An ab initio molecular dynamics study on hydrogen bonds between water molecules.

The quantitative estimation of the total interaction energy of a molecular system containing hydrogen bonds (H bonds) depends largely on how to identify H bonding. The conventional geometric criteria of H bonding are simple and convenient in application, but a certain amount of non-H bonding cases are also identified as H bonding. In order to investigate the wrong identification, we carry out a systematic calculation on the interaction energy of two water molecules at various orientation angles and distances using ab initio molecular dynamics method with the dispersion correction for the Becke-Lee-Yang-Parr (BLYP) functionals. It is shown that, at many orientation angles and distances, the interaction energies of the two water molecules exceed the energy criterion of the H bond, but they are still identified as H-bonded by the conventional "distance-angle" criteria. It is found that in these non-H bonding cases the wrong identification is mainly caused by short-range interaction between the two neighbouring water molecules. We thus propose that, in addition to the conventional distance and angle criteria of H bonding, the distance d(H···H) between the two neighbouring hydrogen atoms of the two water molecules should also be taken as a criterion, and the distance r(O···H) between the hydrogen atom of the H-bond donor molecule and the oxygen atom of the acceptor molecule should be restricted by a lower limit. When d(H···H) and r(O···H) are small (e.g., d(H···H) < 2.0 Å and r(O···H) < 1.62 Å), the repulsion between the two neighbouring atoms increases the total energy of the two water molecules dramatically and apparently weakens the binding of the water dimer. A statistical analysis and comparison of the numbers of the H bonds identified by using different criteria have been conducted on a Car-Parrinello ab initio molecular dynamics simulation with dispersion correction for a system of 64 water molecules at near-ambient temperature. They show that the majority of the H-bonds counted by using the conventional criteria combined with the d(H···H) criterion and the restriction of r(O···H) match what is identified by the binding energy criteria (e.g., E ≤ -10 kJ/mol), while some of them still have a binding energy that exceeds the energy criterion, indicating that the complicated quantum effects in H bonding can only be described by the three geometric parameters to a certain extent.