[Wien] Virual Crystal and Supercell

[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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