Intermolecular potentials from supermolecule and monomer calculations

The use of monomer properties to improve supermolecule calculations of intermolecular potentials is reviewed. For Van der Waals dimers, the MP2 supermolecule method is too inaccurate for most purposes, and the CCSD(T) supermolecule method requires too much computer time for large molecules. Using perturbation theory to analyse the MP2 supermolecule energy shows that the second-order dispersion energy is the main source of the inaccuracy. It is shown that the dispersion energy can be improved by using more accurate dispersion energy coefficients which can be obtained from monomer frequency-dependent polarizabilities. The supermolecule MP2 electrostatic and exchange-repulsion interaction energies can also be recalculated or scaled to a higher level of theory, using monomer charge densities. Applying these corrections to the MP2 supermolecule energy does not require much additional computer time, and gives potential energy surfaces with comparable accuracy to supermolecule CCSD(T) calculations. Possible extensions of the method to different supermolecule methods and to larger molecules are discussed.