Charge instabilities and electron-phonon interaction in the Hubbard-Holstein model

We consider the Hubbard-Holstein model in the adiabatic limit to investigate the effects of electron-electron interactions on the electron-phonon coupling. To this aim we compute at any momentum and filling the static charge susceptibility of the Hubbard model within the Gutzwiller approximation, and we find that electron-electron correlations effectively screen the electron coupling to the lattice. This screening is more effective at large momenta, and as a consequence, the charge-density-wave phase due to the usual Peierls instability of the Fermi-surface momenta is replaced by a phase-separation instability when the correlations are sizable.