Should we pass more properties (e.g. velocities, step number) to `force`/`potential_energy`? about molly.jl

[ehsanirani]

It would be good to hear the properties people are interested in. It would even be nice to pass arbitrary properties, but the issue is how to calculate them in the simulator to pass them to the interaction functions.

I guess you could also store orientations in the atom type, which would make them available in the interaction function, but then the question arises of how to update that property over the simulation.

One idea would be to allow the user to define and have multiple simulators. One could be a classic simulator, and the other one could take care of the additional atom properties. For example, one could define a simulator to just integrate over orientation. It would also be powerful to be able to give the atom-type properties as arguments to the simulator. This way, built-in integrators like Langevin could be used for updating orientation. I think JAX-MD gives you this freedom.

[ehsanirani]

Related to this, have you considered designing the possibility of passing properties other than velocities in the future? For example, in many cases, particles can have orientations, and being able to pass this information to pairwise forces would open up many applications for Molly.jl in Active Matter and Granular Matter physics, for example.

[jgreener64]

It would be good to hear the properties people are interested in. It would even be nice to pass arbitrary properties, but the issue is how to calculate them in the simulator to pass them to the interaction functions.

I guess you could also store orientations in the atom type, which would make them available in the interaction function, but then the question arises of how to update that property over the simulation.

[jgreener64]

We may also want to think about passing the zero force/energy to these functions. At the minute there is a clunky setup where for forces we define the units in the struct and then strip the coordinates to get a zero vector. This means more setup and likely has performance implications.