Energy transfer in molecular crystals: Nondipolar Coulomb exciton transfer interactions in crystalline naphthalene and anthracene

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Abstract
Several approximations to the nondipolar Coulomb exciton transfer interaction are compared using crystalline naphthalene and anthracene as examples. It is found that nondipolar interactions calculated with the atomic multipole model are a good approximation to nondipolar interactions calculated with the exact two-center integrals. In contrast, the atomic monopole and molecular multipole models give poor results. The scaling of the calculated nondipolar interactions with respect to the point dipole–dipole interaction is investigated and it is found that, with the proper choice of scaling factor, the nondipolar interactions are largely independent of the wave function used to generate them. Because the nondipolar interactions decrease very rapidly with increasing intermolecular distance, the sum of the nearest-neighbor interactions is a good approximation to the total interaction. Inclusion of nondipolar interactions in calculations of the exciton energies, oscillator strengths, and factor group splittings of the crystals leads to substantially better agreement of theory and experiment.