Predicting hydrophobic solvation by molecular simulation: 2. New united-atom model for alkanes, alkenes, and alkynes.

Existing united-atom models for non-polar hydrocarbons lead to systematic deviations in predicted solvation free energies in hydrophobic solvents. In this article, an improved set of parameters is proposed for alkane molecules that corrects this systematic deviation and accurately predicts solvation free energies in hydrophobic media, while simultaneously providing a very good description of pure liquid densities. The model is then extended to alkenes and alkynes, again yielding very accurate predictions of solvation free energies and densities for these classes of compounds. For alkynes in particular, this work represents the first attempt at a systematic parameterization using the united-atom approach. Averaging over all 95 solute/solvent pairs tested, the mean signed deviation from experimental data is very close to zero, indicating no systematic error in the predictions. The fact that predictions are robust even for relatively large molecules suggests that the new model may be applicable to solvation of non-polar macromolecules without accumulation of errors. The root mean squared deviation of the simulations is only 0.6 kJ/mol, which is lower than the estimated uncertainty in the experimental measurements. This excellent performance constitutes a solid basis on which a more general model can be parameterized to describe solvation in both polar and non-polar environments.