  Al(110) has been studied for temperatures up to 900 K via ensemble
density-functional molecular dynamics. The strong anharmonicity displayed by
this surface results in a negative coefficient of thermal expansion, where the
first interlayer distance decreases with increasing temperature. Very shallow
channels of oscillation for the second-layer atoms in the direction
perpendicular to the surface support this anomalous contraction, and provide a
novel mechanism for the formation of adatom-vacancy pairs, preliminary to the
disordering and premelting transition. Such characteristic behavior originates
in the free-electron-gas bonding at a loosely packed surface.
