Ab initio study of cis-butadiene valence and Rydberg states using the effective valence shell Hamiltonian method

Abstract

Low-lying σ- and π-electron vertical excitation energies of s-cis-1,3-butadiene are calculated using the ab initio effective valence shell Hamiltonian (ℋv) method. The only experimentally known vertical excitation energy is that to the 1 1B2 state at 5.49 eV, while the ℋv computation in the π-valence space yields 5.62 eV. Calculated excitation energies to various valence and Rydberg states are in good agreement with theoretical multiconfigurational single reference state second-order perturbation theory calculations by Roos and co-workers and with values from other highly correlated computations. The ℋv calculations for cis-butadiene further investigate the dependence of the computations on the nature and the choice of molecular orbitals and provide the first comprehensive study of the convergence with respect to the enlargement of the valence space for π-electron systems. The present computations also represent the first ℋv treatment of the σ→π* and π→σ* excited states in conjugated π-electron systems, along with an analysis of the required degree of σ–π correlation within the valence (or reference) space. Vertical π-and σ-ionization potentials are also produced as a byproduct of the ℋv calculations for neutral cis-butadiene, providing the first predictions of these ionization energies. The computations conclusively reconfirm the high accuracy of the ℋv method.