- The value for the HCN → HNC 0 K isomerization energy has been investigated by combining state-of-the-art electronic structure methods with the Active Thermochemical Tables (ATcT) approach. The directly computed energy difference between HCN and HNC at the HEAT-456QP level of theory is 5236 ± 50 cm–1. This is substantially lower (by ∼470 cm–1 or ∼1.3 kcal/mol) than the recently proposed high-level multireference configuration interaction value of 5705 ± 20 cm–1 of Barber et al. (Mon. Not. R. Astron. Soc.2014, 437, 1828–1835). The discrepancy was analyzed by the ATcT approach, using several distinct steps, which (a) independently corroborated the current single-reference HEAT-456QP result, (b) independently found that the recent multireference-based value is highly unlikely to be correct within its originally stated uncertainty, and (c) produced a recommended value of 5212 ± 30 cm–1 for the HCN → HNC isomerization energy at 0 K, based on all currently available knowledge. The ATcT standard enthalpies of formation at 0 and 298 K for HCN, HNC, and their cations and anions are also presented.