[Wien] Different Rmt's for atoms of one type

[Laurence Marks]

An approximate explanation, but first a caveat: dealing with small
RMT's in Wien2k is not completely trivial.

In Wien2k you have two different basis sets, a radial grid + orbitals
inside the RMT, plane waves outside. If you use different RMTs for the
Fe in effect you make them different. For instance, try setting up a
Fe cell with two different RMTs (i.e. a supercell), converge it then
do aim. You will find that there has been a small charge transfer
between the Fe's, which will go to zero as you increase RKMAX.

For your problem, I would go beyond setrmt (which is OK, but not
always optimal) and use a smaller RMT for the C (maybe 1.3) and
increase that for the Fe. You will have to do this by hand. Be careful
about going too small with the C as if you do you will lose core
charge (although .lcore helps); you also need to be careful about
losing core electrons from the Fe. In both cases look in case.outputm
to see how far the core integrals are from integers. Scale RKMAX so
RKMAX/min(RMT) is roughly the same (e.g. 3.5), although you need to
add a bit when you decrease the RMTs. Probably something like 5-6
should be good for your problem.

How to chose the "right" RMTs? I don't have any great wisdom, others
might have. I tend to use RMTCheck (look in the mailing list, or I can
ask Peter to add it to the contributed software) and check that the
step in the gradients at the RMT for the different atoms are
comparable. In most cases this gives results similar to setrmt. In
principle you can also do calculations for different values with
RKMAX/min(RMT) fixed and look for the minimum energy -- the few times
I've done this the results are about the same. Another method would be
to try and balance the charge lost in the core integrals, which gives
a similar result.

2011/4/21 Maxim Rakitin <rms85 at physics.susu.ac.ru>:
> Thank you Evgenia,
>
> I've already seen that link, but I'd like to get information why it is not
> possible from physical point of view.
>
> Maxim
>
> 21.04.2011 16:53, Евгения Каблиман пишет:
>
> Dear Maxim,
>
> identical elements should have identical RMT's.
> This link might help you http://www.wien2k.at/reg_user/faq/rmt.html .
>
> 2011/4/21 Maxim Rakitin <rms85 at physics.susu.ac.ru>
>>
>> Dear WIEN2k community,
>>
>> Is it correct to perform calculations with different Rmt radii for atoms
>> of one type, e.g. Fe?
>>
>> I need to calculate total energy of a systems containing one interstitial
>> carbon atom in bcc iron matrix. Carbon has quite large MT-radius comparing
>> with H atom, so StructGen sets Rmt(Fe) to 1.59 a.u. and Rmt(C) to 1.41 a.u.
>> for lattice constant 5.394 a.u. In some papers (C. Domain et al., PRB 69,
>> 144112 (2004)) Rmt(C) is even large than I got (~1.6 a.u.) and Rmt(Fe) is
>> also much larger (~2.4 a.u.). But if I use the radii from the article,
>> WIEN2k doesn't allow me to perform further procedures - nn gives me
>> information about overlapping spheres.
>>
>> Can I use Rmt(Fe)=2.4 a.u. in all volume of my supercell except of the
>> nearest iron atoms?
>>
>> Thanks in advance for your answers.
>>
>> --
>> Best regards,
>>   Maxim Rakitin
>>   South Ural State University
>>   Chelyabinsk, Russia
>>   email: rms85 at physics.susu.ac.ru
>>   web: http://www.susu.ac.ru
>>
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>
>
>
> --
> Evgeniya Kabliman.
> Insitute of Materials Chemistry         Vienna University of Technology
> Getreidemarkt 9/165-TC, A-1060 Vienna, Austria
> Tel: +43 1 58801 15674        Fax: +43 1 58801 15698
> evgeniya at theochem.tuwien.ac.at            http://www.imc.tuwien.ac.at
>
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-- 
Laurence Marks
Department of Materials Science and Engineering
MSE Rm 2036 Cook Hall
2220 N Campus Drive
Northwestern University
Evanston, IL 60208, USA
Tel: (847) 491-3996 Fax: (847) 491-7820
email: L-marks at northwestern dot edu
Web: www.numis.northwestern.edu
Chair, Commission on Electron Crystallography of IUCR
www.numis.northwestern.edu/
Research is to see what everybody else has seen, and to think what
nobody else has thought
Albert Szent-Gyorgi