[Wien] 2 ways to calculate bindning energy

[Peter Blaha]

I have not followed in detail the sometimes unclear discussions on that topic,
but I doubt that energy zeros or supercell sizes for single atoms are a "real"
issue (the latter may be both, a fundamental and a numerical issue, when you
need superior precision, but not for "normal" accuracy).

lstart: is a "quasi-exact" (numerical limit) spin-orbit DFT code for free
         SPHERICAL atoms

WIEN2k uses various relativistic approximations, so its energy is not compatible
        with lstart and lstart energies will be too low.

However, you can test supercell calculations for free atoms using NREL in case.struct
This makes energies of lstart and WIEN2k comparable.

You should observe that for good WIEN2k parameters the atomic energy of WIEN2k
is equal (or lower) to lstart-energies. (It can be lower in GGA, since WIEN will
yield non-spherical densities for many non-closed shell atoms
(eg. C: 2s^2 2px,2py but not "spherical" 2p^2)

This way you can "test" your supercell calculations and then include scalar
effects and compare to bulk energies.

Lyudmila Dobysheva schrieb:
>>>>> Fri, 14 Mar 2008 B. Yanchitsky has written:
>> Atom name     Z     E[1]-E[i] (eV)
>>     Be(hcp)    4      -4.01952760
>>     Al(fcc)   13      -9.1210168
>>     Cu(fcc)   29     -32.5998664
>>     Au(fcc)   79     -57.3670440
>> this is just wrong, interatomic potential is something like 0.01-0.1 eV,
>> and been multiplied by number of nearest neighbors, something like 10,
>> gives 0.1-1 eV (1000K-10000K), but not a million of kelvins.
>> I don't think this may be related to DFT, there is some spurious term
>> (electrostatic?) that pushes energy up on large volumes.
> 
> We have discussed this problem, and came to conclusion that this is connected 
> with the energy zero determination. In the high-level mathematical physics, 
> there is a theorem that asserts: increase of the periodical cell size with a 
> single atom does not give the actual zero, and cannot be compared with a real 
> isolated atom moved to infinity. The periodicity itself, irrespective of the 
> dimensions, essentially changes the situation. In order to solve the problem, 
> a real infinity must be inserted into a calculational scheme. Green's 
> function method declares that it can in some variations.
> 
> Best regards
> Lyudmila Dobysheva
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                                       P.Blaha
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Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
Phone: +43-1-58801-15671             FAX: +43-1-58801-15698
Email: blaha at theochem.tuwien.ac.at    WWW: http://info.tuwien.ac.at/theochem/
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