[Pw_forum] total force and number of k-points

[Paolo Giannozzi]

On Sep 2, 2011, at 15:54 , Elie Moujaes wrote:

> I have a simple question: does the total force acting
> on a system depend on the number of k-points  used?

not sure what you mean by "total force", but forces on
atoms depend upon the number of k-points, for sure

> That is in my scf calculation I am using a grid of 4x4x1
> for my system whereas the grid must be denser if phonon
> calculations are taking place I am using 8x8x1. Do I get
> different total force acting on the system and does it mean
> I have to relax the crystal twice one for an scf calculation
> and another for  a phonon calculation?

in principle a system should be relaxed for the same set of
parameters (cutoff, k-points) that are used in the subsequent
phonon calculation. The difference might be small enough to
be negligible, but this is something that should be checked.

P.
---
Paolo Giannozzi, Dept of Chemistry&Physics&Environment,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222