[Pw_forum] pbe+vdW

[Евгений Пермяков]

I'm going to perform a set of calculation using PBE functional for modified
MoS2: a layered compound with layers bound by vdW forces. Since I need to
estimate interlayer distances with at least 0.5A precision, I need to
explicitly account vdW forces, since vanilla PBE grossly overestimates
equilibrium interlayer distance.

However, pw_input is very vague on specifying custom function, sending to
Modules/func.f90, and the file mentions functionals implemented and
specifies components for shorthands, but does not provide a clear example.

example jobs show setups for provided vdW functionals, but not custom ones.

However, I have a large set of previous calculations with rel-PBE used, and
I'm not excited by perspective to re-run them for comparison needs.

So, the questions are:
1) How to specify a custom functional for rel-PBE pseudopotentials with vdW
correction added?
2) How much difference I should expect for energy of hydrogen dissociation
energy when I add vdW?
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