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

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Abstract
Using systematic sequences of correlation consistent basis sets, the accuracy of calculated bond energies De(CH) and equilibrium geometries (re, θe) has been investigated for the CHn and C2Hn 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 re(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 CHn series increased De(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 C2Hn series, correlation of the core electrons increased De(CH) by 0.18 (C2H4) to 1.01 (C2H) kcal/mol, but decreased De(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.

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