Force Field Models for the Simulation of Liquid Water

14 Force Field Models for the Simulation of Liquid Water 

Water models can be conveniently divided into three types. In the simple interaction-site models each water molecule is maintained in a rigid geometry and the interaction between molecules is described using pairwise Coulombic and Lennard-Jones expressions. Flexible models permit internal changes in conformation of the molecule. Finally, models have been developed that explicitly include the effects of polarisation and many-body effects. 

The experimentally determined dipole moment of a water molecule in the gas phase is 1.85 D The dipole moment of an individual water molecule calculated with any of these simple models is significantly higher for example, the SPC dipole moment is 2.27 D and that for TIP4P is 2.18 D. These values are much closer to the effective dipole moment of liquid water, which is approximately 2.6 D. These models are thus all effective pairwise models. The simple water models are usually parametrised by calculating various properties using molecular dynamics or Monte Carlo simulations and then modifying the 

Fig 441 The NCC water model (After Corongiu G 1992 Molecular Dynamics Simulation Jbr Liquid Water Using a Polarisable and Flexible Potential International Journal of Quantum Chemistry 42T209-1235 ) 

The functional form of the NCC model demonstrates the complexity of some empirical models (and this for a molecule that contains only three atoms ). We should also note that the development of empirical models from ab initio quantum mechanical data is an approach that is already well established and looks likely to be a method that is more widely used in the future. 

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