[Pw_forum] Zero of the electrostatic potential ?

[Lorenzo Paulatto]

In data 23 giugno 2009 alle ore 11:29:28, Vladan Stevanovic  
<vladan.stevanovic at epfl.ch> ha scritto:
> I am using QE for simulations of transition metal clusters. My question
> is where the code places the vacuum level in such calculations
> (nonperiodic systems in a big cell) ?

Dear Vladan,
a non-periodic system in a big periodic cell is still a periodic system!  
Hence, the integral of the potential over any possible choice of primitive  
cell has to be zero. This is a sufficient condition to find the zero.

> To formulate differently, is it
> possible to compare directly KS eigenvalues from different calculations ?

In general it is NOT possible to directly compare eigenvalues of different  
calculations, nevertheless they can be compared taking the Fermi energy as  
reference.

best regards

-- 
Lorenzo Paulatto
SISSA  &  DEMOCRITOS (Trieste)
phone: +39 040 3787 511
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