[Pw_forum] What happens at REALLY large ectuwfc?

[Lorenzo Paulatto]

I'm reposting here a message that I mistakenly sent privately to Brad:

have a look at the total-energy components: if the one-electron  
contributions increases  at the expenses of the other terms (mostly  
the hartree term) than you have found a ghost in the pseudopotential.  
As far as I know, all non-local separable pseudopotentials have  ghost  
states for large enough values of the curoff.

I'm not really sure my impression is correct, but I've always had  
problems with large enough cutoffs with any pseudopotential I have  
tried. My rationale is more or less on this line: the pseudopotential  
only have Fourier components up to a certain threshold; plane waves  
over that threshold do not feel any direct effect from the ions. As a  
consequence this very high frequency plane waves gain nothing by  
forming a charge density close to the ions, instead they can just  
spread around to minimize the Hartree energy (which decreases) at the  
expenses of kinetic energy (which increases).

cheers


-- 
Lorenzo Paulatto
SISSA  &  DEMOCRITOS (Trieste)
phone: +39 040 3787 511
skype: paulatz
www:   http://people.sissa.it/~paulatto/

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