[Pw_forum] Baldereschi and MP point.

[Nicola Marzari]

Jiayu Dai wrote:
> Dear users, 
>
> I have a question about the sample points in the Brillouin zone. We usaually use
> the MP point in our calculations, and it works well and fast in cubic crystal.
> However, for disordered materials or liquids, we usually use Gamma point only,
> such as point in CPMD. My question is how do we know the Gamma point is better
> than the Baldereschi mean value point, or on the contrary£¿Which one gives faster
> convergence?
>
> Thanks a lot.
>
> Jiayu


My personal bias is that the Baldereschi point always works better, and that
in an orthormombic cell using 1/4 1/4 1/4 (relative coordinates) is 
almost as
good as using the full 2x2x2 monkhorst pack mesh, shifted, and, for a metal,
much much better than gamma.

Note that

1) if you use only 1/4 1/4 1/4 the forces will not be exactly symmetric 
even for a
symmetric system

2) if you do not use nosym=.true. pwscf will in any case increase the 
k-points to recover
symmetry

3) the wavefunction at 1/4 1/4 1/4 are complex, while at gamma can be 
chosen to be real,
making the code twise as fast (I think) and the memory half. That's why 
e.g. the cp code
uses gamma, even if it is actually the worst choice possible from the 
point of view of the sampling.


nicola


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Prof Nicola Marzari   Department of Materials Science and Engineering
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