[Phonons] Expression for bandstructure including non-analytical correction likely wrong
The non-analytical term correction implemented in the phonons.py
module for computing phonon bandstructures of polar materials is likely wrong:
https://gitlab.com/ase/ase/-/blob/master/ase/phonons.py#L644-645
The expression only uses the q -> 0 limit of the respective expression, which is incorrect at any finite q. I am happy to be proven wrong here but I think it's very likely just wrong.
Reference: Gonze, X. & Lee, C. Phys Rev B 55, 10355–10368 (1997). Sec. IX
Comment 1: Phonopy implements the named approach.
Comment 2: It might very well be that differences are often marginal, but I assume nobody has checked the meaning of often or marginal in this case.