Solid water comprises the most abundant form of ice in the universe. Apart from amorphous structures solid water also appears in a plethora of crystalline (proton-ordered) ice phases under different temperature and pressure conditions. These ice phases capture a wide range of local coordination and thus bonding scenarios between individual water molecules. State-of-the-art density functional theory (DFT) including van-der-Waals interaction and accounting for zero-point energy (ZPE) is believed to provide a good description of many properties ice phases over a wide range of temperatures and pressures [1,2]. Given the computational cost of DFT, it is not surprising that extensive phonon calculations, which yield the ZPE, have only been done for a small amount of ice structures. Computationally convenient force-fields on the other hand are still the method of choice for large systems and/or dynamical simulations.<br/><br/>I will present a systematic comparison between many commonly-used force fields (SPC/E, TIP3P, TIP4P/2005, TIP4P/ice, q-TIP4P/F) and density functionals, including recently developed meta-GGA functionals [3]. Starting from the experimentally determined crystal structures, we perform space-group-constrained structural relaxations. We calculate the full phonon dispersions and concomitant density of states in order to obtain effectively volume-dependent ZPEs within the quasi-harmonic approximation [4]. We find that all studied force fields show much larger ZPE effects than all studied density functionals. We trace this down to significantly different shifts of the O-H-stretch modes, only some of which are compatible with experimental data [5]. I will discuss potential implications of the phononic properties for structural predictions as well as for modeling energy dissipation [6,7] as it occurs during exothermic reactions of ice-covered dust grains in the interstellar medium.

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

2019 Fall National Meeting

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