Ab Initio Methods

The group computed vibration-rotation spectra of small molecules with high accuracy using correlated ab initio methods with large basis sets. In the past research in this area, coupled cluster CCSD(T) calculations were combined with second-order rovibrational perturbation theory to predict the spectroscopic constants of small reactive molecules, with sufficient accuracy to guide their spectroscopic identification and to assist in the analysis of their high-resolution vibration-rotation spectra. More recently, the group had developed and implemented a general variational treatment of nuclear motion that allowed the prediction of rovibrational energies and intensities not only for semirigid molecules, but also for molecules with large amplitude motion and for high rotational excitation. The variational calculations were based on accurate ab initio potential energy surfaces and dipole moment surfaces obtained at the coupled cluster level. Recent applications included the computation of complete rovibrational line lists for ammonia, the explanation of the unexpected intensity anomalies observed for oxadisulfane (HSOH), and purely theoretical predictions for thioformaldehyde with wavenumber accuracy. In the realm of electronic spectroscopy, the Thiel group used high-level ab initio methods to provide theoretical benchmark data for the electronically excited states of representative organic chromophores.