[Wien] Virual Crystal and Supercell

[Peter Blaha]

The VCA is not very reliable for such a problem, where the "mixed" 
elements have "active" electrons at/near EF.

One can use the VCA for elements which dope a system with electrons (or 
holes) like eg. in ionic compounds (eg. some oxides) with Ba/La or K/Ca 
..., where these elements do not participate much in the chemical 
bonding between eg. a Transition metal and oxygen.

The "supercell" approach has the disadvantage of spurious "ordering" 
effects, thus one should use large enough supercells and test various 
distributions of the dopants to see its effect. But for a Cu/Zn system 
it is definitely the better approach.

For DOS properties of metallic alloys the cpa is a fairly good 
approximation, but this is not available in wIEN2k (and you do not learn 
anything about local relaxations,...)

Am 13.07.2012 00:29, schrieb prasenjit roy:
> Dear Wien2K Users,
>                              I want to use virtual crystal
> approximation(VCA) for a very large system, so I applied it on a simple
> system [Ni(28)] to check whether everything is correct or not. To do
> that, I did the following steps.
>   1. Initialized the calculation with Ni(Z=28).
>   2. Changed Z to 28.15
>   3. Changed case.inst to:
> --------------------------------------------------------------
> Ni
> Ar 3
> 3, 2,2.00  N
> 3, 2,2.00  N
> 3,-3,3.00  N
> 3,-3,1.30  N
> 4,-1,1.00  N
> 4,-1,0.85  N
> ****
> ****
> ---------------------------------------------------------------
>   4. Changed NE in case.in2* from 16 to 16.15
>   5. Run the spin polarized calculation.
>                               I also did a supercell calculation where
> the 2x2x2 supercell was containing 32 non equivalent Ni atoms. Then, I
> replaced 5 of them with Cu (Z=29) to make a 15% doped system. Next "x
> spacegroup" changed the spacegroup and reduced the number of symmetry
> operations. At last the scf calculation was performed and solution
> converged.
>                               The problem is, the density of states is
> reasonably different in these two above described cases. The density
> obtained from the VCA looks almost like the pure Ni DOS. Whereas that
> obtained from supercell approach is different. I need to know whether
> the ways of approach are correct or not in both cases. Should I consider
> the supercell approach more reliable over the VCA ? While constructing a
> supercell, is it needed to increase the number of k points also ?
>                               Thank you very much in advance for helping me.
>
> Prasenjit Roy
> Radboud University, Nijmegen
>
>
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-- 
Peter Blaha
Inst.Materials Chemistry
TU Vienna
Getreidemarkt 9
A-1060 Vienna
Austria
+43-1-5880115671