Theoretical chemistry in the service of vibrational spectroscopy: Accurate and affordable (an)harmonic recipes and the way ahead

Sophisticated molecular beam, jet, and ion-trapping experiments, ranging from direct absorption and photoelectron spectroscopy to single- and multiphoton action spectroscopy, provide increasingly detailed insight into the vibrational spectra of neutral and ionic molecules and molecular complexes under well-defined conditions. These microscopic fingerprints are direct probes of the underlying potential energy surface and thus provide important chemical and dynamical insights.

In this seminar talk, I will present different case studies in which complex spectroscopic features in carboxylic acids, their dimers, and microsolvated complexes involving water were disentangled using affordable numerical recipes. Particular attention will be given to the question of when harmonic predictions can be reliably used for spectral assignments and when anharmonic corrections via VPT2 become indispensable.

The limits of this hierarchy become apparent for strongly hydrogen-bonded systems, as the pronounced anharmonicity and large-amplitude motion (LAM) of the hydrogen-bonded protons can and often do lead to the breakdown of conventional VPT. Our available theoretical and computational toolset for modelling such systems is surprisingly limited and has not kept pace with experimental advances over the past two decades.

I pursue a hybrid approach that combines variational strategies for the LAM degrees of freedom with canonical Van Vleck perturbation theory to efficiently treat the anharmonicity of the remaining degrees of freedom. I will outline the essential features of this approach, which provides a route towards modelling the full-dimensional vibrational dynamics of strongly hydrogen-bonded systems.