**Importance of zero-point energy for crystalline ice phases: A comparison
of force fields and density functional theory**

S Rasti and J Meyer, JOURNAL OF CHEMICAL PHYSICS, 150, 234504 (2019).

DOI: 10.1063/1.5097021

Density functional theory (DFT) including van der Waals (vdW)
interactions and accounting for zero-point energy (ZPE) is believed to
provide a good description of crystalline ice phases **B. Pamuk et al.,
Phys. Rev. Lett. 108, 193003 (2012)**. Given the computational cost of
DFT, it is not surprising that extensive phonon calculations, which
yield the ZPE, have only been done for a limited amount of ice
structures. Computationally convenient force fields on the other hand
are the method of choice for large systems and/or dynamical simulations,
e.g., of supercooled water. Here, we present a systematic comparison for
seven hydrogen-ordered crystalline ice phases (Ih, IX, II, XIII, XIV,
XV, and VIII) between many commonly used nonpolarizable force fields and
density functionals, including some recently developed meta-GGA
functionals and accounting for vdW interactions. Starting from the
experimentally determined crystal structures, we perform space-group-
constrained structural relaxations. These provide the starting point for
highly accurate phonon calculations that yield effectively volume-
dependent ZPEs within the quasiharmonic approximation. In particular,
when including ZPE, the force fields show a remarkably good performance
for equilibrium volumes and cohesive energies superior to many density
functionals. A decomposition of the cohesive energies into
intramolecular deformation, electrostatic, and vdW contributions
quantifies the differences between force fields and DFT. Results for the
equilibrium volumes and phase transition pressures for all studied force
fields are much more strongly affected by ZPE than all studied density
functionals. We track this down to significantly smaller shifts of the
O-H-stretch modes and compare with experimental data from Raman
spectroscopy.

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