A THEORETICAL INVESTIGATION OF THE GROUND AND LOW-LYING EXCITED-STATES OF BUTADIENE RADICAL CATION

Results are presented from ab initio calculations on the ground and several low-lying excited states of the butadiene radical cation. In particular, we have calculated geometries for the ground and several excited states at the multiconfiguration self-consistent field level and characterized the planar stationary points. The vertical ionization potentials from the neutral molecule ground state and vertical excitation energies at the computed equilibrium geometry of the cation were also examined. A variety of methods were tested for the calculation of ionization potentials and excitation energies, including multiconfiguration self-consistent field, multireference singles and doubles configuration interaction (with and without size-consistency correction), and multireference averaged coupled pair functional theory. It is found that several of the excited states are strong mixtures of so-called Koopmans' and non-Koopmans' configurations. In most cases, good agreement is found with experiment, but the excitation energy for the second excited pi-state, a state which is multiconfigurational at zeroth order, is apparently overestimated at all levels of theory employed here.