[Pw_forum] Query on phonon dispersion

[Ajit Vallabhaneni]

Dear users,

          I am trying to calculate phonon dispersion and DOS of graphene. I followed the procedure used for silicon in the tutorial with the necessary changes. But the phonon dispersion i obtained is not exactly the same as the one in literature. The ZO branch is particularly distorted and for one of the acoustic branches, the frequencies are slightly -ve near the gamma point. 
Any suggestions or thoughts in this regard are welcome. 

I want to know the answer for the following questions as well

i) Is "16x16x1" MP-grid size dense enough to obtain accurate result for graphene or do i need to increase it further? 
ii) What should be the value for "nbnds" for graphene? I used 8; but i read that for metals you may have to use a higher number. I tried 12 as well but i didnot see much difference in the end result. 
iii) In specifying the k-space coordinates to obtain frequencies at a point in the file "*.matdyn.in", what are the units for the co-ordinates? are they just (2*pi/alat) or crystal (in terms of reciprocal lattice vectors)? 

Thanks
Ajit