Thermal expansion in small metal clusters and its impact on the electric polarizability

The thermal expansion coefficients of Na-N clusters with 8 less than or equal to N less than or equal to 40, and Al-7, Al-13(-), and Al-14(-) clusters are obtained from ab initio Born-Oppenheimer local-density-approximation molecular dynamics. Thermal expansion of small metal clusters is considerably larger than that in the bulk and is size dependent. We demonstrate that the average static electric dipole polarizability of Na clusters depends linearly on the mean interatomic distance and only to a minor extent on the detailed ionic configuration when the overall shape of the electron density is enforced by electronic shell effects. Taking thermal expansion into account brings theoretical and experimental polarizabilities into quantitative agreement.