[Pw_forum] vc-rx

Gabriele Sclauzero sclauzer at sissa.it
Tue Apr 14 09:44:38 CEST 2009

Mehrnaz Anvari wrote:
> Hello
> I have a question about the number of iterations which have to be done 
> for density calculations in each step of vc-rx calculations. I mean do 
> they have to be decreased ?

I'm not sure I really understand what you're speaking about, but if you are asking on how 
many iterations are needed to converge the charge density in an scf calculation (which is 
one step of a vc-relax calculation), well, it depends.
It depends on many parameters, but mainly on the mixing_beta and mixing_mode. You don't 
have to chose on "how many" iterations an scf has to converge, the converge criterion is 
on the "distance" (norm of the difference) of the charge density at two successive 
iterations, and it can be reached after a very variable number of iterations, depending on 
your system and on the initial guess.

> For instance does it have to be converged after 2-3 iterations, after a 
> few steps.

After the first steps of a relax calculation the number of iterations needed usually 
decreases, since the atomic positions do not change dramatically and the in initial guess 
of the charge density is not as bad as it was at the first scf calculation.
Again, the precise number of steps depends on lot of conditions and parameters, you can 
only say that it usually decreases.



> Best Regards
> M.Anvari
> Iran University Of Science & Technology
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