Accuracy and Interpretability: The Devil and the Holy Grail. New Routes across Old Boundaries in Computational Spectroscopy

Abstract

The past decade has witnessed an increasing interaction between experiment and theory in the field of molecular spectroscopy. On the computational side, ongoing developments of hardware and software have moved computational spectroscopy from a highly specialized research area to a general tool for researchers in different fields of chemical science. However, since its dawn, computational spectroscopy has been characterized by the dichotomies of qualitative and quantitative description, and of interpretation and accuracy. Indeed, the analysis of experiments is seldom straightforward because of the subtle interplay of several different effects, which are not easy to evaluate and isolate, and/or the complexity of the system under consideration. Often, the accuracy has to be set aside for a more qualitative analysis that will provide the means for a broad interpretation. In such a scenario, the most recent advances in theoretical treatments as well as computational tools have opened the way to the reconciliation of accuracy and interpretability, resulting in unequivocal analyses and assignments of experimental spectra and their unbiased interpretation. This Review aims at being a comprehensive, authoritative, critical, and readable account of general interest to the chemistry community because of the wealth of qualitative and quantitative information that can be obtained from spectroscopic investigations. Limiting ourselves to rotational and vibrational spectroscopy, emphasis will be put on accuracy and interpretability as well as on the routes toward their reconciliation and integration.

A twist on the reaction of the CN radical with methylamine in the interstellar medium: new hints from a state-of-the-art quantum-chemical study

“…Hyperfine coupling and nitrogen quadrupole coupling constants are instead first-order properties and have been calculated at the CCSD(T)/aug-cc-pCVQZ level (all electrons correlated), with the basis sets for the hydrogen atoms being modified as explained in the Appendix A4. Equilibrium parameters were finally corrected for vibrational effects within the VPT2 approach (Barone 2005;Puzzarini et al 2019) at the B2PLYP-D3(BJ)/may -cc-pVTZ level of theory. The only exception is the electron spin-rotation tensor, for which vibrational corrections have been computed at the B3LYP/6-31+G(d) level due to the lack of the required B2PLYP implementation.…”

The Role of State-of-the-Art Quantum-Chemical Calculations in Astrochemistry: Formation Route and Spectroscopy of Ethanimine as a Paradigmatic Case

“…Anharmonic force fields have also been calculated at the B2 level, thus allowing the evaluation of anharmonic ZPVEs by means of the generalized vibrational-perturbation theory (GVPT2). With respect to standard VPT2, GVPT2 excludes resonant terms from the perturbative treatment and reintroduces them through a reduced-dimensionality variational calculation [36,46,[66][67][68]].…”

Accuracy Meets Interpretability for Computational Spectroscopy by Means of Hybrid and Double-Hybrid Functionals

“…Further developments are, however, needed to deal with new and more sophisticated experimental techniques (Quack and Merkt, 2011;Lane, 2018). For small semi-rigid molecules, the accuracy of state-of-the-art quantum mechanical (QM) methods often rivals that of experimental techniques, but extension to large flexible systems (not to mention condensed phases) faces a number of difficulties (even within the Born-Oppenheimer approximation) ranging from the very unfavorable scaling of those methods with the number of active electrons to the proper description of flat potential energy surfaces (PESs) with stationary points that are ill defined (Puzzarini et al, 2019a).…”