Ab Initio Vertical Ionization Potentials of trans-Butadiene and Cyclobutadiene Using the Effective Valence Shell Hamiltonian Method

Abstract

Moderate-size basis set ab initio calculations for the [pi]-electron vertical ionization potentials of trans- and cyclobutadiene (116 and 128 functions, respectively) are performed using the effective valence shell Hamiltonian (H[sup F]) method. These calculations demonstrate the flexibility and unique features of the H[sup F] method because the ionization potentials emerge from previous computations of the neutral valence-like and Rydberg state excitation energies using the same H[sup F]. The computed ionization potentials agree well with experiment and with previous calculations, including highly correlated ones for trans-butadiene. New predictions are provided for ionizations to quartet and shake-up states. Furthermore, in contrast to current beliefs about quasidegenerate perturbation theory, the third-order H[sup F] calculations do not encounter intruder state problems. 32 refs., 5 tabs.