Determination of noncovalent interaction energies from electronic densities

Starting from the Hellmann-Feynman theorem, an expression is derived for the interaction energy in forming a noncovalently bound complex. It invokes only classical electrostatics, involving the charge distributions of the components as they are in the complex. We propose a method for obtaining these from the total charge distribution. Integration over the electronic densities is carried out by a numerical procedure slightly modified from that of Gavezzotti. We calculate the interaction energies for four molecular dimers at a variety of computational levels. The results are analyzed and compared to the best estimated values available in the literature.