On Thu, 2014-04-24 at 09:03 -0400, Mike Marchywka wrote: > By free volume, you mean places where n and likely grad everything are > numerical noise?
exactly: the regions of space where the charge has decayed to noise. Apparently (the current implementation of) meta-GGA doesn't like the presence of such large regions (and it doesn't seem to like H atoms as well). Even plain GGA diverges in those regions: it is less of a problem, but still, the exact energy values depend upon how the potential is cut off. The divergence comes from gradient corrections to exchange: there is no divergence in atoms with the correct large-r limit of the charge density, but this is not of much help in plane-wave calculations of condensed-matter systems. (disclaimer: this is what I remember. I looked at this problem several years ago for meta-GGA, many years ago for GGA) Paolo > AFAICT, the converge of SCF is largely empirical > as there does not seem to be much theory here. In any case, you would like > to think that the areas lacking "physics" should not be that > big a factor in the overall calculation and if that is not the case there may > be something interesting here to explore :) > > > You can try to decrease the size of the unit-cell in order to minimize the > > free space it contains. Of course, this will be at the expense of the > > inter-polyemer interaction you probably want to avoid. > > > > Nevertheless, this will probably not cure your problem, which to my > > knowledge has no other solution than changing the meta-GGA functional to a > > GGA one. > > I took a look at the "z" parameter which being a ratio could have some issue > with small denominators. With some initial work, > it looks like there are better ways to calculate it than directly from real > quantities such as n and grad mags, > but I am still trying to test what may be trivially wrong math :) > I think my latest result was something to the effect that you could reduce z > to A/(A+B)( both non-negative) > if you calculate everything from psi and the grad of psi expressed in polar > form, not much different from some formula > for current densities, and maybe find a limit when A and B are both zero. > > I had originally hoped to try to test it for trivial issues in JDFTX and c++ > or c with libxc using an expanded interface scheme > to let me pass z or grad psi in polar form but it will require a bit of > effort. Curious if anyone else here has examined > this or has literature references they care to share. Seems disconcerting > that something with physical meaning would have these > kinds of implementation issues - perhaps something else can be learned from > investigating. > > Thanks. > > > Best regards, > > > > Yves > > > > > > > > Le 24 avr. 2014 ? 13:36, Davide Tiana a ?crit : > > > >> > >> Dear all, > >> I've been trying to calculate a polymer using tpss. I already fully > >> optimised it with different functional (pbesol, pw91, hse) and > >> pseudopotential (nc, paw). For some reason (apologise my ignorance if > >> it should be well known) when I try to calculate using tpss nc the > >> system simply does not converge crashing with this error: > >> > >> Error in routine c_bands (1): > >> too many bands are not converged > >> > >> After some reaserch in old pw-forum mails, I tried decreasing > >> mixing_beta=0.2, increasing mixing_ndim=12, as well as mixing mode TF > >> and local-TF. > >> everythings failed. > >> > >> Does anyone have any tips or idea of why the system can't converge? > >> > >> Thanks a lot, > >> Davide > >> > >> > >> > >> _______________________________________________ > >> Pw_forum mailing list > >> Pw_forum at pwscf.org > >> http://pwscf.org/mailman/listinfo/pw_forum > > > > > > _______________________________________________ > > Pw_forum mailing list > > Pw_forum at pwscf.org > > http://pwscf.org/mailman/listinfo/pw_forum > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://pwscf.org/mailman/listinfo/pw_forum -- Paolo Giannozzi, Dept. Chemistry&Physics&Environment, Univ. Udine, via delle Scienze 208, 33100 Udine, Italy Phone +39-0432-558216, fax +39-0432-558222
