Force Field Benchmark of Organic Liquids. 2. Gibbs Energy of Solvation.

Quantitative prediction of physical properties of liquids is a longstanding goal of molecular simulation. Here, we evaluate the predictive power of the Generalized Amber Force Field (Wang et al. J. Comput. Chem. 2004, 25, 1157-1174) for the Gibbs energy of solvation of organic molecules in organic solvents using the thermodynamics integration (TI) method. The results are compared to experimental data, to a model based on quantitative structure property relations (QSPR), and to the conductor-like screening models for realistic solvation (COSMO-RS) model. Although the TI calculations yield slightly better correlation to experimental results than the other models, in all fairness we should conclude that the difference between the models is minor since both QSPR and COSMO-RS yield a slightly lower RMSD from that of the experiment (<3.5 kJ/mol). By analyzing which molecules (either as solvents or solutes) are outliers in the TI calculations, we can pinpoint where additional parametrization efforts are needed. For the force field based TI calculations, deviations from the experiment occur in particular when compounds containing nitro or ester groups are solvated into other liquids, suggesting that the interaction between these groups and solvents may be too strong. In the COSMO-RS calculations, outliers mainly occur when compounds containing (in particular aromatic) rings are solvated despite using a ring correction term in the calculations.