[Wannier] Several questions about distentangle process

[Janiet Wen]

Thank you , Young-Su.



2006/11/10, Young-Su Lee <lee0su at mit.edu>:
>
> Dear Janiet,
>
> >
> >    I have several questions about distentangle process giving in SMV
> > paper.
> > We should set an energy window for distentangle process , while my
> > question
> > is about this energy window's setting. Should all bands in the energy
> > window
> > be included in the distentangle process ?
>
> You can exclude certain bands: that's one of the features in wannier90.


Does it mean that we could do distentangle process in certain energy window
with as minimal bands as possible ?  I think the bands exclude of coz
affect  results, what we should take care is that the bands of same
characteristics should not be excluded . Is it right for my understanding
this exclude feature ?


> > Just count bands in this window , and make
> > sure there are several bands above this window ?
>
> I think that will work.
>
> >
> >  Another question is about  wannierise process for metal cnt. I just
> > wanna
> > repeat yongsu's work on pristine cnt , and obtain the minimal basis
> > for cnt.
> > But there are some problems i donot understand clearly. Just with
> > occupied
> > bands for wannierise, there is no problem both fo semi/metal cnt to
> reach
> > convergence. One of my confusion is why could  wannierise reach
> > convergence without distentangle process in metal cnt since i just set
> > 'num_wan' to the number of valence bands ?
>
> CNT is a zero gap semicondutor and below the Fermi level, the number of
> bands is the same regardless of a k-vector you choose.
>
> However, for a truly metallic system you will never(?) find this
> situation, e.g.
> in SMV paper,  number of bands below E_F varies when you sweep through
> the k-space:so there is no hope of getting WFs without disentanglement.
>
> For CNT, with a bit of fortune, you can still have localized WFs without
> disentanglement.
>
> Here are two things:
>
> First of all, it's not always the case. I once tried (5,0), but it
> didn't work.
>
> Second, those WFs are not good for band structure interpolation.
> If you do band structure interpolation with those WFs, you will find an
> extremely poor agreement
> with one you get from PWSCF code, except the k-points you sampled to get
> WFs.
>
> In a word, you can get WFs from valence band only, but they are useless
> (well, it also depends on what you want to do with them).
>
>
> > What we need to consider
> > is the  degenerate bands in distentangle process , is it right ?
>
> This question is not clear enough for me.



Sorry for my expression.
To my knowledge, besides bandgap, the difference between
insulator/semiconductor and metal is  fermi surface effect in metal
systems.  there are entangled bands near fermi surface in metal systems. So
for distentangle process , it is not due to fermi surface effect , but to
bands degenerate . Is it right for my understanding ?

> Next i
> consider '2.5 orbitals per atom'  to distentangle WFs. Just with exact
> 'nat
> X 2.5' bands for 'nscf' calculation , it is difficult to reach
> convergence
> in wannierise process ,
> Does this problem relate to 'not all bands inculded in the energy
> window ' ?
>
It depends on many sorts of things.
First, you have to plot your band structure from nscf calculation before
conducting any Wannier-related calculations.
Make sure that your 'nbnd' is big enough to cover whole
bonding+antibonding pi-bands.
If you've done this step, I don't know the answer without looking at the
output files.

a poor question is about antibonding pi bands counting ,
Does it equal to the number of atoms ?

I appreciate your kind reply.

Best wishes.

Hai-Ping
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