Born-Oppenheimer dynamics using density-functional theory: Equilibrium and fragmentation of small sodium clusters

The properties of small neutral and positively charged sodium clusters and the fragmentation dynamics of Na₄⁺⁺ are investigated using a simulation technique which combines classical molecular dynamics on the electronic Born-Oppenheimer ground-state potential surface with electronic structure calculations via the local spin-density functional method. Results for the optimal energies and structures of Na_n and Na_n ⁺ (n≤4) are in quantitative agreement with previous studies and experimental data. Fission of Na₄⁺⁺ on its ground state Born-Oppenheimer potential-energy surface, following sudden ionization of selected configurations of an Na₄⁺ (or Na₄) cluster, whose vibrational energy content corresponds to 300 K, is found to occur on a picosecond time scale. The preferred fission channel is found to be Na₃⁺ + Na⁺, with an interfragment relative translational kinetic energy of ∼2 eV, and a vibrationally excited Na ₃⁺. The dynamics of the fragmentation process is analyzed.