[Pw_forum] Applying vdW to only part of a system

[Matthieu Fortin-Deschênes]

You might want to look at this:

https://www.mail-archive.com/pw_forum@pwscf.org/msg28550.html

Matthieu

Ian Shuttleworth <shuttleworth.ian at gmail.com> a écrit :

> 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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