Quantum molecular dynamics simulations of processes in many-atom systems

A novel method is presented far time dependent quantum-mechanical simulations of systems having many degrees of freedom. The method uses classical Molecular Dynamics as a first step, from which separable effective single-mode potentials are computed for each degree of freedom. Single-mode wavepackets (''nuclear orbitals'') are calculated from the effective potential, and the full multidimensional wavefunction is then constructed from the time-dependent orbitals. In the present applications the method is not exact, but an approximation of good accuracy for large realistic systems. The method was applied to study the dynamics following the S-->P excitation of Ba atoms in Ba(Ar)(n) clusters. The P states are nearly degenerate in this system, and rye compute timescales of electronic relaxation, the electronic orbital reorientation (depolarization), and other aspects of the coupled electronic-nuclear dynamics. Results are obtained for clusters of up to n = 20, including all degrees freedom and in 3D geometry. Nonadiabatic electronic transitions in the P-state manifold are found to become significant around t greater than or equal to 1 ps. for the larger clusters (n = 20) electronic relaxation is far more efficient than evaporation of Ba atoms from the cluster. For the smaller clusters (n = 10) the opposite is true. The semiclassical surface hopping method of Tully is tested against our quantum calculations: This method is found to be semiquantitatively very satisfactory, but electronic state populations are off by a factor of similar to 2 for t = 1.5 ps. The semiclassical method yields good results for the evolution in time of the structures of the clusters, but is significantly in error for time-dependent spectroscopic properties.