Hi, Did you to calculate phonons for Na using these pseudopotentials?
Bests, Eyvaz. --- "W. YU" <yuwen_66 at yahoo.com> wrote: > 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 > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum > __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com
