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 Karl Franzens University Graz

Graz University of Technology 

Nuclear Quantum Effects in Ice - New Insights from Computational Modelling
Jörg Meyer
Leiden University
16:15 - 17:15 Tuesday 20 June 2023 

In crystalline phases of water nuclear quantum effects manifest themselves quite prominently in macroscopic thermodynamic properties, like for example density change at low temperatures [1] upon substitution of a light by a heavier isotope or phase transition enthalpies [2]. The former is called the volume isotope effect (VIE). In most materials, substitution with a heavier isotope results in a smaller volume at temperatures approaching the absolute zero. Handwavingly, in a classical picture, this can be rationalized by the smaller vibrational amplitude ascribed to a heavier nucleus experiencing the same chemical interaction potential at the same temperature. In contrast, the volume of the D$_2$O isotopologue of ice Ih is 0.1% larger than its H$_2$O counterpart, which is called anomalous VIE. Earlier computational studies established a link to the zero-point energy of intra- and intermolecular vibrational modes (phonons). However, even the most elaborate force fields developed from basic principles yielded a normal VIE in clear contradiction to experiments. Furthermore, DFT calculations with various exchange-correlation functionals have been struggling to get close to the experimental data. This made it difficult to determine which contributions to the interaction potential result in what effects on the different types of phonon modes and impossible to identify which of them are responsible for this subtle effect.

Motivated by recent high-precision neutron diffraction experiments [3], here we present a computational study based on the quasi-harmonic approximation [4]. We focus on recently developed polarizable many body potentials [5,6] and find that one of them, MB-pol [6], yields the anomalous VIE in good agreement with the most recent high-resolution neutron diffraction measurements – better than many density functionals. We further scrutinize the interaction potentials by decomposing the zero-point pressure into contributions from different vibrational mode groups. This allows us to confirm the remarkable performance of MB-pol by comparing to a hitherto unconsidered “independent” benchmark value for the intramolecular stretching modes of H$_2$O ice I$_h$ obtained from Raman spectroscopy data. According to MB-pol, the librational and stretching are the most important phonon modes responsible for the anomalous VIE, mitigated by short-range three-body effects. This prediction is experimentally verifiable. Our study thus goes one step further towards a deeper understanding of hydrogen-bonding in this most ubiquitous and important system.

References:
[1] S. Rasti and J. Meyer, J. Chem. Phys. 150, 234504, 2019, DOI: 10.1063/1.5097021
[2] V. Fuentes-Landete et al., J. Phys. Chem. Lett. 11, 8268, 2020, DOI: 10.1021/acs.jpclett.0c02368
[3] A. D. Fortes, Acta Cryst. B 74, 196, 2018, DOI: 10.1107/S2052520618002159
[4] S. Rasti et al., J. Phys. Chem. Lett. 13, 11831, 2022, DOI: 10.1021/acs.jpclett.2c03212
[5] E. Ö. Jónsson et al., J. Chem. Theory Comput. 18, 7528, 2022, DOI: 10.1021/acs.jctc.2c00598
[6] G. R. Medders et al. J. Chem. Theory Comput. 9, 1103, 2013; DOI: 10.1021/ct300913g