**Path-integral molecular dynamics predictions of equilibrium H and O
isotope fractionations between brucite and water**

CH Gao and YN Zhang and Q Liu and YH Yang and Y Liu, GEOCHIMICA ET COSMOCHIMICA ACTA, 346, 207-222 (2023).

DOI: 10.1016/j.gca.2023.02.004

Precise prediction of hydrogen (H) isotope fractionations among different substances is a long-standing challenge in isotope geochemistry, as it needs treatments beyond the harmonic approximation. Path-integral molecular dynamics (PIMD) simulations have recently been proved to be valid in predicting equilibrium isotope fractionations for light elements. However, the lack of reliable force fields hinders the application of PIMD to the condensed phases. In this work, the deep potential models trained on the first -principles molecular dynamics (FPMD) data are applied to PIMD simulations to determine the D/H and 18O/16O isotope fractionation factors between brucite and water. To quantitatively assess the influence of quantum effects, the D/H and 18O/16O isotope fractionations between brucite and water are also deter- mined under the framework of harmonic approximation. By comparing the results of the PIMD and those of harmonic calculations, we find that H and O atoms in brucite and water are strongly affected by the anharmonic effects. The accuracy of the harmonic isotope fractionations mainly depends on the cancellation percentage of the anharmonic effects on the reduced partition function ratios (RPFRs) of the two substances. The isotope fractionations predicted by PIMD sim-ulations are close to the experimental results at high temperatures. However, at low temperatures, the predicted isotope fractionations are different from those of experiments. The discrepancies are attributed to the approximations in the density functional theory (DFT) functionals used in this work, i.e., the inac-curacy in predicting the proton positions in brucite and water. Moreover, we find that the H isotope frac-tionations are extremely sensitive to the change of pressure. The direction of H isotope fractionation between brucite and water (at 300 K) will be even inversed as the pressure increases to more than 3 GPa. This strong pressure sensitivity may be a common characteristic of hydrous minerals and water sys-tems. Therefore, the extrapolated H isotope fractionations used in the studies of dehydration of subduct-ing slabs or deep Earth H distribution based on the isotope fractionations under low pressures may need to be rechecked.(c) 2023 Elsevier Ltd. All rights reserved.

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