State-of-the-art coupled custer calculations: molecular and spectroscopic properties

State-of-the-art coupled cluster computations allow the determination of structural, molecular, thermochemical and spectroscopic properties of small- to medium-size molecules to a very high accuracy. Their predictive capabilities have nowadays such an accuracy that they can guide, support and even challenge experimental determinations. To perform benchmark calculations, the coupled cluster methods should be employed in conjunction with extrapolative and additive techniques in order to account for basis set and wave function truncation errors as well as to include important corrections, such as those related to core correlation and relativistic effects. A few illustrative examples will be presented. The comparison of the computed data with experimental results allows us to show how quantum chemical computations are able to either accurately predict experimental data or cast doubts on them. Perspectives for extension of extrapolation techniques to the structures and properties of open-shell systems with an effectiveness and reliability comparable to that well documented for closed-shell systems will be also discussed.