The ABINIT Users Mailing List ( CLOSED )

[Andrew Rappe <rappe@sas.upenn.edu>]

Dear Yu,

It is very kind of Masayoshi to mention our work in the area of VCA.
For your convenience, here are citations to our work (mixing Zr and Ti):

\item N. J. Ramer and A. M. Rappe,
``Application of a new virtual crystal approach for the study 
of disordered perovskites'', 
J.\ Phys.\ Chem.\ Solids {\bf 61}, 315--320 (2000).

\item N. J. Ramer and A. M. Rappe, ``Virtual-crystal approximation
that works: Locating a compositional phase boundary in
Pb(Zr$_{1-x}$Ti$_x$)O$_3$'', Phys.\ Rev.\ B Rapid Comm.\ {\bf 62},
743--746 (2000).

\item\amrsym N. J. Ramer and A. M. Rappe, ``Determination of ferroelectric
compositional phase transition using novel virtual crystal approach'',
AIP Conf.\ Proc.\ {\bf 535}, 95--101 (2000).

Prof. Ramer and I do have some ideas about how to get heterovalent VCA
to work, along the lines of extending our published work, but that
project has not been completed.

Best wishes with your project, and let us know if you have
any further questions.

Sincerely,
Andrew


On Fri, Oct 27, 2006 at 06:17:19PM +0900 or thereabouts, Masayoshi Mikami 
wrote:
> Dear Yu,
> 
> We have little information about your crystal model,
> so my reply may be rather general ... In principle,
> the VCA should be applied for chemically similar ions,
> typically, Sr and Ba etc. I heard that someone VCA applied
> to "Zn and Mg" for "Zn_1-x Mg_x O" --  and it failed --.
> It is quite understandable; Zn (with d shallow core) and
> Mg (without d) are not similar at all.
> Thus, we should not use the VCA for heterovalent ions in general.
> 
> Yet, I come to think that the VCA for heterovalent ions might work
> for some limited cases. One **weird** example ;-) is,
>       http://dx.doi.org/10.1002/pssa.200669547
> Here I used ABINIT. Kindly read the above paper for details.
> In this case, an artificial cation (Z=13.5, ionic charge=+3.5
> as the simple average of Al3+ and Si4+) was treated in the VCA
> implemented in the ABINIT.
> However, I should reiterate that this kind of "luck" may not be
> expected for any kinds of heterovalent solid solutions, in general.
> One should start from some supercells with assumptions of
> the (positional) configuration of heterovalent ions. (I did them first.)
> 
> About the VCA implementation, the following paper may be referred:
>       Ph. Ghosez et al., AIP Conf. Proc. 535, 102 (2000)
> (You can find papers on typical VCA implementations by Google/
>  Google Scholar). There are several ways of averaging.
> ABINIT gives just pseudopotential-averaging.
> Profl Andrew Rappe's Group has other implementations,
> i.e. eigenvalue/eigenfunction-averaging...
> (if I remember correctly... please check their papers on PRB.)
> 
> In ABINIT,
> VCA Examples can be found in Test_v3/t90-t95 of v.4.6.5,
> or, tests/v3/Input/t90-t95 of v.5.2.3.  You might want to
> see the "deliberately unphysical example" in v3/t95;
> the reference output is archived in the ABINIT src.tar.gz ball.
> you will see "how the values are averaged."
> 
> Hope this helps & Good luck (indeed),
> Masayoshi
> 
> On 2006/10/26, at 0:14, Chol-Jun Yu wrote:
> 
> >Dear ABINIT users,
> >
> >recently I am interested in solid solutions. I decided to use the  
> >VCA method.
> >So I generated the pseudopotential of the virtual atom, composed of  
> >the
> >component atoms, using fhi98PP software. Then, when I use it in  
> >order to
> >calculate the property of the solid solution in ABINIT, there is a  
> >problem.
> >
> >Is it possible if the atomic number is a decimal fraction?
> >At this time, how can be estimated the Ewald energy, atomic mass,  
> >number of
> >bands in ABINIT?
> >
> >Best regards,
> >
> >Yu.
> >-- 
> >
> >Computational Materials Engineering(CME)
> >Center for Computational Engineering Science(CCES)
> >Institute of Mineral Engineering(GHI)
> >RWTH Aachen
> >Mauerstrasse 5
> >D-52064 Aachen, Germany
> >Tel: ++49 241 80 94969
> >Fax: ++49 241 80 92271
> >e-mail: yucj@ghi.rwth-aachen.de
> >homepage: http://www.ghi.rwth-aachen.de/www/
> >
> >