Structural and electronic properties of sodium microclusters (n=2–20) at low and high temperatures: New insights from a b i n i t i o molecular dynamics studies

We present the results of extensive computer simulations of several sodium microclusters, using the Car–Parrinello method (unified density-functional theory and molecular dynamics). Dynamical simulated annealing strategies are adopted in the search for low-energy minima of the potential energy surface. A detailed analysis of the results for both structural and electronic properties at temperatures in the 0–600 K range is carried out, which allows us for the first time to gain insight into the structural ‘‘growth’’ pattern, the extent of the validity of (spherical, spheroidal, and ellipsoidal) jellium models, and the effects of temperature. In particular, new and unforeseen structures are discovered for n=10, 13, 18, and 20 and we emphasize the constant presence of arrangements with local pentagonal symmetry for the low-energy isomers as well as the similarity of the structural pattern with that of Lennard-Jones systems. Shape transformations with increasing temperature are observed, ‘‘rigidity’’ and ‘‘nonrigidity’’ of the individual clusters examined, and the presence of distinct isomers is identified for the smaller ones. Closing of electronic shells is confirmed for Na8 and Na20 and—to a minor extent only—for Na18. Hybridization of cluster states of different angular momenta, which represents a deviation from the spherical shell model, is discovered in several cases and discussed in detail, also in correspondence with the presence of anisotropy of the electronic potential. In most cases, this hybridization is observed to increase with increasing temperature, in parallel with the increase of the eccentricity of the cluster shape. In spite of the relatively high atomic mobility, our results do not support a spherical liquid-droplet picture for the atomic distribution.