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This paper presents an extension of the Hubbard Model to Pariser-Parr-Pople form. Althoughthe Hubbard model contains most of the essentials of chemical bonding, it is unable to describe excitedstates with separated charges, such as the lowest 1Bu states of linear polyenes. The PPP model adds longrangeelectron-electron repulsions to the Hubbard model to remedy this defect. If the long range repulsionintegrals are assumed to follow a standard form, all parameters in the model can be evaluated exactly fromhigh accuracy ab initio computations on stretched ethlyene. This yields a model based on the Mataga-Nishimoto form for the long-range integrals which gives excellent agreement with both excitation energiesand ground-state bond lengths, but with a significantly smaller value of the one center electron repulsionU than is usually assumed. A major conclusion of this work is that the exact form of the long-rangeintegrals is not so important, but that the value of the one center integral U must be chosen smaller thantraditional values. The PPP-MN model is recommended for applications because it contains no adjustableparameters, with all parameter values determined directly from ab initio results. (doi: 10.5562/cca2297

From the Hubbard to the PPP Model