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

[Fabio Costa]

Dear QE users/developers


I’m currently seeking for some advice on the limitations of the application of the virtual crystal approximation. This week I came across this report ( https://patentimages.storage.googleapis.com/6d/d6/59/1f0d37b855ea9e/US8981626.pdf ), in which the VCA is used to mix Ti and Nb. As far as I’m aware, this approximation is applied to mix immediate neighbor elements (on the same row of the periodic table) , so I was wondering on how to apply the approximation in such a case.


To exemplify, let's consider Ti and Nb, mixed in a rate of 99:1. The resulting atom would have a fractional Z=22.19, and the Nb 1% would add to the system fractions of electrons to the 4p, 4d and 5s orbitals. As Ti’s highest orbital is the 3d, Is it right to assume that all these fractional charges from Nb’s orbitals would migrate to Ti’s 3d orbital?


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)”. 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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