MONTE-CARLO AB-INITIO SIMULATION OF THE ABSORPTION-SPECTRUM OF NA4

The influence of the classical vibration of the nuclei on the visible absorption spectrum of the Na4 cluster is investigated. The classical motion on the ground state potential energy surface at a given temperature is obtained from a Metropolis Monte Carlo simulation. This dynamical process makes possible a theoretical determination of the widths of the structures in the absorption spectra. The absorption spectrum is integrated from ab initio CI calculations of excited states and oscillator strengths along the random trajectory. A first simulation was achieved at T = 150 K using an ab initio CI determination of the ground and the excited states. The efficiency of the Metropolis process is improved with the use of an approximate representation for the ground state. Simulations are performed for several temperatures T = 50,150, and 500 K. The simulated spectra are discussed in relationship with the experimental spectrum of Kappes et al.