[Q-e-developers] [Pw_forum] Increasing the FFT grid spacing/ using wavefunction to provide initial charge density

[Paolo Giannozzi]

In my opinion it is neither simple not very useful. What might be useful in
my opinion is the possibility to
1. restart from lower-cutoff charge density AND wavefunctions, and
2. interpolate wavefunctions to a denser grid of k-points, adding the
missing bands if needed.
The charge density alone often typically doesn't make a big difference.
Moreover the charge density, now written in real space, should be written
in G-space, like wavefunctions, so that one can use a similar logic (or
illogic).

Paolo

On Thu, Sep 29, 2016 at 4:42 PM, Nicola Marzari <nicola.marzari at epfl.ch>
wrote:

> On 29/09/2016 16:36, Lorenzo Paulatto wrote:
> > If you want to have a try at implementing this rouself you have to start
> > from./Modules/xml_io_base.f90, subroutine read_rho_xml (line 668 in the
> > current development version), in the subroutine you'll find a line
> >          IF ( nr1 /= nr(1) .OR. nr2 /= nr(2) .OR. nr3 /= nr(3) ) &
> >             CALL errore( 'read_rho_xml', 'dimensions do not match', 1 )
> >
> > You should change the code in order to have it read even in the case
> > where the dimensions do not match, as long as nr(1)>nr1, ..., but read
> > in a temporary buffer, take it to G space (with the old FFT grid, this
> > is the hard part), pad it with zeros to the new grid size and take back
> > to real space with the fine grid.
> >
> > Not impossible, but does require some effort and some testing.
> >
>
>
> Thanks Lorenzo, much appreciated.
>
> Maybe this could be a topic for the hackaton in January - it is,
> to be honest, very easy compared to anything else done in QE, and
> would be super-super useful - a lot of calculations could start with
> lower cutoffs and especially low FFT grids, and ramp up only at
> the very end - if used well it could boost computation time significantly.
>
>
>                                 nicola
>
>
>
> --
> ----------------------------------------------------------------------
> Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL
> Director, National Centre for Competence in Research NCCR MARVEL, EPFL
> http://theossrv1.epfl.ch/Main/Contact http://nccr-marvel.ch/en/project
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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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