Valence bond (VB) diagrams provide a convenient basis for correlated states in systems with one valence state per site. The exact Pariserndash;Parrndash;Pople (PPP) solution, including all configuration interactions, is presented for octatetraene (OT), a cyanine dye, and various cyclic systems. We demonstrate rapid computer generation of symmetry adapted VB basis functions, of transformation coefficients due to electron transfer, and of iterative solutions of sparse matrices for any choice of spinhyphen;independent intersite interactions. The 2thinsp;1Agstate of OT is correctly placed for standard Ohno parameters. The pgr;hyphen;electron bond orders and charges of correlated states are contrasted to Huuml;ckel and approximate PPP solutions. The length dependence of the strong 1thinsp;1Agrarr;1thinsp;1Butransition is found for even chains and rings of up toN= 8 sites. Extrapolations asNminus;1yield a limiting PPP gap around 3 eV, or about 1 eV above the absorption maximum in polyacetylene, the nominally infinite polyene. The role of correlations in narrowhyphen;band organic solids and the extension of VB methods to larger systems are discussed.
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