QUANTUM MOLECULAR-DYNAMICS OF LARGE SYSTEMS BEYOND SEPARABLE APPROXIMATION - THE CONFIGURATION-INTERACTION CLASSICAL SEPARABLE POTENTIAL METHOD

A nonseparable method for time-dependent quantum simulations of large polyatomic systems is presented and applied to the dynamics of the I2A r17 cluster, following electronic excitation of the iodine molecule. T he new method is an extension of the classical separable potential (CS P) approximation, in which the evolution of each mode is governed by a time-dependent mean potential due to the other modes and the total wa ve packet is a product of single mode wave functions. The computationa l effectiveness of the CSP approach stems from the use of classical mo lecular dynamics (MD) trajectories, carried out at the outset oi the p rocedure, for obtaining the effective single-mode potentials. The pres ent method generalizes the CSP scheme by a configuration interaction ( CI) treatment, in which the total wave packet is represented as a line ar combination of separable terms? with coefficients determined from t he time-dependent Schrodinger equation. The single mode wave functions for each configuration are propagated along effective potentials that are generated using individual classical trajectories. The classical; MD simulation is also used for simplifying the dynamical equations for the CI coefficients. Thus. the selection of correlations that are inc luded quantum mechanically is guided by classical mechanics, which is the basis for the computational efficiency of this approach. The CI wa ve packet for the I2Ar17 system with 51 vibrational degrees of freedom was propagated for 500 fs following I-2 (B<--X) excitation. About 150 0 configurations proved sufficient for convergence of the CI series. T he separable approximation to the wave function holds for 60 fs and be gins to break down upon the first collision of the iodine atoms with a rgons. After the second iodine-argon collision this breakdown is almos t complete, and at t = 500 fs the CSP term represents less than 5% of the correlated wave packet. Both absorption and resonance Raman spectr a are, however, well described by the separable CSP method, since they are determined within the first 60 fs. The CI-CSP method offers very good accuracy due to inclusion of important correlation effects betwee n different modes, while remaining computationally feasible for system s up to 100 degrees of freedom and more. (C) 1997 American Institute o f Physics.