Store additional properties in ASE calculator

[frostedoyster]

These changes allow for a user to calculate and store non-standard properties in the ASE calculator. For relatively experienced users, this is also a fix for #655 (one can set "energy", "forces", "stress" as properties_to_store, these will be calculated at each step, and then the inherited get_property function will not re-trigger a calculation but just read from the calc.results dict).

Contributor (creator of pull-request) checklist

• Tests updated (for new features and bugfixes)?
• Documentation updated (for new features)?
• Issue referenced (for PRs that solve an issue)?

Reviewer checklist

• CHANGELOG updated with public API or any other important changes?

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[Luthaf]

Why did you go with this approach instead of automatically storing energy/energies when computing the forces and stress? The fact that one has to explicitly set these is not ideal, and I feel like a solution to #655 that works without explicit user action would be nicer.

[frostedoyster]

This was not intended as a fix for #655 (although it is if one knows the internals). It's just to propagate potentially more outputs to the user

[Luthaf]

Ok, the initially intended API for doing outputs outside of ASE specification was the run_model function. Why is it not suitable in your case?

[frostedoyster]

It looks to me like it recomputes everything...
IMO the only way not to recompute if one wants additional properties is to save stuff to the calculator.results dict, because that is what's checked to avoid recomputations (if the Atoms object hasn't changed)

[Luthaf]

It looks to me like it recomputes everything...

No, because this function is not integrated with the rest of ASE interface (get_property & friends). This should be used like this:

calculator = MetatensorCalculator(...)
outputs = {...}

calculator.run_model(atoms, outputs)

Since the user is explicitly calling this function themself, it will only recompute if the users call it again manually.

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In general there are two separate issues IMO: #655 is about storing multiple that ASE knows about (storing energy when computing forces, etc.); while run_model is about computing properties ASE does not know about (i.e. mtt::last_layer_features, nmr_shielding, etc.).

We need a fix for #655, but I don't think that properties_to_store is the best solution. I would rather do this automatically without any user intervention.

[frostedoyster]

The fact that it is not integrated with the rest of ASE is why the recomputations are necessary. If I need a quantity at some step of MD, I have to recompute, while in principle the same quantity would have been available together with the energy computation at near-zero overhead