Euphonic: Anharmonicity

[rebeccafair]

Euphonic currently uses only harmonic lattice dynamics, but introducing anharmonicity could increase the range of systems that Euphonic can simulate accurately. Tasks:

• Review anharmonicity opportunities #189. Done, Anharmonicity for Neutrons document created, further tasks taken from Review anharmonicity opportunities #189 (comment)
• Identify candidate systems. MgO is a simple system with some experimental data in https://doi.org/10.1103/PhysRevB.99.220304
• Generate MgO higher order force constants #193
• 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
• Obtain phonon lifetimes and apply to Euphonic spectrum as Lorentzian broadening. This depends on Feature request: phonon linewidths Euphonic#240
• 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.