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

Abstract

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.

Picking up Good Vibrations through Quartic Force Fields and Vibrational Perturbation Theory

“…Finally, QFFs can be transformed into continuous functions and actually utilized in vibrational structure computations that require large potential energy surfaces. Vibrational configuration interaction (VCI) computations can employ QFFs if the QFFs can be extrapolated to the natural limits of their functions, i.e., they must have proper limiting behavior. The use of Morse-cosine coordinate transformations have been shown to provide such limiting behavior, , and similar trigonometric functions brought to bear for the angles and torsions may be better suited for certain molecular systems .…”