[Pw_forum] degauss

[Stefano Baroni]

wait, wait, wait: please, wait!
being a metal/ insulator / semiconductor (which is the same as being an insulator) is a property of infinite systems, NOT of isolated molecules. Please, be sure to understand this concept before proceeding. If you don't, take some time to revert to some basic solid-state textbook. If you do not know where to start from, ask our advice. (exercise #1)
this being said, the same *trick* is used by QE (as well as by many other codes) to deal with metallicity in infinite systems and quasi-degeneracy in finite ones. I think that this issue has been discussed many times in this mailing list. Please, search its archive and try to retrieve the relevant info (exercice #2). in a nutshell, convergence to self-consistency may be slowed down (or even hindered) if the ground state is quasi-degenerate with one of the excited states. during the self-consistent cycle, the algorithm may jump from one state to the other quite easily, thus hindering convergence. the use os a ghaussian spearing (or of a smearing whatsoever) allows the code to average between the ground- and the quasi-degegenerate excited-state, thus easing the convergence. try to understand this (exercise #3). if you fail, revert to us with further questions as focused as possible.

SB

PS: do not forget that we all expect to know something about the identity of our internet-pals (please, provide your affiliation)

On May 8, 2010, at 10:04 AM, raheleh vaziri wrote:

> Dear All,
>  
>  I don't know that my molecule is metal, insulator or semiconductor , thus, what is suitable value for degauss and mixing_beta?
>  
> thank you for any help
>  
> Raheleh Vaziri.
>  
>  
> 
> 
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---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center - Trieste
http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)

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