[Wien] NaCl Phonon calculations

[Sergio Yanuen Rodriguez]

Dear Wien2k users

I am trying to do phonon calculations for NaCl because this system is
already studied and after this apply this method to something more
complicated, I generate the case.d45 with PHONON for a 2X2X2 supercell,
then I transfer this file to Wien 2k and calculate the forces, but when I
run analyse_phonon the sum of this forces are not small, and the results I
get do not seem alright. I compared this results with another ones and
they do not agree specially for the acoustic modes. Since I am getting
negative frequencies at Gamma where they must have 3 (nearly) zero
frequencies.

Does any one know which parameter is important to adjust to make the sum
of the forces small? or do I have to do a volume optimization before
calculating the forces? Or any suggestions

I really appreciate any help.

Sergio Y. Rodriguez
Physics Department
Texas A&M University