[Pw_forum] Grimme C_ij for epitaxial graphene

[Martin Andersson]

Hi Matthieu,

		I was curious as to which substrates you are putting graphene on. Because if you are looking at either ionic solids or metals, the Grimme corrections are going to be a bit too strong, but it can be fixed. We have had success for calculating adsorption energies on minerals by reducing the C6 parameter for the cations in the substrate by about an order of magnitude (we decreased the parameter by 33 for Ca in calcite in (Okhrimenko et al Langmuir 29, 11062-11073,(2013)). The idea came from Ehrlich et al ChemPhysChem 12, 3414-3420,(2011). For cations in ionic solids (e.g. oxides, minerals), the polarisability is significantly smaller  because of the smaller electron cloud of a cation compared to the corresponding neutral atom.

		For metals, I had good success in Andersson MP Journal of Theoretical Chemistry 2013, Article ID 327839,(2013), where I modified Grimme corrections to make it work better for metals by using a shorter cutoff to simulate screening by the conduction electrons and switching the C6 parameters for the metal to the parameter of the noble gas in the row above.

		In either of the two cases, you only need to modify the mm_dispersion.f90 file and recompile. I have a couple of different mm_dispersion.f90 files lying around if you need one...

		Or you could of course also switch to a suitable vdw functional if you like instead.

		Cheers and good luck,

————————————————————————
Martin P. Andersson
Associate Professor
Nano-Science Center, Department of Chemistry
University of Copenhagen, Denmark

Tel: +45 3532 0280
Mobile: +46 733 893091
E-mail: ma at nano.ku.dk
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> On 12 May 2016, at 20:22, Matthieu Fortin-Deschênes <matthieu.fortin-deschenes at polymtl.ca> wrote:
> 
> Hi all,
> 
> I'm trying to calculate some basic properties of graphene on various  
> substrates using Grimme correction for VdW interactions. The Grimme  
> corrections seem to deteriorate the results (lattice parameter and  
> binding energy) of the graphene as well of the substrate compared to  
> experimental results.
> 
> I was wondering if it's possible to set C_ij=0 for i=j (no correction  
> for C-C interactions, as well as substrate-substrate interaction), but  
> non-zero C_ij for graphene-substrate interactions. As far as i know,  
> C_i are defined for each element and C_ij is calculated from  
> sqrt(C_i*C_j). I would like to set all C_ij by myself.
> 
> Thank you
> 
> Matthieu Fortin-Deschênes
> Polytechnique Montreal
> 
> 
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