where X and Y are the successive cardinal numbers of the two basis sets used, and α is a constant specific to the family of basis set used. We adopted the value of α for ground-state Ahlrichsbasis set extrapolations, namely α = 10.39 for DZ-TZ and α = 7.88 for TZ-QZ extrapolations. The electron-correlation contribution E C was obtained from the differences between CCS and SCS-CC2 or CCSDR(3) excitation energies. Those contributions were extrapolated to the CBS limit with the familiar formula for the correlation energy (E C ): 188