Exact and approximate solutions of the Peierls–Hubbard model

Abstract
The exact eigenvalues for the ground state and the excitation spectra were obtained for a system with two electrons on two sites, taking into account the transfer energy, the Coulomb repulsion on the same site, and the intersite correlation energy. The vibrational modes couple linearly to the electronic density. We discuss the interplay of these interactions resulting in different phases of distorted and undistorted eigenstates with corresponding localized and delocalized electronic wave functions and accompanying charge transfer. Several simple approximation schemes generalizable to larger systems are evaluated. The comparison with the exact results shows that their validity depends strongly on the system parameters. The according limits of applicability are discussed.

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