MULTICONFIGURATION WAVEFUNCTIONS FOR THE $\Pi$-ELECTRON STATES OF ETHYLENE

Abstract

Ab initio calculations have been carried out on the $N(\Pi^{2})$, $T(\Pi \Pi^{*})$ and $V(\Pi \Pi^{*})$ states of ethylene in a planar nuclear configuration. A ``double zeta’’ gaussian basis, augmented by diffuse functions, was used. A wide variety of configuration interaction wavefunctions (including as many as 1605 configurations), optimized by the iterative natural orbital method, were computed. The calculations predict vertical excitation energies of $\sim 4.2$ eV for the $T(\Pi \Pi^{*})$ state and $\sim 8.1$ eV for the $V(\Pi \Pi^{*})$ state. The spatial extent of the ground and excited triplet states is that of a normal valence state with $\langle x^{2} \rangle \sim 12$ $bohr^{2} $. The excited singlet state, on the other hand, is found to be quite diffuse or Rydberg-like, $\langle x^{2} \rangle \sim 35 bohr^{2}$; electron correlation decreases $\langle x^{2} \rangle$ by 30\% from the Hartree-Fock value (52 $bohr^{2}$).