one place.
LAMMPS: No current LAMMPS implementation in GMSO for Coulombic interactions, or long range pppm / Ewald summation implementation for LAMMPS.

I could implement the pppm through an expression in an XML file. I think the better approach would be to work with the GMSO team to make a similar writer for LAMMPS Coulomb interactions and use the existing option to include charges and add the option to include the resolution and order parameters to the XML pair potential section, without them being included directly in the expression.

[StephMcCallum]

We have verified that GMSO already has the ability to include charges in the XML. Once the MuPT to GMSO interface is complete, I will add example notebooks to mupt-examples repo.