[Pw_forum] tpss: too many bands are not converged

Mike Marchywka marchywka at hotmail.com
Thu Apr 24 15:03:53 CEST 2014

> From: yves.ferro at univ-amu.fr
> Date: Thu, 24 Apr 2014 14:18:10 +0200
> To: pw_forum at pwscf.org
> Subject: Re: [Pw_forum] tpss: too many bands are not converged
> Dear Davide,
> Mete-GGA functional such as TPSS are known to have convergency problems and numerical instabilities during scf cycles, specifically when the unit-cell contains a lot of free volume, such as in your case probably.

By free volume, you mean places where n and likely grad everything are numerical noise?  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. 


> 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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