[Pw_forum] SCF correction compared to forces is too large, reduce conv_thr

[Asha gupta]

Dear Stefano,
    i am using the following:
&system:  ibrav=0, nat= 98, ntyp=4, ecutwfc = 30.0,ecutrho =
180.0,occupations='smearing', smearing='methfessel-paxton',
degauss=0.003,nbnd=500,
&electrons: conv_thr = 1.0e-5,mixing_beta = 0.1,
K_points sampling: 4  4  4  0  0  0,
the system is insulator and non-magnetic for sure. the number of bands are
little higher than what is actually required for an insulator. Tha same k
points sampling for smaller similar system gave  results which tallies well
reported data.
  Shall i stop the calculation or let it run to completion? after compleion,
then i can take the new refined data as my input and reduced conv_thr and
re-run the calculation again.
regards
artee
department of physics
Bhavnagar university
india

On 3/22/09, Stefano de Gironcoli <degironc at sissa.it> wrote:
>
> dear artee sharma,
>
> conv_thr=1.0e-5 is a rather large value, acceptable if you just want
> the energy but not if you want forces. 1.0e-6 is the default and for
> relaxations one rather tends to reduce it...
>
> Convergence for large systems (especially inhomogeneous metallic ones)
> may be slow. mixing_mode='local-TF' usually helps in these cases. are
> you using it ?
>
> Even when the first scf convergence is painfully slow the subsequent
> ones should be better as potential and wfcs are extapolated from
> previous steps. check that you are indeed using these extrapolations
> (should however be the default for relaxation, so probably you do).
>
> Sometime when a system does not want to converge it's trying to tell
> you something ... Is it a magnetic or metallic system ? are you using
> smearing ? are you sure the number of bands and your k-point sampling
> is accurate enough ?
>
> hope this helps,
>   stefano
>
> Stefano de Gironcoli - SISSA and DEMOCRITOS
>
>
> Quoting artee sharma <mailtoartee at gmail.com>:
>
> > Dear Stefano.
> >    thanks for reply. my conv_thr in input file is 1.0e-5? i have a large
>
> > system, takes too much time to convarage, so kept it low. Shall i
> decrease
> > it by further one order?
> >   latest output is showing:
> >   Total force =     0.023049     Total SCF correction =     0.002483
> > so i think my system has not converged yet, force is high.
>
> > regards
> > artee sharma
> > Department of physics
> > Bhavnagar University
> > India
> >
> >
> >
> >
> > On Sat, Mar 21, 2009 at 11:46 AM, Stefano de Gironcoli
> > <degironc at sissa.it>wrote:
> >
> >> I think it might be better to stop it. According to the message the
> forces
> >> you are calculating may be not accurate and any further relaxation based
> on
> >> them is not likely to improve your geometry.
> >>
> >> However, are you sure your system is not yet relaxed enough ?
> >> What is your conv_thr and your force threshold ?
> >>
> >> stefano de Gironcoli - SISSA & DEMOCRITOS
> >>
> >> PS: don't forget to  provide your affiliation
> >>
> >>
> >>
> >>
> >>
> >> Quoting artee sharma <mailtoartee at gmail.com>:
> >> **
> >>
> >>
> >> ----------------------------------------------------------------
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> >>
> >>
> >
>
>
>
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