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

[Nicola Marzari]

On 29/09/2016 22:45, Paolo Giannozzi wrote:
> 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


Ciao Paolo,


one of the few times I beg to disagree, but mind you that I was thinking
at a case where you do a lot of ionic steps to relax a geometry (a 
fairly common
case in large systems where you eg put a flopyy molecule on a surface or a
nanostructure). You could easily have ~10-20 ionic
iterations, i.e. 50-100 electronic loops. Many pseudopotentials
do a decent jobs at lower cutoffs (and especially at low fft cutoffs,
e.g. a dual of 2-3 for norm-conserving, and a dual of 4-6 for ultrasoft),
so a lot of calculations could be done rapidly initially, with some final
refinement. (to be honest, I used to do this all the time 25 years ago or
so, with castep)

We'd be very happy to test this before Jan on 1-2 systems, if you need
more justification.

Of course one would need to do the fft interpolation - i.e. go through
the charge density in g-space, but that can be done independently from
the way the charge density is saved (there is an argument to have it in real
space to facilitate post-processing. Of course, adding it in reciprocal
sapce would only add a minor cost in space, and so maybe one should save 
both?)

The more we look at the performance of QE and the more we realize
that "other codes" are faster because the pseudopotentials are significantly
softer, and the parameters quite loose. We could easily explore an approach
in which in a relaxation we start loose and we perfect only at the very end.

Of course, this is most relevant for those calculations where there are 
a lot
of ionic steps to get to the minimum.

				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