The physical effects required for the correct description of the ionic V state of the ethene molecule are discussed in the frame ab initio quantum chemistry. The importance of the dynamic sigma polarization (absent in methods where the sigma skeleton is treated at a mean-field level) has been recognized by many authors in the past. A new physical effect is here described, i. e. the spatial contraction of the π and π* molecular orbitals (or of atomic p orbitals) originated from the reduction of the ionicity due to the dynamic sigma polarization. Such an effect is a second order effect (it appears only as a consequence of the dynamic sigma polarization) but it cannot be ignored. Many of the difficulties found in the past in the calculation of the vertical excitation energy of this state are attributed to an incomplete description of this contraction, while the few successes have been obtained when it has been fortuitously introduced by ad hoc procedures or when it is described in a brute force approach. Various strategies are proposed to allow for the spatial contraction of the p atomic orbitals. If this effect is considered at the orbital optimization step, it is shown that no Rydberg/valence mixing occurs and a simple perturbation correction (to the second order in the energy) on the π → π* singly excited configuration gives stable results with respect to the computational parameters and in a good agreement with the experimental findings and with the best theoretical calculations. Moreover, our results confirm the indication of Müller, Dallos and Lischka that the transition to the V state of ethene conforms to the Franck-Condon principle and that it is not necessary to appeal to a non vertical transition to interpret the experimental data. The strategy reported for ethene can be in principle generalized to the π → π* ionic excited states of other molecules.
File in questo prodotto:
Non ci sono file associati a questo prodotto.