Since the early 1900s, scientists have been discovering ice polymorphs by compressing water. These ice polymorphs are much denser than the ice found on on Earth (ice Ih) and many of them are found in icy planets. The diversity in the structure of these polymorphs provides a comprehensive framework for understanding the complex network of hydrogen bond arising at different thermodynamic conditions. We find that the ice structures are stabilized via concerted effects of hydrogen bonds and van der Waals forces; and a theoretical method which can describe the intricate balance between these two governing forces is critical in predicting the correct atomic structure and the phase transition properties. We have used density-functional theory and quantum Monte Carlo to examine the balance between van der Waals forces and hydrogen bonding in ambient and high-pressure phases of ice. At higher pressure, the contribution to the lattice energy from vdW increases and that from hydrogen bonding decreases, leading vdW to have a substantial effect on the transition pressures between the crystalline ice phases. We expect that the model of ice/water used in our work will enable to predict phase diagram of water at extreme pressures and temperatures that are relevant to Earth’s mantle and to icy planets and ice-rich exoplanets.

References:

• “Hydrogen bonds and van der Waals forces in ice at ambient and high pressures”. B. Santra, J. Klimeš, D. Alfè, A. Tkatchenko, B. Slater, A. Michaelides, R. Car, and M. Scheffler. Phys. Rev. Lett. 107, 185701 (2011)
• “On the accuracy of van der Waals inclusive density-functional theory exchange-correlation functionals for ice at ambient and high pressures”. B. Santra, J. Klimeš, A. Tkatchenko, D. Alfè, B. Slater, A. Michaelides, R. Car, and M. Scheffler. J. Chem. Phys. 139, 154702 (2013).

We explore how anharmonicity, nuclear quantum effects (NQE), many-body dispersion interactions, and Pauli repulsion influence thermal properties of dispersion-bound molecular crystals. Accounting for anharmonicity with ab initio molecular dynamics yields cell parameters accurate to within 2% of experiment for a set of pyridinelike molecular crystals at finite temperatures and pressures. From the experimental thermal expansion curve, we find that pyridine-I has a Debye temperature just above its melting point, indicating sizable NQE across the entire crystalline range of stability. We find that NQE lead to a substantial volume increase in pyridine-I (≈40% more than classical thermal expansion at 153 K) and attribute this to intermolecular Pauli repulsion promoted by intramolecular quantum fluctuations. When predicting delicate properties such as the thermal expansivity, we show that many-body dispersion interactions and more sophisticated density functional approximations improve the accuracy of the theoretical model.

Reference:

• "Thermal Expansion in Dispersion-Bound Molecular Crystals" . H.-Y. Ko, R. A. DiStasio, Jr., B. Santra, and R. Car Physical Review Materials, 2, 055603 (2018)