Although the correlation-consistent basis sets provide a gradual and monotonic progress, their associated computational cost grows faster than the rate of convergence. As a rule of thumb , it is globally said that an improvement of the energy accuracy by a factor of 10 necessitates a computational effort increased by a factor of 10 4 , and the convergence of the correlation energy, be it MP2, CCSD, CCSD(T), ..., remains so slow that basis set limit estimates can only be reached by extrapolation . As a consequence, expressions based on the comparison with very large basis calculations or, most commonly, with explicitly correlated methods (e.g. MP2-R12/F12, CCSD(T)-R12/F12) have been suggested, but the computational overhead for obtaining such accurate references have often restricted the size of the validation systems to a few dozen electrons.