Computation of the short range repulsion energy from correlated monomer wavefunctions in van der Waals dimers containing He, Ne, and N2

The first order Coulomb plus exchange energy between two closed shell monomers is computed in a non-orthogonal natural orbital formalism. SD-CI monomer wavefunctions were used in order to account for the intramolecular correlation effect. Approximations to the SD-CI wavefunctions and some restraints to the dimer orbital basis are investigated for He2. These result in a method which is applicable to larger dimers. This method is applied to dimers consisting of He, Ne, and N2. It is shown that it is possible to obtain reliable potential energy surfaces from this short range contribution, combined with damped dispersion energy terms.