Considerations on correlation effects in π-electron systems

Conventional models use antisymmetrized configurations of molecular orbitals (MOs) expressed as linear combinations of atomic orbitals (LCAOs) to describe correlation effects. In the free electron (FE) model and beyond the antisymmetry condition is neglected. This leads to a strong simplification and to physical transparency in describing correlation effects by electron repulsion without losing the essence, determining spectroscopic properties. Examples are studied where the correlation effects in the ground state and in the excited state are not very different (strong absorption band in dyes and polyenes) and examples where this is not the case (first absorption band in porphyrin, which is diminished in intensity to a tenth by electron correlation, transition into the 21Ag state in polyenes, which is shifted below the 11Bu state by electron correlation). The appropriate evaluation of the electron repulsion integral is crucial in quantitative approaches. The evaluation procedures in the FE approach and in conventional models are critically considered. The nearly free electron (NFE) model approach (numerical solution of 1D Schrِdinger equations for appropriate model potentials and straightforward numerical evaluation of interaction integrals) avoids difficulties in using and overstressing perturbation theory.