An Anisotropic Polarizable Water Model: Incorporation of All-Atom Polarizabilities into Molecular Mechanics Force Fields

Abstract

An anisotropic polarizable water model has been constructed using atomic polarizabilities of oxygen and hydrogen and the SPC molecular mechanics force field. The presence of polarizable interaction sites on all atoms allows treatment of dipole-dipole interactions between bonded atoms and simplifies the incorporation of polarization treatments into existing molecular mechanics force fields. The resulting many-body potential yields the correct dipole moments for the water monomer and dimer as well as molecules in the bulk. Furthermore, the model predicts structures, energies, and dynamical behavior for bulk water which are in good agreement with experimental findings. Several potential energy functions for water are currently used in computer simulations of pure water and water-solute systems. Most simulations use potentials that are pairwise additive.' Calculations with many-body potentia1s2-l6 are not common partly due to the substantial increase in computational requirements demanded by such potentials. These potentials often express the many-body component of the potential energy function in terms of atomic and molecular polarizabilities. The development of such functions and their use in simulations requires development of new molecular mechanics force fieldsz4 or reparametrization of terms in the existing force Other approaches to polarization use either additional variable point chargesg-12 or multipole expansions of the electrostatic intera~tion.~~J~ Additional many-body potentials will undoubtedly be devised as ever increasing computational ~apabilitiesl~-~ ~ encourage their use. There is a need for an all-atom, many-body potential energy function which combines polarization effects within a framework compatible with existing molecular mechanics force fields. First, many-body polarization effects are presumed to be important for describing the energetics of interfacial phenomena,20J some types of ionic2q22-26 and hydrophobic solvati~n,~~-~~ and cluster chemistry32-35 to name a few applications, although it is not yet well established for which systems polarization must be included e~plicitly.29~~~ Second, by retaining other (nonelectrostatic) components of standard molecular force fields in the polarizable potential, it may be possible to build upon the large effort that has gone into the construction of existing molecular mechanics potential functions. With the polarizable potential it becomes possible to examine the many-body nature of the electrostatic interactions. Preservation of the nonelectrostatic pairwise terms simplifies the analysis involved when comparing predictions obtained with the polarizable potential to those calculated using pairwise potentials.