[Pw_forum] tpss: too many bands are not converged

[Paolo Giannozzi]

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 à 13:36, Davide Tiana a écrit :
> >
> >>
> >> 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
> >>
> >>
> >>
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-- 
Paolo Giannozzi, Dept. Chemistry&Physics&Environment, 
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222