[Pw_forum] box grid in cp

[Silviu Zilberman]

Dear Nichols,

The core charge is in principle finite only at the core region (as 
defined by r_cut) and vanishes out side the core. Numerically the charge 
is represented in a Fourier series which may give rise to small charge 
oscillations outside the core and even to negative charge density, but 
only if the cut-off is too low. Having these small boxes removes the 
charge oscillations problem (at least outside the box) and also offers 
some numerical advantages in going to higher cut-offs.

The small boxes should be set as small as possible, but large enough to 
contain the core of the largest element in your system. The formula for 
determining the box size is quite simple:

nr1b=[(2*r_cut)/Lx*nr1]
nr2b=...
nr3b=...

where r_cut is the cut-off radius for the largest element and Lx is the 
physical length of your box along the x axis. You have to round your 
result to the nearest larger integer.

Silviu.

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.
> 1613 Denise Dr. Apt. D
> Forest Hill, MD 21050
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