Nonorthonormal CI for molecular excited states. I. The sudden polarization effect in 90° twisted ethylene

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Abstract
The accurate and efficient calculation of properties of excited states, especially those involved in near-degeneracies and valence-Rydberg mixings, may depend crucially on the zeroth order orbitals employed for the description of a few important configurations. When self-consistent field orbitals specific for the states of interest are employed, one becomes involved with nonorthonormal basis sets, which circumstance has conceptual as well as computational implications. In this work, we have developed a nonorthonormal configuration interaction (NONCI) method and have applied it to the calculation of the ‘‘sudden polarization’’ effect in the zwitterionic excited states of ethylene, in the spirit of a state-specific theory. A very small NONCI, (6×6), yields similar results to those from previous large CI calculations. In particular, the sudden rise of the dipole moment as a function of the pyramidalization angle of the CH2 group of the 90° twisted Z state is predicted while it is shown that the inclusion of NON at 0° angle (D2d symmetry) is sufficient to recover the correct electronic charge distribution (zero dipole moment).