Wang frenkel

[MartinGirard]

Description

This draft PR would add the Wang-Frenkel potential to hoomd (equation (2) in Wang & Frenkel, https://pubs.rsc.org/en/content/articlelanding/2020/cp/c9cp05445f).

Motivation and context

This is largely motivated by coarse-grained IDP models, specifically by MPIPI (https://www.nature.com/articles/s43588-021-00155-3), which uses this type of potential.

How has this been tested?

This is work in progress, unit tests will be added to hoomd. At the time of writing this draft PR, I would want some feedback on the implementation. Specifically, in eq (2) of Wang et al, the cutoff radius appears in the potential. I have chosen to change "r_c" there by an adjustable parameter, and remove "alpha".

This is driven by performance considerations. Using the cutoff value at runtime would require computing an extra power, which is usually expansive (log + exp).

An alternative is to enforce exponents to be half integers (>= 1/2), which suits my needs and considerably simplify calculations. The other available implementation (lammps) uses a similar restriction.

Checklist:

• I have reviewed the Contributor Guidelines.
• I agree with the terms of the HOOMD-blue Contributor Agreement.
• My name is on the list of contributors (sphinx-doc/credits.rst) in the pull request source branch.
• I have summarized these changes in CHANGELOG.rst following the established format.