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WIREs Comput Mol Sci
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Computational challenges in Astrochemistry

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Cosmic evolution is the tale of progressive transition from simplicity to complexity. The newborn universe starts with the simplest atoms formed after the Big Bang and proceeds toward ‘astronomical complex organic molecules’ (astroCOMs). Understanding the chemical evolution of the universe is one of the main aims of Astrochemistry, with the starting point being the knowledge whether a molecule is present in the astronomical environment under consideration and, if so, its abundance. However, the interpretation of astronomical detections and the identification of molecules are not all straightforward. In particular, molecular species characterized by large amplitude motions represent a major challenge for molecular spectroscopy and, in particular, for computational spectroscopy. More in general, for flexible systems, the conformational equilibrium needs to be taken into account and accurately investigated. It is shown that crucial challenges in the computational spectroscopy of astroCOMs can be successfully overcome by combining state‐of‐the‐art quantum‐mechanical approaches with ad hoc treatments of the nuclear motion, thus demonstrating that the rotational and vibrational features can be predicted with the proper accuracy. The second key step in Astrochemistry is understanding how astroCOMs are formed and how they chemically evolve toward more complex species. The challenges in the computational chemistry of astroCOMs are related to the derivation of feasible formation routes in the typically harsh conditions (extremely low temperature and density) of the interstellar medium, as well as the understanding of the chemical evolution of small species toward macromolecules. Within the transition state theory, for instance, it is possible to obtain new astrochemical information by identifying the intermediate species and transition states connecting them in a plausible formation route. Depending on the sophistication of the model, different quantities may be needed. Nevertheless, accuracy can be critical, thus requiring state‐of‐the‐art computational approaches to derive geometries, energies, spectroscopic properties, and thermochemical data for each relevant structure along the reaction path.

The astroCOMs (or potential astroCOMs) addressed in this review: molecular formula and structure together with the relevant information.
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Simulated infrared (IR) spectra of C13H9+ and C13H9. Anharmonic wavenumbers and intensities were computed at the B3LYP/6–31 + G(d,p) level of theory and the stick spectra convoluted by means of Lorentzian broadening functions with half‐widths at half maximum of 2 cm−1.
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Reaction pathways of the progression of chain reactions of C2H4 with (a) C7H7+ and (b) C6H5NH+ to elucidate the formation mechanism of (N‐)heterocycles.
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First steps of the NH2 + formaldehyde (H2CO) reaction mechanism. Relative electronic CCSD(T)/CBS + CV energies (in kJ mol−1) in black. Minima and transition states are labeled according to the notation of Ref In the inset: the relative energy of the TS5 transition state at different levels of theory is given (for details,see text).
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Simulation of the rotational spectra of glycine Ip and IIn at T = 200 K in the 80–250 GHz frequency range. Spectroscopic parameters are taken from the Cologne Database for Molecular Spectroscopy (CDMS) database, while the population ratio is based on an energy difference of either 700 cm−1 (left, Ref ) or 312 cm−1 (right, Ref ).
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The potential energy profile of acetyl cyanide along the torsional angle (see, the inset) evaluated at the CCSD(T)/CBS + CV level using CCSD(T)/cc‐pVTZ optimized geometries.
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Comparison between the simulated (color) spectra of oxirane and the observed (black) emission spectra of planetary nebula NGC 7027 (Sloan et al.). Selected wavelength ranges: (a) the 3.2–3.8 μm and (b) 10.5–13.0 μm. The observed identified line fluxes reported by Salas et al. are marked by the gray lines. Computations with hybrid models are denoted as QM/QM’, with CCvxz (x = t,q) standing for CCSD(T)/cc‐pVXZ. Theoretical spectra have been convoluted with a Lorentzian function with half‐width at half‐maximum of 1 cm−1.
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Comparison of the best‐estimated (see footnote 1 of Table , in red), scaled (in black) and experimental (Ref , in blue) rotational spectrum of oxirane in the (a) ∼85 GHz and (b) ∼104 GHz frequency regions.
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Theoretical and Physical Chemistry > Spectroscopy
Electronic Structure Theory > Ab Initio Electronic Structure Methods
Electronic Structure Theory > Density Functional Theory

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