[Pw_forum] Negative frequencies in hcp structure

[Michael Malorny]

Dear pwscf users,

At the moment I am performing studies on the disordered system CdS/CdSe
with wurtzite structure. I have severe problems in calculating the
dynamical properties of the bulk systems CdS and CdSe:

I obtain badly deformed translational branches in the Gamma-A direction
after interpolating over a 664 q-mesh leading to negative frequencies in
Gamma-A direction (up to -45 cm-1 and reaching from Gamma point to half
way to A point). The calculated dynamical matrix between Gamma and A
yields negative frequencies.

In the Gamma-K-M and Gamma-M direction I have minor problems with negative
frequencies in the vicinity of the Gamma point. This could be due to
strongly negative frequencies I obtained calculating the dynamical matrix
at Gamma (up to -30 cm-1) in conjunction with applying the ASR.

In general, the interpolated frequencies do not agree with directly
calculated frequencies at the same q-point.

Some facts that should help you to understand my situation:

1. The structure has been relaxed and should be okay (I maintain vanishing
forces and stress tensors). The structural results lie within an errorbar
of around 1.5% with respect to experimental results.

2. I use Vanderbild USPP (GGA) which I found enclosed to the Vanderbild
package (and LDA transformed versions of them).

3. Calculating the same materials (CdS, CdSe) with the same USPPs but
applying the fcc structure yields no problems at all (no negative
frequencies, dispersion looking ok).

4. Calculating hcp SiC with PPs found on pwscf.org applying the very same
procedure I used for CdS and CdSe yeilds no problems at all (no negative
frequencies, dispersion looking ok).

Further calculation details can be given.

In my opinion I have a PP problem. :/
In case that's true: Does anyone have a set of working PP for me?

I'll be very grateful for all suggestions!

Regards

Michael Malorny