An analysis of the dynamic sigma polarization in the V state of ethene

The importance of the dynamic sigma polarization (absent in methods where the skeleton is treated at a mean-field level) for the correct description of the V state of the ethene molecule has been recognized by many authors in the past. In this article, this physical effect is analyzed and it is seen as arising from the sum of two contributions: the polarization of the C-C bond and of the C-H bonds. In both cases it is described in a valence bond scheme and the types of excitations needed in a molecular orbital frame to introduce such effects are identified. The effect of the dynamic polarization on the spatial extent of the V state (<x^2>) is described. The analysis here reported has been used in a recent article (Angeli, J Comp Chem 2009, 30, 1319) for the accurate calculation of the V state and of its vertical excitation energy.