Al(110) has been studied for temperatures up to 900 K via ense
mble density-functional molecular
dynamics. The strong anharmonicity displayed by this surfa
ce results in a negative coefficient of
thermal expansion, where the first interlayer distance decr
eases 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 n
ovel mechanism for the formation of
adatom-vacancy pairs, preliminary to the disordering and p
remelting transition. Such characteristic
behavior originates in the free-electron-gas bonding at a l
oosely packed surface
