Ab initioconfiguration interaction (CI) calculations, including all single and double excitations from a twohyphen;configuration reference state, have been performed for the ground state potential energy surface for the reactionNCNOrlarr2;CN+NO.A comparison is made with previous CI results obtained with single configuration RHF and UHF reference states. It is established that the reference, or zeroth order wave function, must have the correct spin symmetry properties, i.e., it must be an eigenfunction ofS2,as well as the correct behavior at the dissociation limit in order to give meaningful CI results. This conclusion should apply, equally well, for the choice of zeroth order wave function for other methods of obtaining accurate correlated molecular wave functions. Finally, the value of the dissociation energy is discussed.
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