[Pw_forum] About postprocessing

[Stefano Baroni]

On Nov 21, 2006, at 12:26 AM, Giuseppe Piero Brandino wrote:

> Dear espresso user,
> i have some questions about the pp.x program.
> Asking the code to write the potential, for example
>
> plot_num=11=the V_bare + V_H potential,
>
> does it print the potential or the potential energy?
> I know that it prints it in Rydberg, so an energy.

right

> How do i get the potential? simply dividing by e

right

> or dividing by the total (ionic and electronic) charge density
> in each point?

this is a bit confusing to me. why would you divide by the local  
charge density? an energy divided by a charge density makes a  
potential times a volume, which is not what you expect, and  
furthermore I am not aware of any sensible use of the resulting  
quantity.

> Something else,
> is there a simple way to get the total charge density (electronic +  
> ionic) from pp.x?

The ionic *pseudo*charge density is ill-defined when the  
pseudopotential is non local (as it usually the case). On the other  
hand, the physical (all-electron) valence+core charge density is not  
simply the sum of the valence pseudo-charge+ionic charge. What would  
be needed here  is the sum of the all-electron ionic charge density +  
the valence charge density obtained from the reconstructed wave- 
functions. This is what is done the PAW formalism (see PE Bloechl,  
Phys. Rev. B 50, 17953 - 17979 (1994)). Work is in progress to  
implement PAW in Quantum Espresso. Meanwhile, there might be tools  
available to obtain a (more or less) quick-and-dirty recostruction of  
the charge, but others may be more appropriate than me to address  
this point.

Stefano Baroni

---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
Trieste
[+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)

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