Sub 20 cm^-1 computational prediction of the CH bond energy - a case of systematic error in computational...

The bond dissociation energy of methylidyne, D₀(CH), is studied using an improved version of the High-Accuracy Extrapolated ab initio Thermochemistry (HEAT) approach as well as the Feller-Peterson-Dixon (FPD) model chemistry. These calculations, which include basis sets up to nonuple (aug-cc-pCV9Z) quality, are expected to be capable of providing results substantially more accurate than the ca. 1 kJ mol^-1 level that is characteristic of standard high-accuracy protocols for computational thermochemistry. The calculated 0 K CH bond energy (27 954 ± 15 cm^-1 for HEAT and 27 956 ± 15 cm^-1 for FPD), along with equivalent treatments of the CH ionization energy and the CH⁺ dissociation energy (85 829 ± 15 cm^-1 and 32 946 ± 15 cm^-1, respectively), were compared to the existing benchmarks from Active Thermochemical Tables (ATcT), uncovering an unexpected difference for D₀(CH). This has prompted a detailed reexamination of the provenance of the corresponding ATcT benchmark, allowing the discovery and subsequent correction of a systematic error present in several published high-level calculations, ultimately yielding an amended ATcT benchmark for D₀(CH). Finally, the current theoretical results were added to the ATcT Thermochemical Network, producing refined ATcT estimates of 27 957.3 ± 6.0 cm^-1 for D₀(CH), 32 946.7 ± 0.6 cm^-1 for D₀(CH⁺), and 85 831.0 ± 6.0 cm^-1 for IE(CH).