A new molecular dynamics model in which the point charges on atomic sites are allowed to fluctuate in response to the environment is developed and applied to water. The idea for treating charges as variables is based on the concept of electronegativity equalization according to which: (a) The electronegativity of an atomic site is dependent on the atom's type and charge and is perturbed by the electrostatic potential it experiences from its neighbors and (b) Charge is transferred between atomic sites in such a way that electronegativities are equalized. The charges are treated as dynamical variables using an extended Lagrangian method in which the charges are given a fictitious mass, velocities and kinetic energy and then propagated according to Newtonian mechanics along with the atomic degrees of freedom. Models for water with fluctuating charges are developed using the geometries of two common fixed-charge water potentials: the simple point charge (SPC) and the 4-point transferable intermolecular potential (TIP4P). Both fluctuating charge models give accurate predictions for gasphase and liquid state properties, including radial distribution functions, the dielectric constant, and the diffusion constant. The method does not introduce any new intermolecular interactions beyond those already present in the fixed charge models and increases the computer time by only a factor of 1.1, making this method tractable for large systems.

The five-site transferable interaction potential ͑TIP5P͒ for water ͓M. W. Mahoney and W. L. Jorgensen, J. Chem. Phys. 112, 8910 ͑2000͔͒ is most accurate at reproducing experimental data when used with a simple spherical cutoff for the long-ranged electrostatic interactions. When used with other methods for treating long-ranged interactions, the model is considerably less accurate. With small modifications, a new TIP5P-like potential can be made which is very accurate for liquid water when used with Ewald sums, a more physical and increasingly more commonly used method for treating long-ranged electrostatic interactions. The new model demonstrates a density maximum near 4°C, like the TIP5P model, and otherwise is similar to the TIP5P model for thermodynamic, dielectric, and dynamical properties of liquid water over a range of temperatures and densities. An analysis of this and other commonly used water models reveals how the quadrupole moment of a model can influence the dielectric response of liquid water.

We established the charge and structure of the oil/water interface by combining ζ-potential measurements, sum frequency scattering (SFS) and molecular dynamics simulations. The SFS experiments show that the orientation of water molecules can be followed on the oil droplet/water interface. The average water orientation on a neat oil droplet/water interface is the same as the water orientation on a negatively charged interface. pH dependent experiments show, however, that there is no sign of selective adsorption of hydroxide ions. Molecular dynamics simulations, both with and without intermolecular charge transfer, show that the balance of accepting and donating hydrogen bonds is broken in the interfacial layer, leading to surface charging. This can account for the negative surface charge that is found in experiments.

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The fluctuating charge (FQ) force field, a polarizable potential model in which point charges on atomic sites fluctuate in response to the environment, is applied to the aqueous solvation of acetamide and trans- and cis-N-methylacetamide (NMA). Two parameters are assigned to each kind of atom, corresponding to an electronegativity and a hardness. The FQ model accurately reproduces both the gas-phase and aqueous-phase charge distributions of these molecules and is therefore effective in treating the influence of functional group substitutions, conformation changes, and solvent on the charge distribution. The FQ model does this in a way that is computationally efficient by propagating the charges in time using an extended Lagrangian method. Solvation free energy calculations are done using both an explicit (FQ) solvent and a dielectric continuum solvent. Both solvent models predict a negligible free energy difference between trans- and cis-NMA, in agreement with experimental estimates. The explicit solvent calculations find that the free energy difference between NMA and acetamide is 0.5 ± 0.8 kcal/mol, in good agreement, but of opposite sign, with the measured value of −0.3 ± 0.4 kcal/mol. The FQ/dielectric continuum calculations find that this free energy difference is sensitive to the details of the solute cavity, varying from 0.4 to 1.5 kcal/mol.