Structural and electronic properties of sodium microclusters (n = 2-20) at low and high temperatures: new insights from ab-initio molecular-dynamics studies

Extensive computer simulations are presented of several Na microclusters, using the Car-Parrinello method (unified d.-functional theory and mol. dynamics). Dynamical simulated annealing strategies are adopted in the search for low-energy min. of the potential energy surface. A detailed anal. of the results for both structural and electronic properties at 0-600 K is carried out, which allows 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 temp. In particular, new and unforseen structures are discovered for n = 10, 13, 18, and 20. The const. presence is emphasized 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 temp. are obsd., "rigidity" and "nonrigidity" of the individual clusters examd., 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 obsd. to increase with increasing temp., in parallel with the increase of the eccentricity of the cluster shape. In spite of the relatively high at. mobility, the results do not support a spherical liq.-droplet picture for the at. distribution. [on SciFinder (R)]