Theory of excitonic couplings in dielectric media : foundation of Poisson-TrEsp method and application to photosystem I trimers.

The Poisson-TrEsp method (where TrEsp stands for transition charges from electrostatic potentials) has been successfully applied to calculate excitonic couplings in a variety of pigment-protein complexes. It relies on an isomorphism that allows for relating the excitonic coupling between transition densities in dielectric media to their Coulomb coupling. This isomorphism was derived by Hsu et al. (J. Chem. Phys. 114, 3065, (2001)) using time-dependent density functional response theory. In this article, we provide an alternative and simple derivation by first-order perturbation theory. An application of Poisson-TrEsp to photosystem I trimers reveals that the local field correction/screening factor depends on the mutual orientation of the pigments and on the dielectric boundaries rather than on distance. A mean correction factor of f = 0.69 is determined for this system.