Friday, September 30, 2011

Why you might worry about classical models of water

A nagging question for the whole field of (classical) molecular dynamics simulations of biomolecules is whether they can have an adequate description of water. This is particularly important because almost all biomolecular processes involve subtle interactions of the biomolecule of interest with its aqueous environment.

I learnt a lot from reading the article On the origin of the redshift of the OH stretch in Ice Ih: evidence from the momentum distribution of the protons and the infrared spectral density, by C. J. Burnham, G. F. Reiter, J. Mayers, T. Abdul-Redah, H. Reichert and H. Dosch.

They highlight several problems and state:
Clearly there is something missing from water models. All of the above difficulties have been consistently tackled by experimentalists, but they remain either unrecognized or unacknowledged by much of the simulation community.
Here are the 4 main difficulties they list concerning the differences between the properties of a water monomer and the water molecule in ice Ih [most common phase of ice]:

1. the magnitude of the measured anharmonicity parameter of the OH stretch X_OH (=difference between 1/2 of the second overtone frequency and the fundamental) of the OH stretch in the condensed phase is increased from 87 cm-1 in the gas-phase to 134 cm-1 in ice Ih. This behavior cannot be reproduced by a simple anharmonic oscillator (such as a Morse oscillator), for which elongation of the OH stretch by the ice H-bond results in a decrease in |XOH|.

2. the enormous observed increase (25 fold) of the integrated IR intensity in the OH stretch mode in ice compared to that of the gas-phase monomer. Most water models (even including polarizable ones) predict almost no increase at all.

3. The gas-phase molecular dipole moment derivative with respect to the OH stretch is observed to be in a direction some 25 degrees outside of the OH stretch vector. In contrast, it is observed that this derivative becomes nearly parallel to the OH vector in ice.

4. The HOH angle increases from the gas-phase value of 104.5 to 107 degrees in ice. This is in contrast to virtually all empirical water models, which predict a lowering of the HOH angle from the gas-phase value.

The authors propose their own (classical) solution to these problems.
I am not in a position to judge the validity or reasonableness of the solution.
However, I have a prejudice that ultimately the origins of these problems is that the H-bond has a significant covalent and quantum character.

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