London dispersion forces by range separated hybrid density functional with second order perturbational corrections (RSH+MP2): the case of rare gas complexes

A satisfactory account of the van der Waals (London dispersion) forces is, in general, not possible by the Kohn-Sham method using standard local, semi-local GGA or meta-GGA density functionals. The recently proposed range-separated hybrid (RSH) approach, supplemented by second order perturbational corrections (MP2) to include long range dynamic correlation effects offers a physically consistent, seamless description of dispersion forces. It is based on a rigorous generalization of the Kohn-Sham method, where long-range exchange and correlation effects are treated by wave function methods, while short-range electron exchange and correlation are handled by local or semi-local functionals. The method is tested on a series of rare gas dimers in comparison with standard WFT and DFT approaches. In contrast to the most successful xc-functionals, which describe at best the vdW minimum, the RSH+MP2 approach is valid also in the asymptotic region and the potential curve displays the correct 1/R6 behaviour at large internuclear separations. In contrast to usual MP2 calculations, the basis set superposition error is considerably reduced, making RSH+MP2 an ideal tool for exploring the potential energy surface of weakly bound molecular complexes.