[Pw_forum] scf calculation for isolated hydrogen atoms

[Gabriele Sclauzero]

Lorenzo Paulatto wrote:
> On Fri, 12 Feb 2010 15:41:44 +0100, Gabriele Sclauzero 
> <sclauzer at sissa.it> wrote:
>> I still believe that the main problem in Dimpy's result is the self 
>> interaction error for the 1s level, and even if you align the 1s level 
>> to that reference energy you would still get a big error.
> 
> I'll try to be clearer... I've attached input and output of a h1 
> calculation for PAW (h.in, h.out): even though the total energy is ~ -1 
> Ry, the Fermi energy is -4.1040 eV. Then I've repeated the calculation 
> for the same system (hb.out), but I've slightly reduced the smearing 
> (from 0.05 Ry to 0.03 Ry), and guess what? The total energy is the same 
> to 8 significant digits, the energy levels are the same, but the Fermi 
> energy is totally different: -2.5074 eV.

Maybe I was not clear enough as well... the reference energy I was referring to is not the 
Fermi energy, but some value of the potential in the middle of the supercell.
Anyway, maybe we have gone too far with this discussion, since we agree on the fact that 
one should not look at Ef for an isolated atom system.

GS

> 
> So, provided that Ef depends dramatically on technical and totally 
> irrelevant details of the calculation, why would someone expect it to be 
> equal to the first ionization energy? Even for a metal, where Ef *do 
> has* a physical meaning, there is no relation between it and the 
> ionization energy, why should this occur for an isolated atom?
> 
> I've been wondering if I had misunderstood the question, so I went back 
> to check:
>>>> I got the fermi energy level at -3.5661eV, while it should be -13.6eV
> 
> the answer is **it should not**, there is no reason for Ef to be -13.6 
> eV. If you are using PAW the *total* energy will be ~ -13.6 eV, 
> otherwise it depends on the pseudopotential generation details. The 
> Fermi energy has nothing to do with it.
> 
> cheers
> 
> 
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