[Pw_forum] occupation problem when system changes from insulator to metal

sbraccia carlo sbraccia at sissa.it
Tue Dec 14 10:50:16 CET 2004

Dear W. YU,

if the system you are considering is an insulator (did you heck it ?) 
and if the smearing is small enough the two results should be equal (if 
not, send me all the input and output files).

If the system is a metal you cannot use fixed occupations. You must use 
either tetrahedra or the smearing. In both cases you have to monitor the 
convergence of your results as a function of the number of k-points (and 
of the smearing parameter "degauss") to be sure that they are accurate 


W. YU wrote:
> Dear sbraccia carlo,
> Thank you very much for your reply and detailed
> explanations. I am now calculating the Cu3N phonon
> dispersions. It is of ABO3 perovskite structure with
> the A site empty. It is generally believed that under
> pressure, one of the M point phonon mode corresponding
> to the rotation of the BO6 octahedra will soften. I
> got the same result without smearing. But when I
> include smearing, the results are different. The
> phonon freqencies at the same k points become much
> smaller(several tens of wave numbers), and softening
> happens not only at M point but also along Delta axis.
> This is intuitively not correct. Which one should I
> trust?
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