Benchmark calculations with correlated molecular wave functions. VIII. Bond energies and equilibrium geometries of the CHn and C2Hn (n=1–4) series

Abstract

Using systematic sequences of correlation consistent basis sets, the accuracy of calculated bond energies D_e(CH) and equilibrium geometries (r_e, θ_e) has been investigated for the CH_n and C₂H_n series (n=1–4). Perturbation theory (MP2, MP3, MP4), coupled cluster [CCSD, CCSD(T)], and single and multireference configuration interaction (HF+1+2, CAS+1+2) methods have been investigated. Except for the vinyl radical, all of the calculated bond energies showed significant basis set dependence with average errors (standard deviations) of 5.6 (±3.0) kcal/mol for the cc-pVDZ set, 1.4 (±0.8) kcal/mol for the cc-pVTZ set, and 0.5 (±0.4) kcal/mol for the cc-pVQZ set with CCSD(T) wave functions. For the vinyl radical the total variation with basis set was just 0.6 kcal/mol. Strong basis set dependence was also observed for the equilibrium geometries, e.g., for r_e(CH) the average error decreased from 0.020 Å (cc-pVDZ) to 0.003 Å (cc-pVTZ) to 0.002 Å (cc-pVQZ). The effect of including the core electrons in the correlated calculations was also investigated for the two series. Inclusion of core correlation in the CH_n series increased D_e(CH) by 0.13 (CH) to 0.61 kcal/mol (CH₂) and decreased the equilibrium CH bond lengths by approximately 0.0015 Å. For the C₂H_n series, correlation of the core electrons increased D_e(CH) by 0.18 (C₂H₄) to 1.01 (C₂H) kcal/mol, but decreased D_e(CH) in C₂H₂ by 0.25 kcal/mol. Predictions are also made for the equilibrium geometries of C₂H, H₂CC, and C₂H₃, as well as the CH bond strength of vinylidene and the acetylene–vinylidene isomerization energy.