Comparison of different computational methods for water structure optimisation

We have compared several computational techniques with the aim to compute the radial distribution function (RDF) as a good characterization of water structure. In particular, we have used molecular mechanic (AMBER99), semi-empirical (AM1, PM3, PM6) and ab initio (DFT) technique. It has been shown that molecular mechanic gives very poor results in the case of water RDF. Ab initio techniques which are in general accepted as very exact methods, in the case of water underestimate intermolecular interaction. Unexpectedly, the semi-empirical method with PM6 parameterisation gives the best results in comparison with RDF measured by X-ray scattering experiment.