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.

The determination of accurate equilibrium molecular structures plays a fundamental role for understanding many physical–chemical properties of molecules, ranging from the precise evaluation of the electronic structure to the analysis of the role played by dynamical and environmental effects in tuning their overall behavior. For small semi-rigid systems in the gas phase, state-of-the-art quantum chemical computations rival the most sophisticated experimental (from, for example, high-resolution spectroscopy) results. For larger molecules, more effective computational approaches must be devised. To this end, we have further enlarged the compilation of available semi-experimental (SE) equilibrium structures, now covering the most important fragments containing H, B, C, N, O, F, P, S, and Cl atoms collected in the new SE100 database. Next, comparison with geometries optimized by methods rooted in the density functional theory showed that the already remarkable results delivered by PW6B95 and, especially, rev-DSDPBEP86 functionals can be further improved by a linear regression (LR) approach. Use of template fragments (taken from the SE100 library) together with LR estimates for the missing interfragment parameters paves the route toward accurate structures of large molecules, as witnessed by the very small deviations between computed and experimental rotational constants. The whole approach has been implemented in a user-friendly tool, termed nano-LEGO, and applied to a number of demanding case studies.

The recent implementation of the computation of IR intensities beyond the double-harmonic approximation [Bloino, J.; Barone, V. J. Chem. Phys. 2012, 136, 124108] paved the route to routine calculations of infrared spectra for a wide set of molecular systems. Halogenated organic compounds represent an interesting class of molecules, from both an atmospheric and computational point of view, due to the peculiar chemical features related to the halogen atoms. In this work we simulate the IR spectra of eight halogenated molecules (CH 2 F 2 , CHBrF 2 , CH 2 DBr, CF 3 Br, CH 2 CHF, CF 2 CFCl, cis-CHFCHBr, cis-CHFCHI), using two common hybrid and doublehybrid density functionals in conjunction with both double-and triple-zeta quality basis sets (SNSD and cc-pVTZ) as well as employing the coupled-cluster theory with basis sets of at least triple-zeta quality. Finally, we compare our results with available experimental spectra, with the aim of checking the accuracy and the performances of the computational approaches.

A recently developed model chemistry (jun-Cheap) has been slightly modified and proposed as an effective, reliable, and parameter-free scheme for the computation of accurate reaction rates with special reference to astrochemical and atmospheric processes. Benchmarks with different sets of state-of-the-art energy barriers spanning a wide range of values show that, in the absence of strong multireference contributions, the proposed model outperforms the most well-known model chemistries, reaching a subchemical accuracy without any empirical parameter and with affordable computer times. Some test cases show that geometries, energy barriers, zero point energies, and thermal contributions computed at this level can be used in the framework of the master equation approach based on the ab initio transition-state theory for obtaining accurate reaction rates.

Accuracy and interpretability are often seen as the devil and holy grail in computational spectroscopy and their reconciliation remains a primary research goal. In the last few decades, density functional theory has revolutionized the situation, paving the way to reliable yet effective models for medium size molecules, which could also be profitably used by non-specialists. In this contribution we will compare the results of some widely used hybrid and double hybrid functionals with the aim of defining the most suitable recipe for all the spectroscopic parameters of interest in rotational and vibrational spectroscopy, going beyond the rigid rotor/harmonic oscillator model. We will show that last-generation hybrid and double hybrid functionals in conjunction with partially augmented double-and triple-zeta basis sets can offer, in the framework of second order vibrational perturbation theory, a general, robust, and user-friendly tool with unprecedented accuracy for medium-size semi-rigid molecules.

The determination of accurate equilibrium molecular structures plays a fundamental role for understanding many physical-chemical properties of molecules, ranging from the precise evaluation of the electronic structure to the analysis of dynamical and environmental effects in tuning their overall behavior. For this purpose the so-called semiexperimental approach, based on a nonlinear least-squares fit of the moments of inertia associated with a set of available isotopologues, allows one to obtain very accurate results, without the unfavorable computational cost characterizing high-level quantum chemical methods. In the present work the MSR (Molecular Structure Refinement) software for the determination of equilibrium structures by means of the semiexperimental approach is presented, and its implementation is discussed in some detail. The software, which is interfaced with a powerful graphical user interface, includes different optimization algorithms, an extended error analysis, and a number of advanced features, the most remarkable ones concerning the choice of internal coordinates and the method of predicate observations. In particular, a new black-box scheme for defining automatically a suitable set of nonredundant internal coordinates of A symmetry in place of the customary Z-matrix has been designed and tested. Finally, the implementation of the method of the predicate observations is discussed and validated for a set of test molecules. As an original application, the method is employed for the determination of the semiexperimental structure for the most stable conformer of glycine.

A gas-phase nitrogen-nitrogen noncovalent interaction has been unveiled in the nitroethane-trimethylamine complex in an environment free from solvent and matrix effects using rotational spectroscopy in supersonic expansion. Different quantum chemical models (NOCV/CD and NBO) agree in indicating that this interaction largely prevails over the C-H⋅⋅⋅O and C-H⋅⋅⋅N hydrogen bonds. Furthermore, a SAPT analysis shows that electrostatic and dispersion interactions play a comparable role in stabilizing the complex. The conformational landscape exploration and stationary points characterization have been performed using state-of-the-art quantum-chemical computations providing significant insights on structure determination.

By combining rotational spectroscopyinsupersonic expansion with the capability of state-of-the-art quantumchemical computations in accurately determining structural and energetic properties,the genuine nature of as ulfur-sulfur chalcogen bond between dimethyl sulfide and sulfur dioxide has been unveiled in agas-jet environment free from collision, solvent and matrix perturbations.ASAPT analysis pointed out that electrostatic S···S interactions play the dominant role in determining the stability of the complex, largely overcoming dispersion and CÀH···O hydrogen-bond contributions.Indeed, in agreement with the analysis of the quadrupole-coupling constants and of the methyl internal rotation barrier,the NBO and NOCV/CD approaches show am arked charge transfer between the sulfur atoms.B ased on the assignment of the rotational spectra for 7i sotopologues,a na ccurate semiexperimental equilibrium structure for the heavy-atom backbone of the molecular complex has been determined, whichis characterized by aS ···S distance (2.947(3) )w ell belowt he sum of van der Waals radii.