Computational challenges in Astrochemistry

Abstract

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. This article is categorized under: Theoretical and Physical Chemistry > Spectroscopy Electronic Structure Theory > Ab Initio Electronic Structure Methods Electronic Structure Theory > Density Functional Theory

On the Choice of Coordinates in Anharmonic Theoretical Vibrational Spectroscopy

“…The localization subsets obtained for different choices of the maximum acceptable error e max are listed in Table IV. For e max = 100 cm −1 , three different subsets are obtained: the C-H stretching vibrations (modes 9-12), the out-of plane vibrations (modes 2-4), and all other vibrations (modes 1, [5][6][7][8]. This matches the manual assignment considered in Sect.…”

“…The combination of experiment and theory allows, e.g., for the elucidation of gas-phase structures of biomolecules [1][2][3][4] and the identification of complex organic molecules in interstellar space. [5,6] While the calculation of vibrational spectra of molecules with up to a few hundred atoms is nowadays routinely possible within the harmonic approximation (see, e.g., Refs. 7-9), the inclusion of anharmonicities is hampered by several computational bottlenecks and is in general only possible for rather small molecules.…”

UV Irradiation and Near Infrared Characterization of Laboratory Mars Soil Analog Samples: the case of Phthalic Acid, Adenosine 5’-Monophosphate, L-Glutamic Acid and L-Phenylalanine Adsorbed onto the Clay Mineral Montmorillonite in the Presence of Magnesium Perchlorate

“…All computations have been performed by means of density functional theory employing the B3LYP functional (Becke, 1993) with the double-zeta basis sets SNSD (Barone et al, 2014), and including Grimme's dispersion correction D3 (in conjunction with Becke-Johnson damping) (Ehrlich et al, 2011;Grimme et al, 2011Grimme et al, , 2010. This approach has been well tested for structural and spectroscopy properties of biological molecules of astrochemical interest (Barone et al, 2015;Biczysko et al, 2018;Fornaro et al, 2014;Fornaro et al, 2015a;Fornaro et al, 2015b).…”

“…In GVPT2 computations, the default criteria for anharmonic resonances have been exploited, and variational corrections have been applied to both energies and IR intensities. Moreover, considering that the large amplitude-motion (LAM) vibrations, which cannot be well-described by perturbative approaches, may contaminate the overall VPT2 treatment and higher frequency vibrations, LAM-free VPT2 scheme have been used in this work (where all LAMs anharmonic constants are excluded from the VPT2) (Biczysko et al, 2018). For all molecules, vibrations with harmonic frequencies below 200 cm -1 have been considered as LAMs.…”

UV Irradiation and Near Infrared Characterization of Laboratory Mars Soil Analog Samples

“…All computations have been performed by means of density functional theory employing the B3LYP functional (Becke, 1993) with the double-zeta basis sets SNSD (Barone et al, 2014), and including Grimme's dispersion correction D3 (in conjunction with Becke-Johnson damping) (Ehrlich et al, 2011;Grimme et al, 2011Grimme et al, , 2010. This approach has been well tested for structural and spectroscopy properties of biological molecules of astrochemical interest (Fornaro et al, 2014;Fornaro et al, 2015a;Barone et al, 2015;Fornaro et al, 2015b;Biczysko et al, 2018). In GVPT2 computations, the default criteria for anharmonic resonances have been exploited, and variational corrections have been applied to both energies and IR intensities.…”