[Pw_forum] Strange phonon branches appear in phonon dispersion

Marcel Mohr marcel at physik.tu-berlin.de
Wed Dec 12 12:33:14 CET 2007


>> 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
>>
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