[Wien] Reference energy for eigenvalues

[Peter Blaha]

You cannot compare eigenvalues between bulk-calculations and a free atom.

The free atom has a well defined energy-zero (V=0 at r--> infinity).
An "infinite bulk" has even at infinity some atoms and thus NO   V=0.

The only way would be to simulate a surface with a thick vacuum. You can then
check V-zero in the middle of the vacuum region and subtract this value from
EF (gives the work function).

However, you can compare "binding energies" (which will be rather bad as absolute numbers,
DFT), or more interesting, "core-level shifts". For this purpose you relate the eigenvalues of two different
calculations to their fermi energies, and the difference is already a good estimate of the shifts.

You can also apply Slaters transition state theory using half a core-hole to get better absolute energies
(still with respect to EF).

I very much doubt that a 2 and 8 atom GaAs cell has so different eigenvalues. Something is wrong
(with the 8-atom cell)

Oleg Rubel schrieb:
> Dear Wien2k Community,
> 
> I need to compare eigenvalues calculated with Wien2k for different  
> compounds. I found a number of discussions in the mail-list regarding  
> the issue of the reference energy. They all concur that the reference  
> energy is an average potential in the interstitial region. In one of  
> the postings  
> (http://zeus.theochem.tuwien.ac.at/pipermail/wien/2004-September/003582.html)  
> Peter Blaha suggests UST0 returned by
> CALL REAN3(NKK,KZZ,CVALUE,IFF1,IFF2,IFF3,CFFT,UST,UST0)
> be the value of the potential in the interstitial region (if I  
> understand it correctly).
> 
> I wander what are the units of UST0? It seems that UST0 is  
> proportional to the volume of the unit cell. In GaAs I found:
> UST0 = 231.58 (UNIT CELL VOLUME =     298.59750)
> UST0 = 926.31 (UNIT CELL VOLUME =    1194.38998)
> corresponding to 2-atom and 8-atom basis. Obviously, in case of the  
> average potential, it should not change at all for the same compound.  
> Nevertheless, eigenvalues of 3d electrons are -2.30 and -0.73 Ry in  
> both cases.
> 
> An isolated "As" has the eigenvalue of 3d electrons at -2.95 Ry. I  
> would expect the difference between -2.30 and -2.95 Ry to be mainly  
> due to the change of the reference energy. But how to bring  
> eigenvalues to the same scale?
> 
> Please help!
> 
> Thank you in advance,
> 
> Oleg Rubel
> 

-- 

                                       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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