Based on an idea that has earlier been proposed for Hartree-Fock calculations[71,72], a general empirical dispersion correction has been proposed by Stefan Grimme for density functional calculations . A modified version of the approach with extension to more elements and more functionals has been published in ref. . The most recent implementation  is less empirical, i.e. the most important parameters are computed by first principles, and it provides a consistent description across the whole periodic system.
The first version (DFT-D1) can be invoked by the keyword $olddisp in the control file. The second version (DFT-D2) is used if the keyword $disp is found. For the usage of DFT-D3 just add keyword $disp3 to the control file. Only one of the three keywords is expected to be present.
If DFT-D3 is used, the total energy is given by
|EDFT-D3 = EKS-DFT - Edisp||(6.9)|
The first sum runs over all atom pair, CnAB denotes the nth-order dispersion coefficient for atom pair AB, rAB is their interatomic distance, and fd, n is a damping function.
Please have look at
http://toc.uni-muenster.de/DFTD3 for more detailed information.