[Pw_forum] different forces between last 2 steps

Nicola Marzari nicola.marzari at epfl.ch
Tue Nov 3 12:26:17 CET 2015

On 03/11/2015 10:13, Jaret Qi wrote:
> Dear QE users,
> Why total forces are different in the last relaxation step and the final
> step that pw.x performs once the criteria satisfies, does this mean
> relaxation not good?
> take a look :

Hi - the code tells you exactly what has happened:

>       A final scf calculation at the relaxed structure.
>       The G-vectors are recalculated for the final unit cell
>       Results may differ from those at the preceding step.

This happens because different unit cells at the same cutoff have different
seta of corresponding plane waves - for an explanation you could check
G Francis and MC Payne circa 1991 (JPCondMatt) or G-M Rignanese
et al PRB circa 1996.


Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL
Director, National Centre for Competence in Research NCCR MARVEL, EPFL
http://theossrv1.epfl.ch/Main/Contact http://nccr-marvel.ch/en/project

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