The Atomic Exciton Model for the Excited States of p-Electron Systems

A new model is presented to describe the π-π* excitations of π-electron systems in terms of intra-atomic excitations. The atomic exciton model combines features of conventional exciton and valence bond theory, reducing to the former in the non-exchanging limit, and the latter in the ground-state limit with covalent structures. The model is ideally suited to the approximate or exact incorporation of exchange, and highlights the opposition of excitation and electron interchange in determining the energetics of the excitation manifold, eliciting thereby the essential difference between ground and excited states. Applications to some simple π-systems are considered, providing new insight into their excited states.