[Pw_forum] Shifting Fermi energy for Fermi surface calculations

[Paolo Giannozzi]

On Dec 23, 2009, at 20:52 , Nicki Frank Hinsche wrote:

> For my isolating system the Fermi energy would be in the gap,
> therefore there wont be an eigenvalue for given k-points at the Fermi
> energy. If I shift the energy to a value right where states are, then
> I would expect of course some contribution to an iso-energetic
> surface. But how to change this 'Fermi energy' in the right way. Is it
> enough to change the parameter in input_FS? Of course I know, that the
> system isn't charge-neutral anymore after artificially shifting the
> Fermi energy.

input_FS is the file containing input data for the program (written  
by Eyvaz)
that calculates the Fermi surface, given the band structure  
calculated in a
previous run, IIRC. That code requires in input the Fermi energy (it  
might
read it from the bands data, but apparently it doesn't). If this is  
your problem:
you can change the Fermi at will, since the band structure is fixed.

You can change the Fermi energy in a self-consistent way by adding or  
removing
charge to the system (variable "nelec"). This will modify the band  
structure wrt the
neutral case, but not by much if you add or remove just a fraction of  
an electron.
The code will take care of adding a neutralizing background (needed  
in order to
have a finite value of the energy).

Paolo
---
Paolo Giannozzi, Dept of Physics, University of Udine
via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222