[Pw_forum] box grid in cp

[Nicola Marzari]

Dear Nichols,

ecutrho defines a corresponding resolution on the
real space FFT mesh (as expressed by nr1, nr2 and nr3, that
the code left on its own sets automatically). In the ultrasoft
case we refer to this mesh as the "hard" mesh, since
it is denser than the smooth mesh that is needed to
represent the square of the non-norm-conserving wavefunctions.

On this "hard", fine-spaced mesh, you need to determine the size
of the cube that will encompass the largest of the augmentation
charges - this is what nr1b/2b/3b are.

So, nr1b is independent of the system size, but dependent on the
size of the augmentation charge (that doesn't vary that much)
and on the real-space resolution needed by augmentation charges
(rule of thumb: ecutrho is between 6 and 12 times ecutwfc).

In practice, nr1b et al. are often in the region of 20-24-28;
testing seems again a necessity (unless the code started
automagically to estimate these).


				nicola




Nichols A. Romero wrote:

> Hi,
> 
> This is in reference to CP in Quantum-Espresso 3.0.
> 
> What exactly is nr1b, nr2b, nr3b?
> 
> Are they at all related to ecutwfc or ecutrho?
> 
> Thanks,
> --
> Nichols A. Romero, Ph.D.
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