Accurately modelling IR spectra of silicate nanoclusters of astronomical interest

Silicate nanoclusters are likely to be abundant in numerous astrophysical environments.The primary means to obtain information about astronomical silicate dust is through its infrared (IR) spectra, and comparison with experimentally prepared bulk silicates. However, silicate nanoclusters have not been experimentally produced yet, and their IR spectra should otherwise be determined to provide a better understanding of the extent of their astronomical importance.

Herein, we assess various computational methods for obtaining IR spectra of silicate dust grains from their atomistic structure and their atomic motions. First, we obtain the most stable silicate nanoclusters from global optimization searches. IR spectra of these nanoclusters are then obtained using density functional theory (DFT) based calculations within the harmonic oscillator approximation. To check if anharmonic effects play a significant role, we obtain IR spectra from finite temperature ab initio molecular dynamics (AIMD) simulations.

Finally, we also study the temperature effect on the broadening of the obtained IR spectra peaks in larger nanosilicate grains with a range of crystallinities. In this case less computationally costly classical MD simulations are necessary due to the large number of atoms involved.