Vibrational spectra and physico-chemical properties of astrophysical analogs

We undertake in this paper a theoretical study based on DFT methodology of amorphous solids formed by methane, water and nitrogen in a ratio of 1 : 3 : 3. By varying the size of the cell containing this mixture of molecules, we study the effect of the corresponding cell volume and density on the predicted IR spectra, in particular on the hydrogen bond modes. Also the relative stability of the structures as a function of the density is studied. We have enclosed a large density range, from a very low value that simulates a gas-phase mixture, to values corresponding to solids under fairly high internal stress, with an intermediate range that could be expected to cover the values of mixtures at astronomical conditions. The variation of the energy at constant temperature with the volume of the unit cell fits well to a Morse function, which allows finding an equation of state for the material in the range of volumes studied here.