Dear all, I met some problems with phonon calculations and I hope someone could give me some help.
I did some calculations on a system with NaCl structure. I used three types of pseudopotentials: norm-conserving LDA, ultrasoft LDA, ultrasoft GGA. The lattice constants are all in good or reasonable agreement with experiments or other full potential calculations. When it comes to phonon dispersion curves, the NC LDA pseudopotentials gave smooth curves and the agreement with experiment is relatively good. For the US LDA calculation, the dispersion curves are smooth and the agreement with experiment is acceptable, but the smallest achievable values of the accoustic branches at Gamma point are about 40 wavenumber. With the increase of the ecut and ecutrho, these values became as large as 70 wavenumber. Now my question is: aren't they supposed to go to zero with the increase of ecut and ecutrho? If the answer is yes, does this mean the pseudopotential has some flaw or it is completely untrustable? As for the US GGA, I found negtive frequencies around gamma point with the same ecut and ecutrho as the LDA case. So I used larger ecut and ecutrho, the negtive frequencies became positive, but there are some kohn-like anomalies in the accoustic branches and the agreement with experiment became very poor for the accoustic branches. I though this might be caused by long range interactions. So I took a 888 q point grid instead of the original 444 one. This time, besides negtive frequencies around the gamma point and the anomalies, the accoustic branches even became zigzaged! I really couldn't figure it out. Does anybody has similar experience? Could anyone tell me what could be the most possible cause for this? PS: accoustic sum rule has been imposed throughout these calculations. ONLY the accoustic branches have these problems. The optical branches seem to be insensitive to the changes of the q point grid and cutoffs. Thanks a lot, W. YU __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com
