High-level thermochemistry for the octasulfur ring: A converged coupled cluster perspective for a challenging second-row system

Sulfur clusters are challenging targets for high-level ab initio procedures. The heat of formation of the most common and energetically stable S₈ allotrope (α-sulfur) has not been the subject of a high-level ab initio investigation. We apply the Weizmann-n computational thermochemistry protocols to the S₈ sulfur cluster. We show that calculating the heat of formation with sub-chemical accuracy requires accurate treatment of post-CCSD (T), core-valence, scalar relativistic, and zero-point vibrational energy contributions. At the relativistic, all-electron CCSDT(Q)/CBS level of theory we obtain an enthalpy of formation at 0 K of ∆_fH^◦₀ = 24.44 kcal mol^–1, and at 298 K of ∆_fH^◦₂₉₈ = 23.51 kcal mol^–1. These values suggest that the experimental values from Gurvich (∆_fH^◦₀ = 25.1 ± 0.5 kcal mol^–1) and JANAF (∆_fH^◦₀ = 24.95 ± 0.15 and ∆_fH^◦₂₉₈ = 24.00 ± 0.15 kcal mol^–1) represent overestimations and should be revised downward by 0.5–0.7 kcal mol^–1. We also show that computationally economical composite ab initio protocols such as G4, G4(MP2), and CBS-QB3 are unable to achieve chemical accuracy relative to our best CCSDT(Q)/CBS heat of formation for S₈.