Ab initiostudy of cyclobutadiene using the effective valence shell Hamiltonian method

Abstract

The effective valence shell Hamiltonian (Hν) method is used to examine the electronic structure of cyclobutadiene. These computations are designed to help guide future experimental studies on this elusive compound, as well as aid in understanding the general features of effective Hamiltonian calculations. Calculations are performed with two qualitatively different valence spaces. The first valence space mimics the valence spaces used in semiempirical methods in that only four π valence-like molecular orbitals are used. The second valence space includes four additional diffuse π molecular orbitals. A [4s5p1d/2s1p] Cartesian Gaussian basis set is used (116 total functions). Our results agree with available spectroscopic vertical ionization potentials to within 0.1 and 0.3 eV for the b2g and b1u orbital ionizations. The calculated vertical excitation energies and ground state automerization barrier height agree well with previous ab initio calculations. For the first time, however, the lowest lying optical transition from the X 1Ag ground state is predicted to occur at 5.99 eV to a Rydberg 1B3u state.