[Pw_forum] Applying vdW to only part of a system

[Ian Shuttleworth]

Hi all

First, thanks for all the replies.

My thoughts are that I want vdW for the adsorbate-adsorbate interaction,
the adsorbate-substrate interaction but not for the bulk
substrate-substrate interaction.

When setting up a surface simulation, you can first determine a bulk
lattice constant. Then, using this lattice constant, you define a slab
which is the truncated bulk and relax the surface layers and add the
adsorbates. So, for a material whose bulk lattice constant is reasonably
well described by non-vdW (e.g. PBEsol) you then overbind if you add vdW
to bulk component of the simulation, and contract the surface lattice
constant as a result.

I've found that if I get a +1 to +2% bulk lattice parameter overestimation
with PBE, I get an equivalent underestimation if I 'turn on' the vdW
correction. I haven't looked at any other parameters such as formation
energies yet, as I was interested to get some feedback about this lattice
constant problem first,

Does this sound reasonable? Or am I thinking about the problem incorrectly?

Again, very kindest thanks for the responses,

Ian





Hi Ian, It is possible with dispersion corrections based on the asymptotic
formula (like XDM or D2, as described by Evren). Why would you want to do
that, though? In general, dispersion improves the description of metals and
ionic solids and, one would guess, surfaces. There was a recent paper by
Perdew and Ruzsinszky on the topic, if memory serves. And I also know for a
fact that, for instance, B86bPBE-XDM reproduces alkali halide and alkaline
earth oxide cubic cell lengths with errors of 0.1 angstrom of less once
vibrational effects are taken into account.

Best,

Alberto

--
Dr. Alberto Otero de la Roza
Department of Chemistry,
University of British Columbia, Okanagan
Kelowna, British Columbia, Canada V1V 1V7

* Ian Shuttleworth <shuttleworth.... at gmail.com> [2017-09-11 16:33:30 +0100]:
> Is it possible to apply vdW forces to only a part of system?
>
> I am thinking of examples where, for example, a vdW adlayer (e.g. benzene)
> sits on a surface, and surface doesn't need vdW corrections.
>
> If I add the vdw_corr toggle (or if I set an appropriate value
> to 'input_dft') I've understood that vdW are applied to the entire system -
> if this is right, how could I make this option more selective?
>
> With thanks
>
> Ian

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