(222ap) Molecular Interactions and the Response to Nanoscale Broken Symmetries

The solvation of ions in water presents many conceptual and computational challenges to current models of ion and water interactions. The solvation free energy is usually described using a two step process: the formation of a cavity in water that accommodates the neutral ion core along with its associated ion-water dispersion interactions and the additional free energy resulting from charging the core to the full charge of the ion. The simplest Born model treats water outside the cavity as a continuum dielectric that responds linearly to the inserted charge, and predicts that the resulting electrostatic free energy contribution is independent of the sign of the charge. However, it has long been recognized that nonlinear electrostatic effects induced by the initial insertion of the ion core must be taken into account, in addition to other possible nonlinearities associated with the charging process. In order to understand where the various asymmetries and nonlinearities arise in the ion solvation process, we examine the structural and electrostatic consequences of various nanoscale broken symmetries that arise in ion hydration. We pay particular attention to donor-acceptor asymmetries of the hydrogen bond network of water, and the dependence of these asymmetries on the form of the molecular interaction potential by studying solvation in the SPC/E and TIP5P models of water.  The results obtained from empirical, point-charge models are then compared with data from ab initio density functional theory-based simulations.