Langevin Dipoles Model for ab Initio Calculations of Chemical Processes in Solution:  Parametrization and Application to Hydration Free Energies of Neutral and Ionic Solutes and Conformational Analysis in Aqueous Solution

A new parametrization of the Langevin dipole (LD) model is developed for ab initio calculations of chemical processes in aqueous solution. This parametrization is implemented in both the iterative (ILD) and noniterative (NLD) versions of the LD model. The training set for the new parametrization encompassed solvation free energies of 44 neutral and 39 ionic solutes that contained the C, O, N, P, S, F, and Cl atoms. The performance of the model is assessed by examining its ability to represent the overall training set, pKa differences of structurally related compounds, and conformation-related changes in the solvation energy of 1,2-ethanediol (glycol). The effects of solute polarization and electron correlation are also discussed. The overall performance of the model is found to be comparable or slightly better than the PCM continuum model of Tomasi and co-workers. However, the simplified explicit representation of solvent molecules of the LD model may allow one to gain a somewhat clearer insight into the molecular origin of different solvent effects than that obtained by continuum models. The present version of the model can be used as a stand alone program with the standard output from the Gaussian or related programs.