Revised charge equilibration potential for liquid alkanes.

We present a revised liquid alkane force field based on the charge equilibration formalism for incorporating electrostatic nonadditive effects arising from local polarization. The model is a revision of earlier work by Patel and Brooks, specifically addressing deficiencies in the dihedral potential, electrostatic, and Lennard-Jones (van der Waals) parameters of the force field. We discuss refinement of the alkane backbone torsion potential to match high-level ab initio relative conformational energetics for pentane, hexane, and heptane. We further discuss refinement of the electrostatic and Lennard-Jones (van der Waals) parameters to reproduce the experimental polarizability, liquid density, and vaporization enthalpy of hexane. Finally, we calculate bulk liquid properties including densities, vaporization enthalpies, self-diffusion constants, isothermal compressibilities, constant pressure heat capacities, and NMR T 1 relaxation times for a series of linear alkanes ranging from hexane to pentadecane based on the current revised model. We also compute free energies of hydration for pentane, hexane, and heptane. The revised force field offers a significantly improved overall description of these properties relative to the original parametrization. The current alkane force field represents a platform for ongoing development of a CHARMM (Chemistry at Harvard Molecular Mechanics) polarizable force field for lipids and integral membrane proteins.