The interaction of oligo(ethylene glycol) with water: Testing an atomistic force field for transferability

An atomistic force field fitted to ab initio MP2 level energies and geometries of 1,2-dimethoxyethane (EG1) and EG1-water complexes (D. Bedrov, M. Pekny and G. D. Smith, J. Phys. Chem. B, 1998, 102, 996) is tested for transferability to methoxy terminated tri(ethylene glycol) (EG3) and its complexes with water. The test is based on a comparison of the force field predictions for EG3 and EG3-water complexes with the respective results of recent ab initio calculations using density functional theory (DFT) (R. L. C. Wang, H. J. Kreuzer and M. Grunze, Phys. Chem. Chem. Phys., 2000, 2, 3613). The force field well reproduces the structure and stability sequence of EG3 and EG3-water conformers but yields noticeably higher EG3-water binding energies. Most of the difference in binding energy is associated with differences between the ab initio methods used in parametrization (MP2) and testing (DFT) of the force field. The agreement between the force field and ab initio DFT predictions can be improved by introducing an additional fitting parameter, like dielectric permittivity, which attenuates the electrostatic terms of the EG3-water potential.