Origin of spontaneous electric dipoles in homonuclear niobium clusters

Surprisingly large spontaneous electric dipole moments recently observed in homonuclear niobium clusters below 100 K (Moro el. al. Science 300, 1265 (2003)) are explained using first-principles electronic structure calculations. The calculated moments for Nb(n) (n <= 15) closely follow the experimental data in which large dipole moments are seen for n = 11-14. We establish that the dipoles are strongly correlated with the geometrical asymmetry of the clusters. The magnitude of the dipole moment is roughly proportional to the spread in the principal moments of inertia and its direction tends to align along the axis of the largest principal moment. Charge deformation densities reveal directional, partially covalent bonds that enhance the formation of asymmetric geometries. Classical simulations of the deflection of a cluster in a molecular beam reveal that the electronic dipole may persist at higher temperatures, but is masked by the rotational dynamics of the cluster.