Models of Water Derivation and Description

Molecular level computer simulations based on molecular dynamics and Monte Carlo methods have become widely used techniques in the study and modeling of aqueous systems. These simulations of water involve a few hundred to a few thousand water molecules at liquid density. Because one can form statistical mechanical averages with arbitrary precision from the generated coordinates, it is possible to calculate an exact answer. The value of a given simulation depends on the potential functions contained in the Hamiltonian for the model. The potential describing the interaction between water molecules is thus an essential component of all molecular level models of aqueous systems. 

Reviews in Computational Chemistry, Volume 13 Kenny B. Lipkowitz and Donald B. Boyd, Editors Wiley-VCH, John Wiley and Sons, Inc., New York, 1999 

This chapter, which examines the development of water models based on potential functions, describes the physical features of water models used in molecular simulations of water and aqueous systems and delineates the common, often unstated, approximations. The widespread and growing use of simulations in many different areas of research makes it important that the user understand the potentials because they contain basic information about the simulations. Since a discussion of applications of these models to a wide variety of aqueous systems would constitute a whole chapter by itself, we concentrate here on the assumptions used to construct a given model and on how well the model predicts/extrapolates key properties that may or may not have been used in fixing various parameters that enter the model. 

An alternative approach was to include explicit, higher order electrostatic moments in the pairwise interactions. This approach has not been extensively developed for use in molecular simulations because of the complex set of moments needed to obtain sensible results, particularly to mimic hydrogen bonding. A notable exception is the polarizable electropole model, which relies on a central polarizability as well as higher order moments to capture the electrostatic part of the interactions. The computational effort required for a multipole-based representation of the electrostatics is much greater than is involved in the use of distributed charges to represent the electrostatic interactions. If, on the other hand, the number of partial charge sites is substantially increased, a local expansion of multipole moments can become computationally economical.  

Current research in water potentials tends to focus on incorporating explicit many-body polarization terms in the water-water energy. This avoids the pairwise additive approach, i.e., the effective media approximation inherent in pairwise additive water potentials, and allows for a better parameterization of the true water-water interaction. Two main avenues for treating polarization effects have developed in the last decade an explicit treatment of classical polarization and fluctuating charge models. The effort expended to find suitable water models will slowly pay off in an enhanced awareness of how to improve current molecular force fields for interactions of other types (e.g., between organic solutes, biomolecules, etc.). 

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Wallqvist, A. and Mountain, R.D. 1999. Molecular models of water Derivation and description. Rev. Comput. Chem. 13, 183-247. 

Anders Wallqvist and Raymond D. Mountain, Molecular Models of Water Derivation and Description. 

A more extensive review was published by Wallqvist and Mountain (1999) entitled Molecular Models of Water Derivation and Description. This is a highly technical review of the merits and achievements of the various potentials. Three quotations from this review are worthwhile noting ... 

WaUqvist A, Mountain RD Molecular models of water derivation and description. In Lipkowitz KB, Boyd DB, editors Reviews in computational chemistry, vol. 13,... 

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