Opposing Electronic and Nuclear Quantum Effects on Hydrogen Bonds in H₂ O and D₂ O

The effect of extending the O-H bond length(s) in water on the hydrogen-bonding strength has been investigated using static ab initio molecular orbital calculations. The "polar flattening" effect that causes a slight σ-hole to form on hydrogen atoms is strengthened when the bond is stretched, so that the σ-hole becomes more positive and hydrogen bonding stronger. In opposition to this electronic effect, path-integral ab initio molecular-dynamics simulations show that the nuclear quantum effect weakens the hydrogen bond in the water dimer. Thus, static electronic effects strengthen the hydrogen bond in H₂ O relative to D₂ O, whereas nuclear quantum effects weaken it. These quantum fluctuations are stronger for the water dimer than in bulk water.