Include arbitrary external potential in HPMC

[alessandrosimon]

Hello,
right now there's rather little information in the official documentation on
how to add an external potential to a MC simulation. The documentation says one should
include the potential in the intgrator's external_potentials list:
simulation.operations.integrator.external_potentials = [extPot]

However, it's not clear to which class extPot should derive from, i.e.
External from hoomd.hpmc.external or ExternalField from hoomd.hpmc.external.field.
Also, which functions should be implemented in the subclass (and what's the signature)?
Thank you!

[joaander]

The Python user facing code must subclass hoomd.hpmc.external.External and be backed by a C++ implementation that subclasses ExternalPotential:

hoomd-blue/hoomd/hpmc/ExternalPotential.h

Lines 13 to 22 in fe4836b

	/** Functor that computes interactions of particles with external fields.
	ExternalPotential allows energetic interactions to be included in an HPMC simulation. This
	abstract base class defines the API for the external energy object, consisting of a the energy
	evaluation fuction.
	Provide a ExternalPotential instance to IntegratorHPMC. The external potential energy will be
	evaluated when needed during the HPMC trial moves.
	*/
	class ExternalPotential

The hpmc-energy-template repository shows a fully working C++ example: https://github.com/glotzerlab/hpmc-energy-template/blob/trunk/src/ExampleExternalPotential.cc

hoomd.hpmc.external.field.ExternalField is no longer used as of HOOMD-blue 5.0.0. That file should be removed.

If you are asking how to do this in pure Python then the answer is that it is not possible. I am currently working on a new code that will make user-defined data and potentials possible at every level of the implementation without the need to write C++ code. I will make a complete announcement it when it is ready to share.

[alessandrosimon]

Ok,that makes sense. Thank you for the quick reply!