Photochemical reaction paths of cis-dienes studied with RASSCF: The changing balance between ionic and covalent excited states

The balanced description of ionic and covalent molecular excited electronic states still presents a challenge for current electronic structure methods. In this contribution, we show how the restricted active space self-consistent field (RASSCF) method can be used to address this problem, applied to two dienes in the cis conformation. As with the closely related complete active space self-consistent field (CASSCF) method, the construction of the orbital active space in the RASSCF methodology requires the a priori formulation of a physical or theoretical model of the system being studied. In this article, we discuss how the active space can be constructed in a guided and systematic way, using pairs of natural bond orbitals as correlating partner orbitals (oscillator orbitals) and semi-internal correlation. The resulting balanced description of the covalent and ionic valence excited states - with the ionic state correctly lower in energy at the Franck-Condon geometry - is suitable to study the photochemistry of these and other molecules.