The roles of electronic exchange and correlation in charge-transfer-to-solvent dynamics: Many-electron nonadiabatic mixed quantum/classical simulations of photoexcited sodium anions in the condensed phase
Authors: Glover William J. , Larsen Ross E. , Schwartz Benjamin J. 
Published: October 2008

The charge-transfer-to-solvent (CTTS) reactions of solvated atomic anions serve as ideal models for studying the dynamics of electron transfer: The fact that atomic anions have no internal degrees of freedom provides one of the most direct routes to understanding how the motions of solvent molecules influence charge transfer, and the relative simplicity of atomic electronic structure allows for direct contact between theory and experiment. To date, molecular dynamics simulations of the CTTS process have relied on a single-electron description of the atomic anion—only the electron involved in the charge transfer has been treated quantum mechanically, and the electronic structure of the atomic solute has been treated via pseudopotentials. In this paper, we examine the severity of approximating the electronic structure of CTTS anions with a one-electron model and address the role of electronic exchange and correlation in both CTTS electronic structure and dynamics. To do this, we perform many-electron mixed quantum/classical molecular dynamics simulations of the ground- and excited-state properties of the aqueous sodium anion (sodide). We treat both of the sodide valence electrons quantum mechanically and solve the Schrödinger equation using configuration interaction with singles and doubles (CISD), which provides an exact solution for two electrons. We find that our multielectron simulations give excellent general agreement with experimental results on the CTTS spectroscopy and dynamics of sodide in related solvents. We also compare the results of our multielectron simulations to those from one-electron simulations on the same system [C. J. Smallwood et al., J. Chem. Phys. 119, 11263 (2003)] and find substantial differences in the equilibrium CTTS properties and the nonadiabatic relaxation dynamics of one- and two-electron aqueous sodide. For example, the one-electron model substantially underpredicts the size of sodide, which in turn results in a dramatically different solvation structure around the ion. The one-electron model also misses the existence of an entire manifold of bound CTTS excited states and predicts an absorption spectrum that is blueshifted from that in the two-electron model by over 2 eV. Even the use of a quantum mechanics/molecular mechanics (QM/MM)-like approach, where we calculated the electronic structure with our CISD method using solvent configurations generated from the one-electron simulations, still produced an absorption spectrum that was shifted ∼1 eV to the blue. In addition, we find that the two-electron model sodide anion is very polarizable: The instantaneous dipole induced by local fluctuating electric fields in the solvent reaches values over 14 D. This large polarizability is driven by an unusual solvation motif in which the solvent pushes the valence electron density far enough to expose the sodium cation core, a situation that cannot be captured by one-electron models that employ a neutral atomic core. Following excitation to one of the bound CTTS excited states, we find that one of the two sodide valence electrons is detached, forming a sodium atom:solvated electron contact pair. Surprisingly, the CTTS relaxation dynamics are qualitatively similar in both the one- and two-electron simulations, a result we attribute to the fact that the one-electron model does correctly describe the symmetry of the important CTTS excited states. The excited-state lifetime of the one-electron model, however, is over three times longer than that in the two-electron model, and the detachment dynamics in the two-electron model is correlated with the presence of solvent molecules that directly solvate the cationic atomic core. Thus, our results make it clear that a proper treatment of anion electron structure that accounts for electronic exchange and correlation is crucial to understanding CTTS electronic structure and dynamics.