  Predicting the asymmetric structure and dynamics of solvated hydroxide and
hydronium in water has been a challenging task from ab initio molecular
dynamics (AIMD). The difficulty mainly comes from a lack of accurate and
efficient exchange-correlation functional in elucidating the amphiphilic nature
and the ubiquitous proton transfer behaviors of the two ions. By adopting the
strongly-constrained and appropriately normed (SCAN) meta-GGA functional in
AIMD simulations, we systematically examine the amphiphilic properties, the
solvation structures, the electronic structures, and the dynamic properties of
the two water ions. In particular, we compare these results to those predicted
by the PBE0-TS functional, which is an accurate yet computationally more
expensive exchange-correlation functional. We demonstrate that the
general-purpose SCAN functional provides a reliable choice in describing the
two water ions. Specifically, in the SCAN picture of water ions, the appearance
of the fourth and fifth hydrogen bonds near hydroxide stabilizes the pot-like
shape solvation structure and suppresses the structural diffusion, while the
hydronium stably donates three hydrogen bonds to its neighbors. As a result,
hydroxide prefers the stepwise proton transfer (PT) while hydronium prefers the
concerted PT behavior. Consequently, hydroxide diffuses slower than hydronium
in water, which is consistent with the experiments.
