The electromagnetic interaction energy of a molecular aggregate consisting of pointlike molecules in the presence of an electromagnetic field is derived. The corresponding Hamiltonian consists of three parts; Ho describes the aggregate in the absence of the electromagnetic field,H 1 describes the interaction of the molecules with the external field, and H 2 corresponds to the induced interaction between the molecules. Based on this Hamiltonian we derive a self-consistent equation of motion for a quasiparticle, which we refer to as a polarized exciton. Th~ equation has the same form as the one in classical dipole theory .The polarized exciton model is based on a time-dependent perturbative treatment and corresponds to the assumption H o~ H 1 + H 2. Our model is compared to standard exciton theory , which is based on the assumption Ho~ H 2 ~ H 1 .In particular the differences and similarities are illustrated for a direct example, a finite linear chain. We advocate the use of polarized excitons to fully account for the physics in these systems.
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