Layering at free liquid surfaces.

We show that simple liquids, with appropriate choices of the isotropic pair interaction, may exhibit surface layering above the melting temperature. Results for the liquid surface have been obtained by Monte Carlo simulations in slab geometry. Surface layering appears at temperatures below approximately one-third of the critical temperature for very different choices of pair interaction. The high melting temperature of the Lennard-Jones crystal preempts the observation of the oscillatory density at the free liquid interface, while model pair interaction potentials, built to reproduce some properties of mercury and the alkali metals, have low melting temperature, uncovering the region of surface layering.