Hi, can someone help me about rotating phonon eigenvectors? Say I have a primitive cell with 3 atoms, so for each point in reciprocal space, there are 9 eigen frequency and eigenvectors. Each eigenvector is a 9-dimensional complex vector. I calculated eigenvectors from a transverse branch for two points in reciprocal space say 0.5ax+0.5ay+0.0az and 0.7ax+0.7ay+0.7az from a software. However, due to the degeneracy nature of transverse modes, the two eigenvectors are not parallel to each other. (longitudinal modes are) So I am wondering how can I apply a rotation to one vector to make it parallel with the other?