New Insights into the Volume Isotope Effect of Ice Ih from Polarizable Many-Body Potentials

Abstract

The anomalous volume isotope effect (VIE) of ice Ih is calculated and analyzed based on the quasi-harmonic approximation to account for nuclear quantum effects in the Helmholtz free energy. While a lot of recently developed polarizable many-body potential functions give a normal VIE contrary to experimental results, we find that one of them, MB-pol, yields the anomalous VIE in good agreement with the most recent high-resolution neutron diffraction measurementsbetter than DFT calculations. The short-range three-body terms in the MB-pol function, which are fitted to CCSD(T) calculations, are found to have a surprisingly large influence. A vibrational mode group decomposition of the zero-point pressure together with a hitherto unconsidered benchmark value for the intramolecular stretching modes of H2O ice Ih obtained from Raman spectroscopy data unveils the reason for the VIE: a delicate competition between the latter and the librations.

MBX: A many-body energy and force calculator for data-driven many-body simulations

“…78 By construction, MB-pol is fully transferable across all phases, 79,80 accurately reproducing the properties of small gas-phase clusters, [81][82][83][84][85][86][87][88][89][90][91][92] liquid water, [93][94][95][96][97][98][99] the air/water interface, [100][101][102][103][104] and ice. [105][106][107][108][109][110] Remarkably, MB-pol was shown to be the first and, currently, only water PEF able to correctly predict the phase diagram of water. 111 More recently, an updated version of MB-pol, MB-pol(2023), which was trained on larger training sets of many-body interactions, was shown to achieve even higher accuracy for simulations of water in both gas and liquid phases.…”