[Pw_forum] scf calculation for isolated hydrogen atoms

[Lorenzo Paulatto]

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.

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

-- 
Lorenzo Paulatto

*** Note: my affiliation has changed! please send future
correspondence to: <Lorenzo.Paulatto at impmc.upmc.fr> ***

post-doc @ IMPMC/UPMC - Université Paris 6
phone: +33 (0)1 44 27 74 89
www:   http://www-int.impmc.upmc.fr/~paulatto/

previously:
phd student @ SISSA  &  DEMOCRITOS (Trieste)
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www:   http://people.sissa.it/~paulatto/
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