CCL:G: DFT and dispersion

 Sent to CCL by: "Dr. Lars Goerigk" [lars.goerigk(~)chem.usyd.edu.au]
 Dear Arne,
M06-2X was fitted to noncovalently bound systems in their equilibrium geometries and due to that fit it somehow incorporates medium-range dispersion interactions. However, as also discussed in the literature, it is expected that the asymptotic long-range behavior is still not correct and that it can also benefit from dispersion corrections. On the issue DFT-D3 in combination with the Minnesota functionals you can refer to PCCP 2011, 13, 6670. There, the parameters for DFT-D3 for those functionals were published and discussed.
DFT-D2 parameters for the Minnesota functionals were published by the Martin group in JPCA 2008, 112, 12868. The s6 had to be smaller than unity, though, which still does not give the correct asymptotics, opposed to DFT-D3, where it was fixed to unity.
If you have the Gaussian code available you could try to implement DFT-D3 (based on the code Stefan Grimme made available on his website). If you do not have the code, you would have to use DFT-D2. Please note that in G09 Rev A, DFT-D2 only works with B97-D and omega-B97X-D. That bug was fixed however and in Rev. B01 and higher you can now also use DFT-D2 for those functionals, which are mentioned in Grimme's DFT-D2 paper from 2006 (i.e. also for B3LYP, TPSS, PBE, BP86). You have to use the IOp(3/124=3) then. In combination with IOp33(3=3) you also get a printout to check, if the correct s6 value was applied. Other functionals are not supported and I do not know if it is possible to read in an s6 specified by the user (maybe you should contact the Gaussian support to find that out). IOp(3/124=1) adds the dispersion correction to any functional, but only with an s6=1, which is too large for the Minnesota functionals.
If you can't use M06-2X, you could try TPSS in combination with D-corrections. Based on publications and my own experience, this gives very good structures.
Another possibility would be to get ORCA, which is for free. I am not 100% sure if DFT-D3 is already implemented; have a look at the ORCA website for that. 
 You could also check if other programs have already implemented DFT-D3.
 I hope these information could help you.
 With best wishes,
 Lars
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 Dr. Lars Goerigk
 School of Chemistry
 The University of Sydney
 Sydney, New South Wales 2006
 Australia