[Wien] insulator in virtual crystal approximation

[Moritz Hoesch]

Dear Wien-Users,

I'm using Wien2k and the virtual crystal approximation (VCA) to  
simulate the metallic state in highly Boron doped diamond. K.W. Lee  
and W.E. Pickett [PRL93 (2004) 237003] used VCA in the Wien2k code  
and got a metallic hole Fermi surface with a Fermi energy of about  
0.8 eV at 2.5 at% doping. When I put 2.5 at% doping, i.e. I replace  
Carbon with Z=6 by a virtual atom with Z=5.975, the calculation  
converges into a insulating solution. At 2.9% and above I get a  
metallic band structure with E_F of about 1 eV, i.e. the Fermi level  
is about 1 eV below the (now unoccupied) valence band maximum. I  
don't do any phonons (yet) like Lee and Pickett did, I just want to  
get the ground state of the undistorted lattice. I tried volume  
optimization and found a 4% volume expansion, but at 2.5% doping I  
get an insulating state for all lattice parameters.

How can I reproduce the result of Lee and Pickett? Is there a trick  
for VCA? Where do my 0.025 electrons per atom go? Or is this some  
kind of Mott transition in the GGA?

I use GGA with the 1s as core electron (energy separation -6 Ry). To  
sample the small Fermi surface well I use a dense k-mesh of 1240 k- 
points in the reduced wedge (36 x 36 x 36 in total). I use  
WIEN2k_05.3 (Release 3/4/2005) and I got the same result also with  
Wien97. I attach my .struct and .inst file to this e-mail.

Many thanks for any elucidating remarks

Moritz





-------------
Moritz Hoesch
JSPS Postdoctoral Research Fellow

Synchrotron Radiation Research Center                phone   
+81-791-58-2643 (PHS 3808)
Japan Atomic Energy Research Institute (JAERI)        fax    
+81-791-58-2740

1-1-1 Koto, Mikazuki-cho, Sayo-gun
Hyogo 679-5148 JAPAN

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