[Pw_forum] scf calculation for isolated hydrogen atoms

[Lorenzo Paulatto]

On Fri, 12 Feb 2010 13:32:14 +0100, Gabriele Sclauzero <sclauzer at sissa.it>  
wrote:
>> I tried to calculate the ground state energy for isolated hydrogen atom
>> with the following input,however I got the fermi energy level at
>> -3.5661eV, while it should be -13.6eV.
> [...]
> (maybe PAW can give an AE energy estimate).

With PAW, PBE and spin polarization you can get a total energy for the H  
atom of -0.998 Ry, but this is not related to Dimpy's problem.

The problem is that in periodic boundary conditions absolute values of  
energy (let them be band energies, the Fermi energy or even total energy)  
have no meaning, but only energies differences matter. I.e. if you  
consider an isolated system (H1) in two different choices of unit cell  
(both big enough to ensure convergence) the absolute energies will likely  
be different, yet the energy differences (e.g. LUMO-HOMO) will remain the  
same.

Last, but by any mean no least, I don't see why you should get -1 Ry for  
the Fermi energy of an isolated Hydrogen. Ef (which means you have used a  
fictitious smearing to ease convergence) shall be somewhere between the  
energy of the higher (and only) occupied orbital and the energy of the  
lowest empty orbital. Both these energies depend on the periodic boundary  
conditions, hence the Ef for such a system has no meaning. Furthermore, I  
cannot to see any physical meaning in the Fermi energy of an isolated  
system represented in periodic boundary conditions.

best regards

-- 
Lorenzo Paulatto

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