[Pw_forum] Non-orthogonal modes with dynmat.x at gamma point

[Florian Altvater]

Hi,
I calculated the phonons for an isolated naphthalene molecule as well as
naphthalene crystal at q = (0,0,0). After diagonalizing the dynamical
matrix with dynmat.x and enforcing the appropriate acoustic sum rule -
"zero-dim" and "crystal" respectively - I get 6 and 3 acoustic modes
with frequencies of 0 cm^-1. So far so good.

If I use the displacements printed to dynmat.out (filout) and calculate
the various scalar products of the modes, i.e. c_ij = m_i . m_j, I find
that they are normalized but not orthogonal (c_ij != 0 for some i != j).
Some of the overlaps c_ij are as high as 0.75, so it is not just
numerical noise. By trying to find more information in the source files,
I discovered that matdyn.x prints two files, flvec and fleig, where
flvec prints non-orthogonal normalized vectors. So I thought that this
could be the issue, as it is not clearly stated if dynmat prints
eigenvectors or normalized modes like flvec. However, multiplying with
the masses didn't help either.

Is dynmat.x not printing eigenvectors? If it is/should, what could be
the problem here? How would I debug the problem?

Thanks so much for you help!
Flo