[Pw_forum] more about phonon frequencies are different when treated as insulator or metal

[Stefano de Gironcoli]

when trying to describe Si as a metal what kind of smearing are you using ?
Methfessel and Paxton or gaussian  or something else ?
MP scheme, although in principle"better" than gaussain  for *real* 
metals, suffers
of the fact that the corresponding occupation function is not positive 
definite.
Usually this is not a problem with many k-points but may give spurious 
results
with few points and a small degauss....
Try simple gaussian or Marzari-Vanderbilt cold smearing scheme that combines
a nice (lack of) dependence of the results on the smearing width and a 
positive definite
occupation.

Stefano de Gironcoli

W. YU wrote:

>Dear Paolo and all,
>
>In order to see whether the value of degause was too
>large in my previous post with the same title, I have
>caculated the phonon frequencies of Si at gamma and X
>points using different values of degause from
>0.05-0.005, even a value of 0.005 will affect the
>frequencies to some extend. This value sure will fall
>in the band gap of Si and in theory will have no
>effect even if I treat Si as a metal instead of an
>insulator. But the results are dispointing.
>
>If I increase the number of K points, the results will
>improve. Must I use a much larger number of K points
>when I am treating an insulator as a metal instead of
>an insulator in order get similar results? Why so?
>
>
>
>		
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