Molecular modeling of aqueous solutions: The effects of molecular polarization on classical forcefield development
Dyer, Peter James
The effect of polarization and its importance to the correct reproduction of the behavior of aqueous solutions is presented. First, the effect of the addition of polarization on non-polar solutes in both pure water and aqueous solutions containing electrolytes was studied. For pure water, the use of polarizable solutes leads to improved quantative agreement with experimental measurements. For aqueous solutions containing electrolytes, the inclusion of polarizability increases the solubility of the solute, but is not able to reproduce salting-out effects as a function of ion concentration. It has been found that the salting-out effects, are not due to increasing ordering of the bulk water in the system, but rather due to the use of an overly repulsive ion-solute interaction potential. A classical water model, which features Gaussian charges and polarizability (GCPM), was compared with Car-Parrinello simulations (CPMD), compares the mean and the distribution of the total dipole moment. It was found that GCPM obtained a total dipole moments bound between the values obtained using two popular methods from CPMD. The effect of using a Gaussian distribution in the modeling of the electrostatic interaction of simple ions with polarization in aqueous solutions was studied. It has been shown that kosmotropic ions undergo bulk solvation, while the chaotropic ions undergo surface solvation. Good agreement with ab initio simulations for the induced dipole moment was also obtained. GCPM still shows the same trends as other classical models for water in relation to the ion-water orientation, but it shows a reduced attachment of the surrounding water molecules, indicated by the lower residence times in the first hydration shell. We have constructed a group of classical potentials based on ab initio density functional theory calculations to describe the chemical bonding between the benzenedithiolate molecule and gold clusters. It was found that the forcefield parameters depend on cluster size and geometry, but variations of these have a minimal impact on the global packing structure of BDT self-assembled monolayer on the Au (111) surface.