  Experiments on quasi-one-dimensional systems such as quantum wires and
metallic chains on surfaces suggest the existence of electron-electron
interactions of substantial range and hence physics beyond the Hubbard model.
We therefore investigate one-dimensional, quarter-filled chains with a Coulomb
potential with variable screening length by quantum Monte Carlo methods and
exact diagonalization. The Luttinger liquid interaction parameter K_rho
decreases with increasing interaction strength and range. Experimentally
observed values close to 1/4 require strong interactions and/or large screening
lengths. As predicted by bosonization, we find a metal-insulator transition at
K_rho=1/4. Upon increasing the screening length, the charge and spin
correlation functions reveal the crossover from dominant 2k_F spin correlations
to dominant 4k_F charge correlations, and a strong enhancement of the charge
velocity. In the metallic phase, the signatures of spin-charge separation in
the single-particle spectrum, spinon and holon bands, remain robust even for
rather long-ranged interactions. The charge-density-wave state exhibits
backfolded shadow bands.
