[Wien] Virtual Crystal Approximation (VCA): not neighboring atoms in periodic table

[Qingyun Mao]

Dear Xavier,

Thank you for your reply!

Regards,
Qingyun

Sent from my iPad

On Mar 9, 2013, at 1:14 AM, Rocquefelte <xavier.rocquefelte at cnrs-imn.fr> wrote:

> Many other posts have been sent more recently. Search the word "virtual" and not "VCA". 
> 
> The idea is that you replace an atom defined by a nucleus +Z surrounded by Z electrons, by an effective atom defined by a nucleus Zeff = Z+x  surrounded by Z+x electrons. Such an effective       treatment can only be done for neigboring atoms, which have the same number of core electrons for instance and similar radii...
> Indeed, remember the Slater approach (effective nucleus). The screening is defined by the inner electrons Zeff = Z - Sigma, and the way the nucleus is screened defines all the properties of the       atom. In your case the valence electrons of La and Nb feel a completely different effective nucleus (the screening is too much different) and a mixing of these two atoms cannot be simply       treated by an effective nucleus. In a way VCA is a perturbative approach and could give results only if the atoms are not too far in the periodic table. 
> 
> In addition, as already explained by Peter in previous posts it should be used only for "non-active" electrons in the valence band (like Sr2+/La3+), but not for active ones (like O/F) ...
> 
> In your case, I will use a supercell approach and to insure that you didn't create an artificial order you must try different models and compare them in terms of total energy, DOS, properties... 
> 
> Best Regards
> 
> Xavier
> 
> 
> 
> 
> 
>  valence electrons/core charge
> 
> 
> Le 3/9/2013 1:11 AM, Qingyun Mao a écrit :
>> Dear Prof. Blaha and wien users,
>> 
>> Recently I am trying to do calculations on systems like La(x)Nb(1-x)O(y). I searched in the mailing list and the earliest comments showed up was posted around 2004. I saw comments like: "VCA in WINE2k is only possible between neighboring elements, like Na-Mg or Ba-La." But I have not seen any explanation for that yet.
>> 
>> May I ask the reason why we cannot use VCA for atoms which are not neighbors (like La and Nb) in periodic table? Thanks a lot!
>> 
>> Regards,
>> Qingyun Mao
>> 
>> School of Applied and Engineering Physics
>> Cornell University
>> 
>> 
>> 
>> 
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