A computational strategy for geometry optimization of ionic and covalent excited states, applied to butadiene and hexatriene

We propose a computational strategy that enables ionic and covalent pipi* excited states to be described in a balanced way. This strategy depends upon (1) the restricted active space self-consistent field method, in which the dynamic correlation between core sigma and valence pi electrons can be described by adding single sigma excitations to all pi configurations and (2) the use of a new conventional one-electron basis set specifically designed for the description of valence ionic states. Together, these provide excitation energies comparable with more accurate and expensive ab initio methods--e.g., multiconfigurational second-order perturbation theory and multireference configuration interaction. Moreover, our strategy also allows full optimization of excited-state geometries--including conical intersections between ionic and covalent excited states--to be routinely carried out, thanks to the availability of analytical energy gradients. The prototype systems studied are the cis and trans isomers of butadiene and hexatriene, for which the ground 1A(1/g), lower-lying dark (i.e., symmetry forbidden covalent) 2A(1/g) and spectroscopic 1B(2/u) (valence ionic) states were investigated.