A matrix partitioning approach to the calculation of intermolecular potentials. General theory and some examples

Abstract

A general expression for the interaction energy of two molecules is obtained by using a matrix partitioning method. The wavefunction of the whole system is expanded in terms of antisymmetrized products of free-molecule functions and using a matrix perturbation scheme it is possible to describe the interaction energy in terms of free-molecule quantities (like frequency-dependent polarizabilities and density or spin density matrices) that, in principle, can be evaluated at any level of approximation. By way of example, the interaction potentials are calculated for (N 2) 2 and Ne 2 using wavefunctions of HF and TDHF form. The results are in substantial accord with those available in the literature. Application of these potentials to the calculation of macroscopic properties, however, leads to considerable errors. From the analysis of our results it appears that the dispersion energy is underestimated, probably on account of the neglect of intrasystem correlation energy in the TDHF approximation. The use of more sophisticated methods of evaluation of dynamic polarizabilities will not involve any extension of the approach presented in this work.