# [Pw_forum] Question about electron (pseudo-)charge density calculations in QE

Paolo Giannozzi p.giannozzi at gmail.com
Thu May 26 07:32:16 CEST 2016

\int rho(r) dx dy dz => \sum_{ijk} rho(i,j,k) \Omega/(nr1*nr2*nr3),
where \Omega = unit cell volume, nr*=FFT grid dimensions, i,j,k run over
FFT real-space grid points.

Paolo

On Thu, May 26, 2016 at 1:44 AM, reza vatan <rezavatan64 at gmail.com> wrote:

> Dear all,
>
> I have one hydrogen molecule in a unit cell repeating in 3D space in such
> a way non of the hydrogen molecules can't see each other. I'm trying to
> calculate the "electron (pseudo-)charge density" in the entire unit cell
> using pp.x. The problem is that when I add the charge density of all meshes
> to get the total charge density of the entire unit cell I get different
> values depends on the used grid. For instance, when I use 45x45x45 grid I
> get 86.25 for charge density, but when I use 72x72x72 I get 353.26.
>
> However, since my unit cell size is 20.37x20.37x20.37 bhor^3 and I have 2
> electrons in the unit cell, I think I should get
> 2/(20.37x20.37x20.37)=0.000236623. Does any one know why I'm not getting
> this number?
>
> Thanks in advances,
>
> Best,
> Reza Vatan,
> Electrical Engineering Department,
> Arizona State University.
>
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--
Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222
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