How accurately can structural, spectroscopic and thermochemical properties be predicted by ab initio computations?

Nowadays, ab initio calculations are able to provide very accurate predictions of molecular properties and energetics not only for compounds containing light atoms (such as first-row elements) but also for heavy atom (from second-row elements to transition metals) containing systems. The predictive capabilities of ab initio computations are consolidated in various field: - On one hand, theoretical computations themselves are able to provide very accurate equilibrium geometries by performing highly correlated calculations in conjunction with extrapolation to the complete basis set limit and accurate treatments of core-valence correlation. On the other hand, the combination of experimental ground-state rotational constants and calculated vibration-rotation interaction constants leads to one of the most successful approaches to the determination of molecular equilibrium structures: the most accurate way for obtaining equilibrium geometry for large molecules. - Theoretical predictions recover a fundamental role in the high resolution spectroscopy field. For example, it is even possible to accurately predict the fine- and/or hyperfine-structure of rotational spectra due to electric and/or magnetic interactions; that is to say that spin-rotation and nuclear quadrupole coupling constants can be accurately evaluated by ab initio calculations.. - High-level ab initio calculations can be competitive with experiment in the precise determination of thermochemical properties, such as ionization potential, electron affinity, enthalpy of formation, … By accounting for model and basis set truncations as well as core-valence, spin-orbit and, in case, relativistic effects the chemical accuracy (<1 kcal/mol) can be reliably obtained.