Elastic Wave velocities

[Oier Arcelus]

Dear Forum,

I am trying to use the Elastic modules, to calculate the propagation velocities of elastic waves in my system and I have a few questions. First, the longitudinal and transverse velocities are given from the Voigt-Reuss-Hill average values, but not the concrete directions of propagation and polarization of those waves. Is there any reason for this? Also, different crystal symmetries allow only for specific directions in which sound waves propagate, where pure longitudinal and transverse modes are present (there are groups in which three modes are present, one longitudinal and two transversals). So, which assumptions does pymatgen make to calculate those values regardless of the propagation direction?

Best regards,

Oier.