[Pw_forum] shape of the wave function in reciprocal space

[Lorenzo Paulatto]

Dear Jacopo,
The g vectors that are used respect the cutoff condition |k+G|^2 <
ecutwc/hbar, this defines a sphere (per k point). The box, used for the
FFT, is the smallest that contains all spheres.

Kind regards

-- 
Lorenzo Paulatto
Written on a virtual keyboard with real fingers

On 26 May 2017 2:48 a.m., "Jacopo Simoni" <simonij at tcd.ie> wrote:

> Dear PWSCF users,
>
> I am doing a simple scf calculation for a system of 100 Hydrogen atoms in
> a simple cubic cell.
>
> I am generating the wave functions with wf_collect=.true. for a single K
> vector (0,0,0).
>
> From the scf output I realized that the total number of G vectors printed
> on the file is 847. I am using ecutwfc=20.0 and the shape of the Kohn-Sham
> matrix is
>
> Kohn-Sham wave functions    dimensions=(847, 52)
>
> where 52 corresponds to the number of bands that I am using in the
> calculation.
>
> However, I am wondering why in the gkvectors.xml file I have a list of G
> vectors ranging between -5 and 5 times 2*pi / a (a is the lattice
> parameter) along each primitive lattice direction.
>
> The total number of points in such a regular grid should be 11*11*11 =
> 1331 that is clearly different from 847. What is the meaning of the missing
> 484 G vectors in the grid ? I am a bit confused could you please explain to
> me these results ?
>
> Many thanks in advance,
>
> Jacopo Simoni,
> Los Alamos National Laboratory
>
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