An improved simple polarisable water model for use in biomolecular simulation.

The accuracy of biomolecular simulations depends to some degree on the accuracy of the water model used to solvate the biomolecules. Because many biomolecules such as proteins are electrostatically rather inhomogeneous, containing apolar, polar, and charged moieties or side chains, a water model should be able to represent the polarisation response to a local electrostatic field, while being compatible with the force field used to model the biomolecules or protein. The two polarisable water models, COS/G2 and COS/D, that are compatible with the GROMOS biomolecular force fields leave room for improvement. The COS/G2 model has a slightly too large dielectric permittivity and the COS/D model displays a much too slow dynamics. The proposed COS/D2 model has four interaction sites: only one Lennard-Jones interaction site, the oxygen atom, and three permanent charge sites, the two hydrogens, and one massless off-atom site that also serves as charge-on-spring (COS) polarisable site with a damped or sub-linear dependence of the induced dipole on the electric field strength for large values of the latter. These properties make it a cheap and yet realistic water model for biomolecular solvation.