[Wien] Reference energy for eigenvalues

[Oleg Rubel]

Thank you for the reply.

> Peter Blaha wrote:
>
> 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).

I do not think this is the way I am going to go. My interest is the  
deformation potential of selenium. Also, I would like to distinguish  
between between different bands (not just the energy gap). A crude  
estimate shows that I will need eigenvalues with the accuracy of  
better than 0.001 Ry.

> 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.

In my case, the difference of Fermi energies is a too rough estimate.  
However, what works probably fine is taking one of the deep core  
states (say S1) as a reference when comparing two calculations,  
provided this atom present in both calculations.

For example in trigonal Selenium (semiconductor):

VOLUME = 552.24497
E_FERMI = 0.27371 Ry
E_1S = -916.580021885 Ry

VOLUME = 535.67766
E_FERMI = 0.24834 Ry
E_1S = -916.601436004 Ry

The difference between the Fermi energies (the top of the valence  
band) is 0.02537 Ry. The difference between 1S eigenvalues is 0.02141  
Ry. Can attribute 0.00396 Ry (= 0.02537 - 0.02141) to the deformation  
potential of the valence band edge? If so, I get correct at least an  
order of magnitude for the deformation potential.

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

Please, correct me if I am wrong. Slaters transition state theory is  
intended for treating excitations and allows to improve over DFT-LDA  
energy gap for semiconductors. I do not quite understand its relation  
to the problem of the reference energy.

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

Sorry, I did not express myself clearly. -2.30 and -0.73 Ry are  
eigenvalues of 3d-As and 3d-Ga states in GaAs. They are exactly the  
same in 2- and 8-atom cell. This was my point to say that, although,  
UST0 changes eigenvalues stay the same.

What about units of UST0? I looked at REAN3.f, and it seems that UST0  
is just a first Fourier coefficient, which should contain the average  
value. Could it be that UST0 is the average potential multiplied by  
the number of points in Fourier expansion?


Thank you once again for the suggestions,

Oleg Rubel


>
> 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
> --------------------------------------------------------------------------
> 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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