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<div class="moz-cite-prefix">Many other posts have been sent more
recently. Search the word "virtual" and not "VCA". <br>
<br>
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...<br>
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. <br>
<br>
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) ...<br>
<br>
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... <br>
<br>
Best Regards<br>
<br>
Xavier<br>
<br>
<br>
<br>
<br>
<br>
valence electrons/core charge<br>
<br>
<br>
Le 3/9/2013 1:11 AM, Qingyun Mao a écrit :<br>
</div>
<blockquote
cite="mid:CACPYYaw1XJxA-3JOqqgEt04pQWYQR7PUuF3XF9KX9GgQQXXS_A@mail.gmail.com"
type="cite"><span
style="color:rgb(34,34,34);font-size:13px;font-family:arial,sans-serif">Dear
Prof. Blaha and wien users,</span><br
style="color:rgb(34,34,34);font-size:13px;font-family:arial,sans-serif">
<br
style="color:rgb(34,34,34);font-size:13px;font-family:arial,sans-serif">
<span
style="color:rgb(34,34,34);font-size:13px;font-family:arial,sans-serif">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: "</span><span
style="color:rgb(34,34,34);font-size:13px;font-family:arial,sans-serif">VCA
in WINE2k is only possible between neighboring elements, like
Na-Mg or </span><span
style="color:rgb(34,34,34);font-size:13px;font-family:arial,sans-serif">Ba-La."
But I have not seen any explanation for that yet.</span>
<div>
<span
style="color:rgb(34,34,34);font-size:13px;font-family:arial,sans-serif"><br>
</span></div>
<div><span
style="color:rgb(34,34,34);font-family:arial,sans-serif">May I
ask the reason why we cannot use VCA for atoms which are not
neighbors (like </span><span
style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px">La
and </span><span
style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px">Nb</span><span
style="color:rgb(34,34,34);font-family:arial,sans-serif">) in
periodic table? Thanks a lot!</span></div>
<div><span
style="color:rgb(34,34,34);font-family:arial,sans-serif"><br>
</span></div>
<div><span
style="color:rgb(34,34,34);font-family:arial,sans-serif">Regards,</span></div>
<div><span
style="color:rgb(34,34,34);font-family:arial,sans-serif">Qingyun
Mao</span></div>
<div><span
style="color:rgb(34,34,34);font-family:arial,sans-serif"><br>
</span></div>
<div><span
style="color:rgb(34,34,34);font-family:arial,sans-serif">School
of Applied and Engineering Physics</span></div>
<div><span
style="color:rgb(34,34,34);font-family:arial,sans-serif">Cornell
University</span></div>
<div><span
style="color:rgb(34,34,34);font-family:arial,sans-serif"><br>
</span></div>
<div><span
style="color:rgb(34,34,34);font-family:arial,sans-serif"><br>
</span></div>
<br>
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</blockquote>
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