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

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 $bohr^{2}$).