Accuracy of spectroscopic constants of diatomic molecules from ab initio calculations

The basis-set convergence of $cc-pVXZ$ basis sets is investigated for the MP2 and CCSD equilibrium bond distances and harmonic frequencies of BH, HF, CO, $N2,$ and $F2$ by comparing with explicitly correlated R12 results. The convergence is, in general, smooth but slow—for example, for harmonic frequencies at the quadruple-zeta level, the basis-set error is typically 7 cm⁻¹; at the sixtuple-zeta level, it is about 2 cm⁻¹. For most constants, the convergence can be accelerated by using a two-point linear extrapolation procedure. Equilibrium bond distances, harmonic frequencies, anharmonic contributions, vibration-rotation interaction constants, and rotational constants for the vibrational ground state have been calculated for the same set of molecules using standard wave function and basis-set levels of ab initio theory. The accuracy of the calculated constants has been established by carrying out a statistical analysis of the deviations with respect to experiment. The largest errors for bond distances and harmonic frequencies calculated at the core-corrected CCSD(T)/cc-pV6Z level are 0.4 pm and 13.4 cm⁻¹, respectively. Much smaller errors occur for the anharmonic contributions.