The effect of 1s correlation on D e, r e, and ωe of first-row diatomics

A series of De, re, and ωe ab initio spectroscopic data for N2, O2, F2, CO, NO, and HF is presented, as determined via the coupled-cluster singles and doubles approach including a perturbational estimate of connected triple excitations [CCSD(T)], appropriately implemented with correlation-consistent, polarized-valence and -core one-particle basis sets varying in quality from [4s3p2d1f] to [12s6p5d4f1g] and [6s5p4d3f2g]. In these molecules, 1s correlation causes the following (De,re) shifts in (kcal mol−1, Å): N2(+0.78,−0.0021), O2(+0.24−0.0020), F2(−0.09,−0.0014), CO(+0.93,−0.0024), NO(+0.42,−0.0021), and HF(+0.18,−0.0006). Evaluation of quadratic force constants at the precise experimental re distances reveals that valence CCSD(T) treatments exhibit an inherent 0.2%–0.8% overestimation of harmonic frequencies for the closed-shell species as the one-particle limit is approached, the error for O2(X 3Σg−) and NO(X 2Π) being slightly higher. At fixed reference geometries the inclusion of 1s correlation serves to lower harmonic frequencies by 3 to 9 cm−1 and proportionately reduce the deficiency in valence treatments by 0.1%–0.4%.