[Wien] Charge Convergence is not achieved

[Laurence Marks]

2011/7/7 Shamik Chakrabarti <shamikiitkgp at gmail.com>:
> Dear Peter Blaha Sir,
>                        Indeed by increasing number of K points we got the
> convergence. Sir I have now some basic queries on this topic. You have said
> that
>                     "sometimes you cannot reach (easily) arbitrary
> convergence"
> why in some cases we can not reach convergence up to our desired limit?...is
> it the limitation of DFT?....or it means that the feasibility of the
> solution is only up to the achieved convergence?

This is in fact a deep, and very good question, at least in my opinion.

Unfortunately that does not mean that there is a good answer to it!

With the perfect functional convergence should (I believe, others may
disagree) always be good. With a very imperfect functional it is quite
possible that a DFT calculation will not converge, i.e. it is
unfeasible. Empirically many (but not all) metals do not converge well
with small numbers of k-points, but some others do. Why....I do not
understand as I cannot write down a mathematical analysis to explain
this and do not believe that there is a formal analysis in the
literature, it is just empirical knowledge (folklore).


-- 
Laurence Marks
Department of Materials Science and Engineering
MSE Rm 2036 Cook Hall
2220 N Campus Drive
Northwestern University
Evanston, IL 60208, USA
Tel: (847) 491-3996 Fax: (847) 491-7820
email: L-marks at northwestern dot edu
Web: www.numis.northwestern.edu
Chair, Commission on Electron Crystallography of IUCR
www.numis.northwestern.edu/
Research is to see what everybody else has seen, and to think what
nobody else has thought
Albert Szent-Gyorgi