Variational localized-site cluster expansions

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Abstract
The simplest variational localized-site cluster expansions for Hubbard and Pariser-Parr-Pople (PPP) models are considered and explicit matrix element formulae are developed. Numerical calculations for infinite chains and finite cycles are reported. It is found that even the simplest ansatz yields results in the atomic limit which are better than other conventional techniques. Near the band limit its accuracy decreases, as expected, but the next most simple ansatz does better.