<italic>Ab initio</italic>path-integral molecular dynamics and the quantum nature of hydrogen bonds

Ab initiopath-integral molecular dynamics and the quantum nature of hydrogen bonds

Feng Yexin¹, Chen Ji¹, Li Xin-Zheng^{2, 3, †,}, Wang Enge^{1, 3}

(a) The differences between the shortest heavy-atom distances obtained from the PIMD and MD simulations, denoted byΔ(X–X), are plotted as a function of the HB strength index.Δ(X–X) characterizes the impact of the NQEs on the strength of the HBs. The HB strength is defined as the ratio of theX–H stretching frequency in the H-bonded system to that in the free monomer. (b) The correlation between this HB strength index and the binding energy per HB in the neutral systems. (c) A simplified schematic illustration of the isotope (Ubbelohde) effect. We suggest that regimes of positive, negligible, and negative Ubbelohde effects exist depending on the HB strength. For the HF clusters, labels 1–5 denote the HBs in the dimer to the hexamer. For the H₂O clusters, labels 1, 2, 3a, and 3b refer to the HBs in the dimer, pentamer, and the long and short HBs in the octamer, respectively. For the charged clusters, labels 1–4 refer to,,, and. For the organic dimers, labels 1a, 1b, and 2 mean the red shifted and blue shifted HBs in the formamide and the red shifted HB in formic acid. For the solids, labels 1–3 refer to the HBs in HCl, HF, and squaric acid. The same labels also apply to Fig. 4.^[7]