[Wannier] parameter to control convergence

[Jonathan Yates]

On Wed, 23 Aug 2006, Tadashi Ogitsu wrote:

Hi Tadashi,

  Thanks for telling us about your Boron calculations.

  I'd like to make a fairly general comment. But I think it might be at the 
root of your question.

  If we look at the valence states of an insulator then the MLWF are 
determined by the localisation criteria. The MLWF are "fully occupied", by 
which I mean that the sum of |w(r)|^2 over all of the WF gives the charge 
density. Here it is certainly valid to think about the bonding in terms of 
the MLWF.

  When we use the disentangling procedure (ie for a metal or to describe 
both valence and conduction states in a insulator) the MLWF are defined 
not only by the localisation criteria, but also by the dimension of the 
optimal subspace (ie the number of WF) and the ranges of the two energy 
windows.
  The MLWF are no longer fully occupied, we have "partially-occupied" MLWF. 
So now the sum of |w(r)|^2 over all of the WF is the not the charge 
density. Using these MLFW to interpret the bonding in metals is 
therefore not straight-forward. [which is to say that you need to think 
about how this issue affects your particular problem.]

> Do the occupation given in the pwscf calculations used in Wannier90 code? or 
> in Wannier90, only eigenfunctions from the pwscf output are used and the 
> energy window to construct the Wannier functions (or the definition of 
> subspace for the unitary transformation?) is determined only by 
> dis_*_[min|max] parameters? Thanks a lot!

  You are correct, Wannier90 does not know anything about the occupation of 
the bloch states. The energy windows are determined by the dis_*_[min|max] 
parameters. One must also specify the number of WF to extract.

  There is a keyword called 'fermi_energy'. This is not used in the 
construction of the WF. It is used when generating a plot of the Fermi 
surface using Wannier interpolation [eg /cond-mat/0608257].

  Yours
   Jonathan

-- 
Dr Jonathan Yates:  email: jryates at lbl.gov   phone: 510-642-7302
Material Science Division, Lawrence Berkeley National Laboratory  and
Department of Physics, University of California,
366 Le Conte Hall #7300, Berkeley, CA 94720-7300, USA