Spectroscopy, the molecular response to electromagnetic radiation of different wavelengths, is one of the most powerful experimental tools for interrogating a molecule's structure and dynamics as it interacts with its environment. However, relating a spectroscopic signature to a molecular picture relies on sophisticated computational approaches, which offer a wealth of methods for identifying structures, intermolecular interactions, and their correlation with spectroscopic response. This thesis focuses on the how to correlate a molecule's structure and interactions with its environment via ab initio calculation of spectroscopic parameters. To build a molecular picture of carbon dioxide dynamics in ionic liquids (ILs), quantum chemical calculations on small clusters qualitatively reproduced the experimental ordering for carbon dioxide's asymmetric vibrational stretch peak position which shifts when dissolved in a series of ILs with varying anions. To uncover the physical origin of the shift, the language of decomposition analysis based on absolutely localized molecular orbitals (ALMO-EDA) was translated from energies to vibrational frequencies. Geometric distortion of carbon dioxide, as a result of charge transfer (CT) from the anion into the carbon dioxide, is the driving force for differentiating the carbon dioxide asymmetric stretch shift in different IL anions. After validating these simple models, we further decomposed the CT contribution into geometry and curvature mechanisms, finding that CT is a significant contributor in both the geometry optimization and frequency calculation steps. A comparison between ALMO-EDA and symmetry-adapted perturbation theory (SAPT) showed that while dispersion dominates the binding energy, excellent correlation between both total interaction energies and individual components for ALMO-EDA and SAPT validates the use of DFT, enabling the construction of a semiempirical spectroscopic map. This decomposition presented the first application of an EDA outside the energy realm into molecular properties; however, it is not generally applicable to arbitrary perturbations. A reformulation of the canonical linear response equations for use with ALMOs provides a direct connection between EDA terms and their corresponding contribution to spectra. Results for argon-lithium cation dimer polarizabilities show that allowing CT is equally important in both the underlying ground-state wavefunction and the response calculation, and should not be confused with basis set superposition error.
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