This paper describes an extension of Ziegler's transition state formalism for the calculation of interaction energies in order to include the pseudopotential Hartree-Fock-Slater-LCAO method developed by Snijders and Baerends. Perturbation corrections to the expressions based on averaged pseudopotentials are obtained within a new, self-consistent scheme. Test calculations on a variety of systems are reported. It is found that the pseudopotential method reproduces results obtained with the frozen-core HFS-LCAO method quite well. For first and second row diatomics equilibrium distances agree within 0.02 Å, dissociation energies within 0.2 eV, and vibration frequencies within 20 cm-1. The spectroscopic constants are also in fair agreement with experiment. For systems containing the transition metal Cu, where the binding energy curves have rather shallow wells, dissociation energies are equally accurate, and deviations in equilibrium distances and vibration frequencies are larger.