  It is established that liquid metals exhibit surface layering at the
liquid-vapor interface, while dielectric simple systems, like those interacting
through Lennard-Jones potentials, show a monotonic decay from the liquid
density to that of the vapor. First principles molecular dynamics simulations
of the liquid-vapor interface of several liquid metals (Li, Na, K, Rb, Cs, Mg,
Ba, Al, Tl and Si), and the Na$_3$K$_7$ alloy near their triple points have
been performed in order to study the atomic motion at the interface, mainly at
the outer layer. Comparison with results of classical molecular dynamics
simulations of a Lennard-Jones system shows interesting differences and
similarities. The probability distribution function of the time of residence in
a layer shows a peak at very short times and a long lasting tail. The mean
residence time in a layer increases when approaching the interfacial region,
slightly in the Lennard-Jones system, but strongly in the metallic systems. The
motion within the layers, parallel to the interface, can be described as
diffusion enhanced (strongly, in the case of the outermost layer) with respect
to the bulk, for both types of systems, despite its reduced dimensionality in
metals.
