Liquid water displays some remarkable anomalous properties that are still not fully understood. Among them, there is a splitting observed in the 1b1 band of the X-ray emission (XE) spectrum of liquid water. Previous publications have presented different hypotheses to explain this splitting, such as associating it to the local hydrogen-bonding environment; However, although several theoretical studies have attempted to describe the 1b1 splitting using a variety of methods, no theoretical approach has successfully reproduced it before. In a recent publication, we introduced a novel theoretical/computational QM/MM approach that uses the successful MB-pol model combined with path-integral molecular dynamics (PIMD) simulations and time-dependent density functional theory (TD-DFT) calculations to model the X-ray emission (XE) spectrum of water and ice Ih. To the best of our knowledge, this study reports the first calculations that predict the split of the 1b1 peak in the XE spectrum of liquid water. A systematic analysis in terms of the underlying local structure of liquid water at ambient conditions indicates that several different hydrogen-bonding motifs contribute to the overall XE lineshape in the energy range corresponding to emissions from the 1b1 orbitals, which suggests that it is not possible to unambiguously attribute the split of the 1b1 peak to only two specific structural arrangements of the underlying hydrogen-bonding network; this assessment conflicts with the mainstream hypotheses to explain the origin of the splitting. While these findings represent a further step towards understanding the behavior of liquid water at the molecular level and allow for reconciling apparent inconsistencies between experimental measurements and computer simulations, a quantitative description of the XE spectrum is still missing. The next step in this study is to go beyond the accuracy of densify-functional theory by employing rigorous electronic structure methods, namely equation-of-motion coupled-cluster theory, to describe the XE spectrum. Since the computational cost of this methodology is commonly prohibitively expensive, we will take advantage of the GPU-accelerated TeraChem software. We will investigate whether a more accurate treatment of the electronic structure calculations could contribute to a better description of the XE spectrum of liquid water, which is a problem that has remained unsolved for more than one decade.

Towards an accurate description of the X-ray emission spectrum of liquid water

ACS Spring 2022

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