Studies on the Alternant Molecular Orbital Method. V. A Many-Parameter Energy Expression for States with Different Multiplicities; Application to Benzene

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Abstract
Within the framework of the refined alternant molecular orbital method, a general energy expression is derived for the lowest state of any desired multiplicity of an arbitrary system with closed‐shell structure. A numerical analysis of the lowest singlet, triplet, and quintet state of the pi‐electron sextet in benzene reveals that the many‐parameter alternant molecular orbital wavefunction does, in each case, account for better than 90% of the energy improvement obtained in a full configuration interaction treatment. A detailed comparison of the AMO and CI wavefunctions for both triplet and quintet state is presented. Both methods yield a singlet—triplet separation that is too low, which suggests a basic deficiency in the customary approach to the quantum chemistry of conjugated hydrocarbons.