Ab initio path integral study of isotope effect of hydronium ion

The differences in molecular fluctuation of protonated water composed of hydrogen isotopes are elucidated by ab initio path integral molecular dynamics simulation, in which ‘on-the-fly’ calculation of potential energy surface is done accurately with the MP2 electron correlation to sample quantum nuclear configurations. It turned out that, at the room temperature 298 K, H3O+ is quite flexible in which pyramidal structure is inverted frequently with multi-dimensional molecular distortion, while the flexibility of D3O+ is much more restrained. Our analysis shows that anharmonicity in full-dimensional rovibrational motion is essential in this isotope effect.