Following the ideas behind the Feynman approach, a variational wave function is proposed for the Fröhlich model. It is shown that it provides, for any value of the electron-phonon coupling constant, an estimate of the polaron ground state energy better than the Feynman method based on path...

A q-state frustrated Potts model is introduced which generalizes the Kasteleyn-Fortuin formalism to frustrated systems. For q = 2 the Ising spin is recovered. For q = 1 it gives the frustrated percolation model, which combines frustration and connectivity features and might be relevant to...

Polaron formation is investigated in a one-dimensional chain by taking into account both the local Holstein and the non-local SSH electron-phonon interactions. The study of the adiabatic regime points out that the combined effects of the two interactions are important mainly in the weak coupling...

The percolation properties of suitably defined clusters of parallel spins have been studied for two two-dimensional models with frustration: the Ising spin glass and the fully frustrated Ising model. The critical temperatures and some exponents have been estimated by using standard Monte Carlo...