Molecular structure determination: a testing ground for high-level quantum-chemical computations

structure by means of quantum-chemistry as well as on the accuracy nowadays reachable will be presented. Taking the coupled-cluster singles and doubles (CCSD) level augmented by a perturbative treatment of triple excitations (CCSD(T)) as starting point, emphasis will be given to extrapolation techniques as well as to inclusion of “minor” contributions, such as core-correlation effects, higher-excitation and relativistic effects. The combination of experimental ground-state rotational constants with calculated vibrational corrections to determine reliable equilibrium geometries for polyatomic molecules will be also considered. Due to the computational contribution, these structures are usually referred to as ''empirical'', ''mixed experimental/theoretical'', or ''semi-experimental''. The comparison to pure ab initio structural determinations will be also discussed. Illustrative examples, mostly for small to medium-sized molecules, will be presented.