[gpaw-users] vdW calculations

[Jens Jørgen Mortensen]

On Fri, 2010-03-26 at 13:41 +0200, Janne Blomqvist wrote:
> Jess Wellendorff Pedersen wrote:
> > Dear Jussi,
> > 
> > You are right that the vdW-DF functional uses revPBE setups and these 
> > must therefore be generated manually.
> > 
> > A general status of the FFT implementation of the vdW-DF in GPAW:
> > 1) Forces work, and we expect them to work well.
> > 2) The spatial integral over all electron densities that vdW-interact 
> > with each other is handled by an FFT, which is why the vdW-DF takes more 
> > memory than usual GGA calculations. This is highly dependent on system 
> > size: The extra memory used grows rapidly as one approaches large unit 
> > cells, say 20x20x20 Å. Also, periodic boundary conditions takes less 
> > memory than non-periodic ones.
> > 3) Apart from the memory issue, the functional is reasonably ready-to-go 
> > as is, and as a starting point one can use it without giving any 
> > arguments for the functional. I.e. just do calc=GPAW(..., 
> > xc='vdW-DF',.....). It works in parallel with up to 20 processors. When 
> > in parallel, each processor handles one of the 20 cubic interpolation 
> > splines, so if only 16 processors are used, 12 processors must wait for 
> > 4 processors to handle the remaining 4 splines.
> 
> In my experience, the memory consumption is a big problem in practice. 
> On a big machine with only 1 GB per core (louhi, should be familiar to 
> Jussi!), I have used N*160 cores. As there are 8 cores per node, with 
> 160 cores there is then only 1 vdw job per node, and the vdw job can use 
> all the remaining memory on a node. Sometimes, even that is not enough.
> 
> On way to slightly reduce memory consumption is to use real->complex 
> FFT's instead of complex-complex. Jens-Jörgen posted a patch to trac, 
> and I have a half-done effort at this lying around in my home dir, but 
> AFAIK nothing has been committed to trunk yet.

My patch seemed to work, but I didn't have time to polish and test it
carefully.  If someone could finish that work, that would be very
welcome!

JJ

> I suppose another step would be to domain decompose the arrays, and 
> implement a MPI distributed FFT, but that's a lot of work.
> 
> Performance-wise, I'm happy with the current FFT implementation. 
> Compared to the GGA calculation, the calculation time for the vdw 
> correction seems to be quite insignificant, at least for decent sized 
> systems (I don't actually have numbers, just my seat-of-the-pants feeling).
> 
> > 4) A word of caution, however: The vdW-DF implementation is still not as 
> > robust and ready-to-go as GGAs. One recently realized issue concerns the 
> > value of cutoff-radius used for density-density interactions (rcut in 
> > the code). Default is 125 Bohr, but for some reason this large cut-off 
> > sometimes leads to diverging interaction energies when increasing the 
> > unit cell size. Most often so for small systems such as molecules, rare 
> > gas dimers etc. The reason for  this is still unresolved. For metallic 
> > adsorption systems, however, rcut=125 Bohr is well suited.
> 
> One thing I have noticed is that the effective potential seems a bit 
> weird. With a GGA calculation, the xy-averaged potential (surface calc, 
> see 
> https://svn.fysik.dtu.dk/projects/gpaw/trunk/doc/exercises/surface/work_function.py) 
> looks like one would expect, but the same system calculated with vdw the 
> potential seems 'spiky'.
> 
> > 
> > Sincerely,
> > Jess Wellendorff
> > CAMD ph.d student
> > 
> > 
> > Jussi Enkovaara wrote:
> >> Jussi Enkovaara wrote:
> >>> Hi,
> >>> it seems that vdW functional uses revPBE functional for some parts. 
> >>> As revPBE setups are not
> >>> distributed in gpaw-setups-0.5.3574.tar.gz, these have to be 
> >>> generated manually, am I right?
> >>>
> >>> Are there some other things one has to consider when peforming 
> >>> self-consistent vdW calculations?
> >>
> >> One question already came to my mind, do forces work correctly with vdW?
> >>
> >> Best regards,
> >> Jussi
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> 
> -- 
> Janne Blomqvist
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