Molecular structure determination from quantum-chemical calculations: extrapolation to the complete basis set limit and additivity approximation

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. Concerning the extrapolation to the complete basis set (CBS) limit, different procedures are compared. More precisely, a well-tested and theoretically justified scheme (here denoted as "gradient scheme"), based on extrapolated gradients, is employed for validating the extrapolation procedure directly applied to geometrical parameters ("geometry scheme") by means of different extrapolation functions. Analogously, using an additivity scheme at gradient level for the treatment of the various electron correlation contributions (core correlation, full treatment of triple excitations, quadruple excitations, …), the additivity approximation directly applied to geometrical parameters has been tested. The main advantage of the "geometry scheme" is that it is always applicable and is definitely less computationally expensive than the "gradient scheme".