[Wien] Virtual Crystal Approximation (VCA): not neighboring atoms in periodic table
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 ?crit : 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 ___ Wien mailing list Wien at zeus.theochem.tuwien.ac.at http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien ___ Wien mailing list Wien at zeus.theochem.tuwien.ac.at http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien -- next part -- An HTML attachment was scrubbed... URL: http://zeus.theochem.tuwien.ac.at/pipermail/wien/attachments/20130311/6e21e3d4/attachment.htm
[Wien] Virtual Crystal Approximation (VCA): not neighboring atoms in periodic table
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 ?crit : 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 ___ Wien mailing list Wien at zeus.theochem.tuwien.ac.at http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien -- next part -- An HTML attachment was scrubbed... URL: http://zeus.theochem.tuwien.ac.at/pipermail/wien/attachments/20130309/c3f0d16f/attachment.htm
[Wien] Virtual Crystal Approximation (VCA): not neighboring atoms in periodic table
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 -- next part -- An HTML attachment was scrubbed... URL: http://zeus.theochem.tuwien.ac.at/pipermail/wien/attachments/20130308/580e3785/attachment.htm
