Anomalous ortho-to-para ratio of nuclear spin isomers of H$_2$O at low temperatures

A theoretical model proposed for nuclear spin isomers of H$_2$O molecules located inside the C$_{60}$ fullerene explains an anomalously high stability of ortho-H$_2$O isomers detected in the experiments reported in [B. Meier et al., Nature Commun. 6, 8112 (2015)] at a temperature of $T = 5$ K.