Correlation Effects on ππ* Transition Energies in the Series of Linear Polyenes. I. π Correlation Effects in the Minimal Basis Set

The effect of configuration interaction on the transition energies is developed according to the Rayleigh–Schrödinger perturbation theory. The second-order correction to the ππ* transition energies is separable into three parts: (a) a pure π effect, (b) a σπ effect—both in a fixed minimal basis set, and (c) effects which include the change and the extension of the minimal basis set (mainly dynamical correlation effects). Here the pure π effects are studied as functions of the dimension of the conjugated system. The effect of the di- and triexcited configurations is as important as the effect of the singly excited configurations. The highest singly excited configurations do not play a significant role. The di- and triexcited configurations have a rather small influence on the spacing between the two lowest singlet excited states, while they change significantly the singlet–triplet splitting. The results of various methods for the calculation of transition energies are compared for different sets of parameters. The use of CI limited to singly excited states is especially discussed.