[QE-users] Virtual Crystal Approximation for non-neighboring elements

[Lorenzo Paulatto]

> Is it right to assume that all these 
> fractional charges from Nb’s orbitals would migrate to Ti’s 3d orbital?

Hello, the initial charges used to generate the pseudopotential should 
have no effect on the bulk calculation anyway, all that matters is the 
total charge which is computed from the valence charge

> 
> 
> Moving on to the way to implement this, I tried to do this mixture with 
> virtual_v2.x, but it stops with the message “ different nqlc are not 
> implemented (yet)”. 

The virtual.x code expects the two pseudopotentials to be very similar, 
and in particular to have the same amount of valence orbitals for each 
angular momentum. It is possible that the specific choice of Ti and Nb 
pseudopotentials you have made is not compatible, i.e.e beacause on or 
the otehr did not include semi-core in valence, or omitted some higher 
angular momentum.

For example, if you have taken Ti and Nb from the Andrea Dal Corso 
pslibrary, Nb includes 4s4p4d and 5s5p in valence, while Ti includes 
3s3p3d and 4s4p (the 4p is not marked at the beginning because it is 
empty, but it should be there). Hence the two files should be compatible.

have a look here:
http://www.quantum-espresso.org/pseudopotentials

hth

> Another approach would be to generate a 
> pseudopotential for this Z=22.19 specimen, by just resetting the 
> variables to represent this fractional atom? In this case, would it be 
> right to assign to the 3d state the occupancy 2.19 (instead of the usual 
> 2 for the pure Ti)? The 0.19 extra electron comes from the Nb 3d, 4p, 4d 
> and 5s orbitals.
> 
> 
> Thank you for the support
> 
> Fábio Costa
> IF - Universidade Federal da Bahia
> 
> 
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-- 
Lorenzo Paulatto - Paris