Water model
The electrostatic interaction is modeled using Coulomb's law, and the dispersion and repulsion forces using the Lennard-Jones potential.
The exact geometric parameters (the OH distance and the HOH angle) vary depending on the model.
Since 3-site models achieve a high computational efficiency, these are widely used for many applications of molecular dynamics simulations.
The SPC/E model adds an average polarization correction to the potential energy function: where μ is the electric dipole moment of the effectively polarized water molecule (2.35 D for the SPC/E model), μ0 is the dipole moment of an isolated water molecule (1.85 D from experiment), and αi is an isotropic polarizability constant, with a value of 1.608×10−40 F·m2.
The TIP3P model implemented in the CHARMM force field is a slightly modified version of the original.
In the model of Toukan and Rahman, the O–H stretching is made anharmonic, and thus the dynamical behavior is well described.
However, the BF model doesn't reproduce well the bulk properties of water, such as density and heat of vaporization, and is thus of historical interest only.
The TIP4P model, first published in 1983, is widely implemented in computational chemistry software packages and often used for the simulation of biomolecular systems.
There have been subsequent reparameterizations of the TIP4P model for specific uses: the TIP4P-Ew model, for use with Ewald summation methods; the TIP4P/Ice, for simulation of solid water ice; TIP4P/2005, a general parameterization for simulating the entire phase diagram of condensed water; and TIP4PQ/2005, a similar model but designed to accurately describe the properties of solid and liquid water when quantum effects are included in the simulation.
One exception is the OPC model, in which no geometry constraints are imposed other than the fundamental C2v molecular symmetry of the water molecule.
OPC reproduces a comprehensive set of bulk properties more accurately than several of the commonly used rigid n-site water models.
The CPU time is approximately proportional to the number of interatomic distances that need to be computed.
