[Pw_forum] FFT grid and cutoff energy.

Duy Le ttduyle at gmail.com
Tue May 31 15:51:13 CEST 2011

On Tue, May 31, 2011 at 9:02 PM, Hongsheng Zhao <zhaohscas at yahoo.com.cn> 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.

Not really true. There is no direct relationship between FFT grid and
cutoff energy (I supposed that by cutoff energy you meant Kinetic
energy cutoff for the planewave expansion)
FFT mesh is calculated based on the cutoff for charge density. In any
code, the coder usually set a default relationship between cutoff for
PW expansion and cutoff for charge density. For example, in QE, the
later is 4 time larger.

> 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?
It depends. If that default value is good enough, you don't have to do
an additional test. The default value is good enough for PAW
potential, but too small for US or NC potential.
BTW, I understood that by saying "Independent FFT grid convergence
test" you really meant "Independent *cutoff energy for the charge
density* convergence test"

Duy Le
PhD Student
Department of Physics
University of Central Florida.

"Men don't need hand to do things"
> Regards.
> --
> Hongsheng Zhao <zhaohscas at yahoo.com.cn>
> School of Physics and Electrical Information Science,
> Ningxia University, Yinchuan 750021, China
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