Phonopy calculates group velocity of phonon as follows:

where the meanings of the variables are found at Formulations.

In the previous versions, group velocity was calculated using finite difference method:

Group velocity calculation with the finite difference method is still
able to be activated using `GV_DELTA_Q`

tag or `-gv_delta_q`

option. is described in Cartesian coordinated in reciprocal space. In
the implementation, central difference is employed, and and are taken to calculate group
velocity, where is the Cartesian index in reciprocal
space. is specified in the unit of reciprocal
space distance ( for the default case) by
`--gv_delta_q`

option or `GV_DELTA_Q`

tag.