[Pw_forum] SCF correction compared to forces is too large.

[Guido Fratesi]

Dear Sonu Kumar,

in general this warning means that the forces you are computing are not 
very accurate, BUT relatively to their absolute value.
  Here, the absolute value of the force is very small so even a tiny SCF 
correction appears large, so the warning. You can safely ignore it 
because there's no harm in missing forces by 25% when they are 
negligible. Notice that how large a 'negligible' force depends on you 
and the system under investigation.
  In my experience this situation is not unfrequent at the end of the 
optimization process. I expect you could get accurate results in terms 
of coordinates and total energy also with a bit larger scf threshold 
(1d-8?).

Guido

Il 12/23/2011 05:47 AM, Sonu Kumar ha scritto:
> Dear all QE users,
>
> During optimization, i got following message:
>
>   Total force =     0.000004     Total SCF correction =     0.000001
>   SCF correction compared to forces is too large, reduce conv_thr
>
> Total force is  less than total scf correction. Then why the message
> is contradicting? I used  conv threshold for scf=1.0d-10.
>
> Thank you.
>
> regards,
>
> ==========================================
> Sonu Kumar
> Phd Student,Physics Department
> Indian Institute of Technology ,Delhi-110016, India
> web:-http://www.iitd.ac.in/
> ==========================================
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-- 
Guido Fratesi

Dipartimento di Scienza dei Materiali
Universita` degli Studi di Milano-Bicocca
via Cozzi 53, 20125 Milano, Italy