Accuracy of spectroscopic constants of diatomic molecules from <i>ab initio</i> calculations

Abstract

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.