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

[Nicola Marzari]

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



-- 
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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