[Pw_forum] FFT grid and cutoff energy.

Nicola Marzari nicola.marzari at materials.ox.ac.uk
Tue May 31 15:50:51 CEST 2011

On 6/1/11 2:02 AM, Hongsheng Zhao wrote:
> //Hi all,
> //I've learned that, for all of the PW based DFT codes, such as,
> VASP/PWscf/Abinit/CASTEP, the
> FFT grid is generated based on the cutoff energy. So if we have tested
> the convergence total energy vs cutoff energy, then the
> FFT grid used for our calcultions should also be OK based on the
> selected cutoff energy. In other words, it is unnecessary to do a
> Independent FFT grid convergence test. Am I right?
> Regards.

Yes, indeed. In the old days one would use slightly
coarser meshes than what prescribed, and get away with it (especially
if that allowed for the FFT dimension to be a power of 2, for
which typically FFT used to be much faster); nowadays noone bothers.

FFTs dominate the cost of small calculations (relevant if you need
to do a lot of them) and can be tricky in massively parallel 
calculations, but in general I wouldn't bother.

The CP code in Q-E is slightly more sophisticated, and for ultrasoft
pseudopotentials it uses different FFT grids to transform the
softer wavefunctions, and harder charge density - not sure if other 
codes do the same (the technique is all described in Pasquarello 1993 PRB).


Prof Nicola Marzari    Department of Materials    University of Oxford
Chair of Materials Modelling  Director, Materials Modelling Laboratory
nicola.marzari at materials.ox.ac.uk     http://mml.materials.ox.ac.uk/NM

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