Dispersion energies of interaction between asymmetric molecules

A derivation is given of a formula that describes the contribution of electric dipole-dipole forces to the dispersion energy between two unexcited molecules possessing axial symmetry. The symmetry axes may be oriented in any way relative to one another, and the molecules need not be identical. Our result is valid for the case that each molecule belongs to a substance whose optical dispersion curve can be characterized adequately by a pair of resonance frequencies. The method developed in the present paper, forms the basis for an extension of our theory that includes the dipole-quadrupole forces, and which will be expounded in a subsequent paper. A few special orientations of the symmetry axes are treated numerically with the aid of experimental data relating to some diatomic molecules. In this way estimates are made of the sizes of the errors committed in calculations carried out on the assumption that spherical symmetry obtains.