[gpaw-users] vdW calculations

[Jess Wellendorff Pedersen]

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.
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.

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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