Self-consistent field molecular orbitals are used to describe two interacting conjugated molecules with overlappingporbitals. Perturbation theory is used to determine intermolecular orbitals which could serve as starting point for a self-consistent field calculation for the composite system. The total interaction energy is obtained as an expansion in powers of the overlapSrrbetween interacting atomic orbitals and of the net electronic charge Δqron the atoms. The energy expression contains an important term for net charge interactions. Expressions are also obtained when one of the interacting molecules is excited. Configuration interaction is examined in this case and the behaviour of singlets and triplets compared.