Molecular mechanical forces generally include dispersion interactions through the -C6/r6 term of the Lennard-Jones potential. The C6 coefficients are assigned empirically, so different force fields have widely different parameters for the same atom type. Atomic dispersion coefficients can be calculated accurately using the eXchange-hole Dipole Moment (XDM) model, providing an ab initio check of these parameters. Comparison of the XDM and force field coefficients shows that the dispersion coefficients are systematically 50% higher than the XDM values, which compensates the neglect of sizable -C8/r8 and -C10/r10 dispersion interactions. To correct this issue, we have implemented an improved non-bonded potential into OpenMM, which describes Pauli repulsion using a Buckingham-type exponential term and includes -C8/r8 and -C10/r10 dispersion terms. The dispersion coefficients are assigned from a simple molecular DFT calculation, while the repulsive parameters are fit empirically using the ForceBalance automated parameterization code. We have successfully parameterized a new water model using this procedure, which predicts that -C8/r8 and -C10/r10 dispersion is a significant component of the liquid cohesive energy. These terms also account for roughly 30% of the solute-solvent attraction of aqueous methane. Using this procedure, we are developing a new force field for biophysical simulations that describes dispersion with systematic, QM-derived parameters and the correct distance dependence.

Molecular mechanical force fields with higher-order dispersion terms using the exchange-hole dipole moment model

256th National Meeting (2018)

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