[Wien] Electron density at implanted Be nucleus in Au and Al lattice

[Amlan Ray]

Dear Prof. Blaha,
Recently we have done WIEN2K calculations by putting beryllium atoms at the octahedral sites of face centered cubic lattices Au (lattice parameter 4.08 Angstrom) and Al (lattice parameter 4.05 Angstrom) and determined the electron densities at the implanted beryllium nucleus for the two cases (Be implanted in Au and Be implanted in Al). We first performed the calculations by putting a beryllium atom at every octahedral site of the lattice without forming any supercell. We find that the electron density at Be nucleus implanted in aluminum is higher than that in Au by about 0.35%. The result remains about the same whether we treat 1s state of Be as a core state or valence state. This result qualitatively agrees with the experimental result of electron capture rate of 7Be in Al versus Au. 
 
Then we formed a supercell (using WIEN2K). The supercell included just one more neighboring fcc cells of Au or Al and there was just one beryllium atom at the octahedral site of this supercell. Using viewing program CRYSDEN, we have ensured that the supercells have been formed correctly. However now, we find that the electron density at Be nucleus in Au is higher by about 0.18% compared to that in Al. So this is qualitatively just the opposite result compared to what we found earlier by putting one Be atom in every octahedral site. The result remains unchanged whether we treat 1s state of Be as a core state or valence state.
 
I think the supercell is a better modelling of the implanted ions in Au and Al. (Of course they can go to tetrahedral sites also, but at first we are only considering octahedral sites.) My problem is why I am getting opposite result when the supercells were formed. I think the electron density at Be nucleus should be higher when implanted in Al compared to Au, because Au has much higher electron affinity compared to Al and so Be should lose more valence electrons when implanted in Au and hence less electron density at the Be nucleus. The experimental results also support the same conclusion, but my supercell calculations is giving opposite result for the octahedral site, although I get qualitatively correct result if Be is implanted in every octahedral site. 
 
I request some comments and suggestions about this problem. 
 
With best regards
                                                                                         Amlan Ray
Address
Variable Energy Cyclotron Center
1/AF, Bidhan Nagar
Kolkata - 700064
India

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