Estimating the infrared band strengths of amorphous interstellar ice analogues using density functional theory

Infrared band strengths are needed to obtain the column density of ice mantle molecular components observed towards cold interstellar and circumstellar environments. The values for these ices are often outdated or unavailable. Using density functional theory, we aim to provide a general method for the prediction and confirmation of band strengths for any amorphous ice. Amorphous ices were created using randomised initial positions of molecules in a cubical simulation box with periodic boundary conditions to simulate an infinite amorphous solid. Temperature was controlled using molecular dynamics with a thermostat and maintaining a constant volume. Infrared spectra were subsequently generated according to density functional perturbation theory. Estimations for band strengths are presented for ices of astrophysical interest, including water, carbon dioxide, carbon monoxide, methane, ammonia, and methanol. The newly calculated band strengths are in good agreement with previous values obtained through experimental measurements. This novel method can be applied in general to any amorphous ice, with especially good accuracy for bending vibrational modes. The method can also be applied to ices of unknown band strengths, including unstable species under Earth conditions, and to mixed or layered ices that contain more than one species.