Benchmark calculations for molecules in the gas phase: state-of-the-art coupled-cluster computations

Implementation of very accurate ab initio methods on the one hand and improvements in computer facilities on the other hand allow the determination of structural, thermochemical, and spectroscopic properties of small- to medium-sized molecules to a very high accuracy. The predictive capabilities have become so high that theoretical calculations can guide, support, and even challenge experimental determinations. Therefore, nowadays ab initio calculations are suitable for benchmarking purposes, even for quite large systems. To perform benchmark calculations, highly correlated methods, such as the coupled cluster ones, should be employed in conjunction with extrapolative and additive techniques 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. Some illustrative examples are 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 with that well documented for closed-shell systems are also discussed. Finally, perspectives for wider application of benchmark calculations are addressed.