AB-INITIO CI CALCULATIONS ON BENZENE WITH AN EXTENDED BASIS-SET

An extended basis set of triple zeta plus polarization quality is empl oyed to carry out configuration interaction (CI) calculations of the t hree lowest singlet and triplet excited states of benzene. The CI calc ulation is carried out by taking into account single and double excita tions of pi and sigma electrons. In the CI, composite natural orbitals (CNOs), which are constructed from the natural orbitals of the ground state of ethylene, are used as virtual orbitals. The aim of using CNO s is to reduce the number of virtual orbitals to be used in constructi ng configuration-state functions, thus cutting down CI dimensions with out losing reasonable accuracy. The excitation energies resulting from the CI are in fairly good agreement with experiment. The root mean sq uare of the deviation is 0.22 eV for the six calculated energies and t he largest disagreement is 0.37 eV for the third singlet excited state . To obtain better excitation energies by an ab initio calculation, it seems likely that we need to take into account more electron correlat ion than in the present calculation. (C) 1994 John Wlley & Sons, Inc.