Review anharmonicity opportunities

[rebeccafair]

Anharmonicity is a big topic, it includes mode-mode interaction (e.g. as Phono3py handles), overtones, quasi-harmonic approximation. Before deciding on specific tasks, we need to survey the state of the art, the opportunities and gauge priorities.

• By 16/08/22:

• Keith & Adam to review opportunities and produce document
  • Lifetime broadening easy win
  • Producing higher order force constants in Euphonic format also possible easy win - need to investigate workflows
  • Harder to see where MD fits in

• By 13/09/22

• Document created: Anharmonicity for Neutrons

• By 11/10/22

• All read through document - suggest test cases
• Adam Create task list

[ajjackson]

The proposed "next steps" from discussion of the current document state are:

1. AJJ/KR to compute some 3rd/4th-order force constants for a suitable system with one of our favoured codes
   • i.e. hiphive, TDEP
2. AJJ/KR to perform proof-of-concept import of renormalised harmonic phonons into Euphonic. (This may Just Work™ using Phonopy format as an intermediate, but likely some fiddling will be required.)

• In principle this can be done using tutorial data or cheap force calculators, without waiting for item 1

3. Obtain phonon lifetimes and apply to Euphonic spectrum as Lorentzian broadening. This depends on Feature request: phonon linewidths Euphonic#240
4. Deeper exploration of molecular dynamics sampling. This could act as validation of Lorentzian broadening, as well as provide a way to access anharmonic and multi-phonon contributions simultaneously.

[rebeccafair]

Closing this as done as I think we now have a direction to go in. Tasks from #189 (comment) have been moved to #182