Computational UV spectra for amorphous solids of small molecules

Area of Science:

• Astrochemistry
• Computational Chemistry
• Solid-State Physics

Background:

• Interstellar ices are primarily amorphous solids, challenging to model computationally.
• Existing methods excel at gas-phase and crystalline solids but struggle with amorphous structures.

Purpose of the Study:

• To develop and validate a novel computational methodology for generating amorphous solid structures.
• To construct and analyze electronic spectra of interstellar ice analogs using this new method.

Main Methods:

• Utilized a combination of quantum chemical calculations (DFT methods like ωB97-XD, CAM-B3LYP) and statistical mechanics.
• Employed a random positioning program to generate amorphous structures.
• Applied Boltzmann distribution to weight excitations from sampled molecular clusters for spectrum compilation.

Main Results:

• Achieved semi-quantitative agreement with experimental data for ammonia and carbon dioxide ice analogs using CAM-B3LYP/6-311++G(2d,2p).
• Improved the accuracy of amorphous water ice spectra by increasing cluster size and count (up to 105 clusters of 32 molecules).

Conclusions:

• The developed methodology provides a robust approach for describing electronic spectra of amorphous ice analogs.
• This method can be extended to predict spectra for other types of interstellar ices, aiding in their characterization.