[Wannier] Several questions about distentangle process

Young-Su Lee lee0su at MIT.EDU
Thu Nov 9 22:16:33 CET 2006

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.

> 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 

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 

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.

> 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.

Hope this answers some of your questions.

Young-Su Lee

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