Opposing Electronic and Nuclear Quantum Effects on Hydrogen Bonds in H(2)O and D(2)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 H(2)O relative to D(2)O, whereas nuclear quantum effects weaken it. These quantum fluctuations are stronger for the water dimer than in bulk water.