Hello Dr. Jonas Wiebke, Thank you very much.

I am actually trying do measure the electronic, optical and magnetic properties of a 13 atom Ag structure through Accelrys Material Studio. But I am confused whether I should include TS-DFTD correction for my calculations (As far I know, TS-DFTD correction is mainly to include van der waals force calculation)

It would be nice to know your thinking regarding this issue.

Certainly, there is such a force, but of course small. Some enhancement is expected due to larger polarizability of clusters than of atoms. A charge or a dipole would induce a "mirror" charge/dipole near conductive surface. I am not expert in this particular field, but this is what my intuition says. There are also plasmon resonances in metal particles that might be related to this topic. And at large distances between clusters, Van-der-Waals would dominate.

The dispersion forces cannot be neglected when a highly polarizable atome as Ag is involved. In his PhD that deals with silver bromide clusters (http://tel.archives-ouvertes.fr/tel-00006320/ , in french), Franck Rabilloud estimate their contribution for Ag+ - Ag+ interactions by the difference between CCSD and Hartree-Fock calculations : The effective Van der Waals interaction between two Ag+ ions at a distance of 5 Bohr is estimated at 0,3 eV. Due to the well known wrong charge balance in Hartree Fock calculations, the Van der Waals contribution evaluated in this way is certainly overestimated, however this rough estimate is not in favour of getting rid of Van der Waals interaction when dealing with silver atoms.

Thank you very very much Dr. J. -M. L'Hermite.

Thank you very much for all of your comments. So, can anyone please give me any exact reference that van der waals force is involved/not involved in atomic clusters (like 13 atom silver structure or, alkali metal doped Ag cluster)?

In some papers I have seen that when it is Inorganic-Organic cluster, van der waals force is active there and they used DFTD correction in the calculation. But in case of NEUTRAL CHARGE metal-metal cluster structure (whether it is one metal or, alloy of different metals) will the van der waals force play an important part? Any specific reference for that can anyone please share?

(Thank you Dr. J. -M. L'Hermite for your reference but I can't read french! So, if you can share any English reference, it would be great for me)

Thanking you all in anticipation

The general idea here is that van der waals forces between atoms never just turn off as you always have electronic interactions as described by Jonas.

I would recommend you take a very small cluster (say 7 atoms) and run DFT with and without dispersion correction. I would be very curious to see the outcome for your properties of interest.