We assess the elastic stiffness constants of hexagonal proton-disordered ice Ih as described by density-functional theory calculations. Specifically, we compare the results for a set of nine exchange-correlation functionals, including standard generalized-gradient approximations (GGAs), the strongly constrained and appropriately normed (SCAN) metaGGA functional, and a number of dispersion-corrected versions based on the van der Waals (vdW) and VV10 schemes. Compared to the experimental data, all functionals predict an excessively stiff response to tensile and compressive distortions, as well as shear deformations along the basal plane, with the SCAN metaGGA functional displaying the largest deviations as compared to the experimental values. These discrepancies are found to correlate with underestimates of inter-molecular distances, on the one hand, and overestimates of intra-molecular separations, on the other. The inclusion of non-local vdW corrections according to the vdW approach generally improves these structural parameters and softens the elastic response functions compared to their parent GGA functionals. The dispersion-corrected SCAN-rVV10 functional, however, acts in the opposite direction, further worsening the comparison to experiment. In this view, it appears useful that the database employed to gauge the quality of exchange-correlation functionals for water includes an assessment of their elastic response of ice Ih and possibly other crystalline phases.

Due to their potential role in the peculiar geophysical properties of the ice giants Neptune and Uranus, there has been a growing interest in superionic (SI) phases of water ice. So far, however, little attention has been given to their mechanical properties, even though plastic deformation processes in the interiors of planets are known to affect long-term processes, such as plate tectonics and mantle convection. Here, using density functional theory calculations and machine learning techniques, we assess the mechanical response of high-pressure/temperature solid phases of water in terms of their ideal shear strength (ISS) and dislocation behavior. The ISS results are well described by the renormalized Frenkel model of ideal strength and indicate that the SI ices are expected to be highly ductile. This is further supported by deep neural network molecular dynamics simulations for the behavior of lattice dislocations for the SI face-centered cubic (fcc) phase. Dislocation velocity data indicate effective shear viscosities that are orders of magnitude smaller than that of Earth’s lower mantle, suggesting that the plastic flow of the internal icy layers in Neptune and Uranus may be significantly faster than previously foreseen.

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