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 center dot center dot center dot H) between the two neighbouring hydrogen atoms of the two water molecules should also be taken as a criterion, and the distance r(O center dot center dot center dot 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 center dot center dot center dot H) and r(O center dot center dot center dot H) are small (e. g., d(H center dot center dot center dot H) < 2.0 angstrom and r(O center dot center dot center dot H) < 1.62 angstrom), 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 center dot center dot center dot H) criterion and the restriction of r(O center dot center dot center dot 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. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4705371]