On page 2140 (DOI: 10.1002/jcc.24438) Y. Kanematsu, Y. Takano, and M. Tachikawa discuss the H/D isotope effect on a hydrogen bond between Yz and His190 of Photosystem II (PSII) using multicomponent quantum mechanics. The hydrogen bond showed a decrease in the donor-acceptor distance upon deuteration, i.e. the inverse Ubbelohde effect, along with a strong dependence on the anisotropy of the potential energy profile. Such isotopic behaviors reasonably agreed with those of FHF–, which is one of the strongest hydrogen bonding systems. According to the agreement, PSII was confirmed as a biomolecular system containing the short strong hydrogen bond.