First principles optical constants of water ice: geometry effects and vibrational contributions

In plasmas, atmospheres, and interstellar environments, the relevant material is often inaccessible to direct in situ measurements. Radiative transfer calculations that the available spectroscopic and scattering measurements reduce to knowing, as a function of frequency, how strongly electromagnetic radiation is attenuated and redistributed by the medium. In practice, this requires specifying the radiative properties for each constituent in the medium, from atomic and molecular opacities to the optical constants of condensed phases. A substantial fraction of possible inputs is now supported by community databases and curated tables. These resources are powerful, but they do not address condensed phases such as dust, which require knowledge of their complex optical constants. The lack of these constants presents challenges when modeling these distant environments.

Hexagonal ice (ice Ih) is a particularly significant condensed phase because it appears in atmospheric clouds and snow/ice surfaces, and as mantles or solid phases in cold astrophysical environments. Widely used optical-constant datasets for ice aggregate heterogeneous measurements across preparation methods and temperatures. Calculating optical constants from first-principles would allow for internal consistency and yield insight into temperature, interface, and particle size dependence. 

We use density functional theory calculations to compute the complex dielectric response of ice Ih and the corresponding complex refractive index. We treat electronic and ionic contributions separately and complement our bulk calculations with surface calculations to assess how the interface modifies absorption. We benchmark the results against the Warren and Brandt compilation of heterogeneous ``hexagonal’’ ice measurements. From this comparison, we identify regions where more complex computational techniques are required, establishing a workflow for other phases of ice and other large particles essential for atmospheric and astrophysical modeling.