[Wannier] Several questions about distentangle process

Young-Su Lee lee0su at MIT.EDU
Mon Nov 13 23:06:42 CET 2006

> 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 ?

I would start from including all bands and check  projection information 
of disentangled subspace to the original space.
If certain bands have very very small contribution, you may exclude 
these bands. I'm not totally sure whether it's going to work.
In my opinion, exclude feature will be useful when some bands are 
hampering disentanglement/minimization procedure
while you are sure that they should not be included in the disentangled 
subspce by symmetry.

>   <>
> 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 ?

I think disentanglement will be useful when gap is small even if the 
system is not metallica.
Suppose the system has very very small gap at Gamma (G afterwards).
Then it is likely that HOMO(G+delta) has a character of LUMO(G).
In that case, by including LUMO(G) in the disentangled subspace, we can 
better describe HOMO(G+delta).
I may not the right person to explain this, but this is the best answer 
I can give.

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

Yes. But nbnd must be set higher than 2.5*nat, since sp2 anti-bonding or 
free electron-like states mix with anti-bonding pi-bands.
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