The Importance of Nuclear Quantum Effects on the Thermodynamic and Structural Properties of Low-Density Amorphous Ice: A Comparison with Hexagonal Ice.

We study the nuclear quantum effects (NQE) on the thermodynamic properties of low-density amorphous ice (LDA) and hexagonal ice () at = 0.1 MPa and ≥ 25 K. Our results are based on path-integral molecular dynamics (PIMD) and classical MD simulations of HO and DO using the q-TIP4P/F water model. We show that the inclusion of NQE is necessary to reproduce the experimental properties of LDA and ice . While MD simulations (no NQE) predict that the density ρ() of LDA and ice increases monotonically upon cooling, PIMD simulations indicate the presence of a density maximum in LDA and ice . MD and PIMD simulations also predict a qualitatively different T-dependence for the thermal expansion coefficient α() and bulk modulus () of both LDA and ice . Remarkably, the ρ(), α(), and () of LDA are practically identical to those of ice . The origin of the observed NQE is due to the delocalization of the H atoms, which is identical in LDA and ice . H atoms delocalize considerably (over a distance ≈ 20-25% of the OH covalent-bond length) and anisotropically (preferentially perpendicular to the OH covalent bond), leading to less linear hydrogen bonds HB (larger HOO angles and longer OO separations) than observed in classical MD simulations.