Implement Fitsnap interface for SNAP and ACE

[jan-janssen]

Implement a https://github.com/FitSNAP/FitSNAP based interface to calculate SNAP and ACE descriptors for the whole simulation cell and their directional derivatives for fitting machine learned potentials.

[liamhuber]

Mutable defaults are not recommended.

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fitsnap

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[raynol-dsouza]

Hi @jan-janssen, just wanted to ask if this branch is good to go or still requires some work. I am in need of a standalone descriptor evaluator (without explicitly running LAMMPS or another calculator) for my current project and was referred to this branch by @HaithamGaafer.

[jan-janssen]

@raynol-dsouza It still needs a bit of work, in terms of writing tests and fixing the continuous integration. But I think it should be possible reasonable effort.

[jan-janssen]

I am in need of a standalone descriptor evaluator (without explicitly running LAMMPS or another calculator) for my current project and was referred to this branch by @HaithamGaafer.

This runs fitsnap which internally runs lammps, it is just a more efficient interface. Is this what you need?

[raynol-dsouza]

This runs fitsnap which internally runs lammps, it is just a more efficient interface. Is this what you need?

I am aware of the internal usage of lammps in fitsnap. I know that lammps can compute SNAP (bispectrum) descriptors from just the input parameters for a structure, but for computing ACE descriptors, it requires in addition a coefficients file.

Ideally, I would like to have a means of computing ACE descriptors without input coefficients just like SNAP (where the coefficients are computed internally in the compute). For the time being though, if the interface here does that for me, then it would be a good starting point!