  Despite the inherently quantum mechanical nature of hydrogen bonding, it is
unclear how nuclear quantum effects (NQEs) alter the strengths of hydrogen
bonds. With this in mind, we use ab initio path integral molecular dynamics to
determine the absolute contribution of NQEs to the binding in DNA base pair
complexes, arguably the most important hydrogen-bonded systems of all. We find
that depending on the temperature, NQEs can either strengthen or weaken the
binding within the hydrogen-bonded complexes. As a somewhat counterintuitive
consequence, NQEs can have a smaller impact on hydrogen bond strengths at
cryogenic temperatures than at room temperature. We rationalize this in terms
of a competition of NQEs between low-frequency and high-frequency vibrational
modes. Extending this idea, we also propose a simple model to predict the
temperature dependence of NQEs on hydrogen bond strengths in general.
