>> I am trying to do calculation phonon dispersion and EPC for 1D >> Aluminum chain. I got the results which are strange phonon branches >> appearing. In the case of 2 atom-cell, I got 4 phonon branches, two of >> them are Optical and Acoustical phonon branches and two others, which >> have zero and negative values, are unknown for me. In the case if I >> have 1 atom-cell, that strange branch also appears. Could any body >> explain for me what kind of frequencies here?
6 branches for 2 atom are what one expects (in general 3n branches and 3 acoustical and 3n-3 optical ones). However negative values indicate some error in your calculation, dependent on the magnitude and system. You can impose some sum-rules for the acoustic ones, to force them to be 0 at the Gamma point. Acoustic forces are harder to calculate (smaller forces). In your case I would guess up to 15 cm^1 should be ok for the acoustic ones. Negative optical ones should not occur. Best Marcel Mohr >> >> I was surfing this forum and FAQ and I found many discussion about >> negative frequencies, but I still cannot find the answers for me. >> >> Bac >> > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum > >
