Hi Nicola, Thanks for your comment. Actually, following Davide Ceresoli's advice, I always use "cg" as my minimization algorithm. I just redid my calculation with startingwfc = 'random' instead of 'atomic' and with conv_thr = 1.e-12 instead of 1.e-10. The results of the new calculation are identical to the "old" one (i.e., the one that was used to compute phonons in my example). Anyway, I have other phonon calculations where the symmetry is reduced from Pm-3m to, e.g., P4/mmm (not sure, though, that this symmetry is low enough to avoid the problem you mention below), and the dispersion-curve breaks are present there too.
THX, Serge Nicola Marzari wrote: > > Dear Serge, > > > in the past we encountered a problem in the cubic perovskites, > in which the davidson algorithm would converge the top band > at some k-points to an "excited state". This is, afaik, a problem > with some algorithms (much more so with diis); a simple > solution was to use "atomic+random" as a starting point, and maybe > use "cg" as the minimization strategy in the scf calculation, > with a very tight tolerance. > > Try that, maybe without redoing the phonons, just to see if the > calculations converge to the same ground state, or to a lower one. > > nicola > > > > > Serge Nakhmanson wrote: >> Dear All, >> >> I am learning how to compute phonon dispersion curves across the BZ and >> right now I am trying to reproduce the results of a simple calculation >> for cubic perovskites that Philippe, Karin and company have done a long >> time ago [PRB 60, 836 (1999)]. >> -- ********************************************************* Serge M. Nakhmanson phone: (630) 252-5205 Assistant Scientist fax: (630) 252-4798 MSD-212, Rm. C-224 Argonne National Laboratory 9700 S. Cass Ave. Argonne, IL 60439 *********************************************************
