The results of MD simulations of liquid water with QMPFF2 demonstrate very good agreement with experimental results on the structure of liquid water and a wide set of basic thermodynamic properties, with the use of accurate quantum path integral techniques generally needed to best realize this agreement. This conclusion indicates that careful, physically grounded simulation of intermolecular interaction in vacuum, taking into account its main features and avoiding oversimplification, does allow accurate simulation of the properties of the condensed phase. The approach is further supported by successful results of MD simulations with QMPFF2 of the solvation energies of small organic molecules in water (unpublished data). The results are especially promising because further progress is possible and its direction is clear: extremely accurate ab initio calculations can be performed with advanced methods, e.g., ref. 23, followed by further refinement of the functional forms and parameterization of the QMPFF interaction potentials to track the ab initio data.
In this connection, the alternative fully QM parameter free approach based on Car-Parinello molecular dynamics (24) should be mentioned. However, these calculations are too computationally expensive so presently they are applicable only to short-time simulations of simplest molecular systems (25–27) that essentially restricts the performance capabilities of the method in comparison with the force field approach, in particular, preventing an adequate treatment of boundary effects and therefore of the bulk properties. The relation between the two approaches will change in the future with the growth of computer speed and the sizes of simulated systems providing a basis for joint QM/molecular mechanical methodologies. Probably the most exciting feature of our results is that the model is based strictly on ab initio-calculated QM data, unlike empirical water potentials, which are fitted to experimental data and therefore cannot be said to be predicting the properties of water. Our results show that the state of the art has now advanced to where even subtle properties of water such as the anomalous density/temperature dependence can be predicted solely from the basic principles of quantum physics. Following this approach, a complete, accurate theoretical treatment of water may now be in sight, including solution of the water structure problem recently highlighted as one of the 125 outstanding problems of science (28).