Modeling structure and dynamics of solvated molecular ions: Photodissociation and recombination in I2−(CO2)n Original Research Article

We describe a method for simulating the reaction dynamics of a molecular ion embedded in a cluster of polarizable solvent molecules. Potential energy surfaces for ground and excited states are calculated from an effective Hamiltonian that takes into account the strong perturbation of the solute electronic structure by the solvent. The parameters of the model Hamiltonian are obtained from a combination of ab initio calculations and spectroscopic data; intermolecular electrostatic and polarization interactions are treated by the distributed multipole analysis of Stone and co-workers, while short range interactions are modeled with empirical pair potentials. Analytical expressions for the derivatives of the effective Hamiltonian allow for efficient computation of forces and nonadiabatic couplings. The intramolecular degrees of freedom of the solvent molecules are held fixed during the simulations using the method of constraints, and electronic transitions are treated using Tullyʹs surface hopping algorithm. The method is applied to the photodissociation and recombination of I2−(CO2)n. Electronic relaxation in this system is found to occur on multiple time scales, ranging from 2 ps to many tens of ps. The relationship of these results with the experimental measurements of Lineberger and co-workers is discussed.