Approaches to calculation of exciton interaction energies for a molecular dimer

For two model systems, a dimer of N,N'-dimethyl thiacarbocyanine and a dimer of C.l. Pigment Yellow 12, we compare results of several approaches to calculation of the exciton interaction energy. The sum over Coulombic interactions between atomic transition charges is compared to the point-dipole and extended-dipole approximations and to the direct evaluation of the Coulomb interaction integral over transition charge densities, for a range of dimer configurations. Calculations are carried out at semiempirical, ab initio Hartree-Fock, and ab initio configuration interaction-singles levels. Finally, we discuss the relation of these interaction energies to those calculated using a supermolecular approach. We conclude that for the materials studied, semiempirical methods are adequate to describe the excitonic shift.