Rigid water model

Flexible force field models require 30 or more beads in Cartesian path integral descriptions to obtain converged intramolecular energies. Rigid water models require only about five beads due to the weaker intermolecular quantum effects. 

Rigid water models appear to represent real water better than flexible classical models. 

Miyamoto S, Kollman P (1992) SETTLE an analytical version of the SHAKE and RATTLE algorithms for rigid water models. J Comput Chem 13 952-962... 

In water and ice cumox corresponds to intramolecular vibrations at about 500 meV (or 4000 cm" ). Hence, we estimate that n At is about 4 femto-seconds (fs) or n = 10 for a MD simulation step of 0.4 fs for simulations of non-rigid-water models and n = 20 for the rigid-water models. These values of n would allow enough MD steps to resolve the highest frequencies required for the simulations. 

Version of the SHAKE and RATTLE Algorithm for Rigid Water Models. 

TABLE 2 Potential parameters of rigid water models. 

The second comment concerns the usage of rigid water models such as the SPC/E model [81], which by design cannot undergo ionization or react chemically. We note that actual fields considered here are much too weak to polarize significantly, let alone decompose water when any flow of electric current is prevented by proper insulation. Top end insulators like polymer and silica films with dielectric strength of up to 5 x 10 V m can provide more than adequate insulation. 

MD results for the BNS rigid water model at 641 K (Stillinger and Rahman 1972). MD results for the Carravetta-Clementi potential (Kataoka 1989). 

In Section 2.2 we present a direct comparison between the water microstructure predicted by a rigid water model and the recently published NDIS experimental data s at the same state conditions. We use this case to illustrate the power of computer simulation to reveal inconsistencies and/or unphysical results from experimental determinations. In doing so, we discuss some pertinent aspects of the microscopic water behavior, regarding how well current water models represent the actual water structure, the temperature effect on the H-bonding, and the reliable ways to assess the strength of the H-bonding. 

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