Anharmonic Vibrational Treatment Exclusively in Curvilinear Valence Coordinates: The Case of Formamide

From the Automated Calculation of Potential Energy Surfaces to Accurate Infrared Spectra

Advances in the development of quantum chemical methods and progress in multicore architectures in computer science made the simulation of infrared spectra of isolated molecules competitive with respect to established experimental methods. Although it is mainly the multidimensional potential energy surface that controls the accuracy of these calculations, the subsequent vibrational structure calculations need to be carefully converged in order to yield accurate results. As both aspects need to be considered in a balanced way, we focus on approaches for molecules of up to 12-15 atoms with respect to both parts, which have been automated to some extent so that they can be employed in routine applications. Alternatives to machine learning will be discussed, which appear to be attractive, as long as local regions of the potential energy surface are sufficient. The automatization of these methods is still in its infancy, and the generalization to molecules with large amplitude motions or molecular clusters is far from trivial, but many systems relevant for astrophysical studies are already in reach.

Vibrational treatment of hydroxylamine in valence coordinates

A valence coordinate H2NOH ground state potential energy surface accurate for all levels up to 6000 cm−1 relative to trans zero point energy has been generated at the coupled-cluster single double triple-F12/aug-cc-pVTZ level encompassing the trans and cis as well as the N–H2 permutational conformers. All cis and trans fundamentals and a complete set of eigenfunctions up to about 3100 cm−1 have been calculated and assigned using the improved relaxation method of the Heidelberg multi-configuration time-dependent Hartree package and an exact expression for the kinetic energy in valence coordinates generated by the TANA program. The average and maximal error to all observed transitions is about 6.3 and 14.6 cm−1, respectively. Local cis eigenfunctions exist with up to two quanta in the isomerization mode ν9. Although no significant inversion splittings have been found up to the considered 3100 cm−1, they are expected within the fundamental energy range in view of the calculated 4261 cm−1 H2 permutation/inversion barrier height. The cis-NH2 symmetric stretch fundamental shows a Fermi resonance with a splitting of about 10 cm−1.

Calculation of Anharmonic IR and Raman Intensities for Periodic Systems from DFT Calculations: Implementation and Validation

An extension of the CRYSTAL program is presented allowing for calculations of anharmonic infrared (IR) intensities and Raman activities for periodic systems. This work is a follow-up of two papers devoted to the computation of anharmonic vibrational states of solids from DFT (density functional theory) calculations (Erba et al. J. Chem. Theory Comput. 2019, 15, 3755-3765 and Erba et al. J. Chem. Theory Comput. 2019, 15, 3766-3777). The approach presented here relies on the evaluation of integrals of the dipole moment and polarizability operators over anharmonic wave functions obtained from either VSCF (vibrational self-consistent field) or VCI (vibrational configuration interaction) calculations. With this extension, the program now allows for a more complete characterization of the vibrational spectroscopic features of solids within the density functional theory. In particular, it is able (i) to provide reliable positions and intensities for the most intense spectral features and (ii) to check whether a first overtone or a combination band has a nonvanishing IR intensity or Raman activity. Therefore, it becomes possible to assign the transition(s) corresponding to satellite peak(s) around a fundamental transition or the overtones or combination bands that may be as intense as their corresponding fundamental transitions through the strongest mode-mode couplings, as in so-called Fermi resonances. The present method is assessed on two molecular systems, H₂O and H₂CO, as well as on two solid state cases, boron hydrides BH₄ and their deuterated species BD₄ in a crystalline environment of alkali metals (M = Na, K). The solid state cases are particularly insightful as, in the B-H (or B-D) stretching region here considered, they exhibit many spectral features entirely due to anharmonic effects: two out of three in the IR spectrum and four out of six in the Raman spectrum. All IR and Raman active overtones and combination bands experimentally observed are correctly predicted with our approach. The effect of the adopted quantum-chemical model (DFT exchange-correlation functional/basis set) for the electronic structure calculations on the computed spectra is discussed and found to be significant, which suggests some special care is needed for the analysis of subtle spectral features.