[Pw_forum] conv_thr for forces for relaxing ions

[Stefano de Gironcoli]

In PWscf the forces are calculated  from the  Hellman-Feynman theorem 
(strictly true only at the stationary point) plus a correction that 
(approximately) accounts for the residual lack of self-consistency.  As 
such when the correction is large compared to the Hellman-Feynman term 
one should be careful.
In your case I would say that the forces are  now probably very small 
and  therefore the structure is reliable.
I would say that the actual (small) value for the force are probably 
correct within a few percent since the correction term is (according to 
the value quote) of the order of 10 % of the total (and hopefully is not 
completely wrong).
Hope this helps,
stefano de Gironcoli -SISSA and DEMOCRITOS

Madhura Marathe wrote:
>  Dear all,
>
>  During one of the ionic relaxation calculations, I got the error message,
> "SCF correction compared to forces is too large, reduce conv_thr". I am
> using conv_thr = 1.0d-8 which is generally sufficient, so I increased the
> parameter upscale (from 10.0 to 100.0) in &IONS to reduce the the
> threshold during relaxation. This lead to the convergence till the last
> ionic iteration which was converged to sufficient accuracy, when again the
> same error message occurred. The forces then are
>      Total force =     0.000136     Total SCF correction =     0.000016
>      SCF correction compared to forces is too large, reduce conv_thr
>
>  There has been a recent discussion on the topic. From that, I gathered
> that the subsequent relaxations after this error message are not
> reliable.
>  So my question is whether the forces are reliable in this last iteration?
> Or do I need to further reduce the conv_thr and re-run the whole
> calculation?
>
>  Thanks for the help,
>  Madhura.
>
>