Perturbative and variational calculations of charge fluctuations of an extended Hubbard model

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Abstract
A charge-transfer instability that occurs in an extended Hubbard model with strong nearest-neighbor repulsions is examined nonperturbatively using variational wave functions. We show that a single mode approximation applied to a Hartree-Fock wave function accurately reproduces previous weak-coupling results on excitonic spectra. We then apply the method to a Gutzwiller variational wave function and extract information about the dynamic charge fluctuations in the intermediate and strong-coupling regimes.