The AE-CCSDTQ(P)/CBS + MVD1 results now available for HCNO provide highaccuracy bond distances and harmonic vibrational frequencies as benchmark data for this vexing molecule with a linear equilibrium structure. Calibrations of this theoretical method against rigorous spectroscopic parameters for HCN indicate that our re and ωi benchmarks for fulminic acid are accurate to 10 -4 Å and 1.6 cm -1 , respectively. Moreover, the HCN + O( 3 P) ® HCNO reaction energy [ΔEe(rxn) = -De(HCN-O)] converged to the AE-CCSDTQP(H)/CBS + MVD1 level via a composite focal-point analysis (FPA) scheme yields a thermochemical target accurate to approximately 0.1 kcal mol -1 for a challenging homolytic bond cleavage. Our HCNO standards have special chemical significance beyond the quasilinear/quasibent issue. In particular, fulminic acid exhibits one of the shortest C-H bond lengths and highest C-H stretching frequencies known in organic chemistry, and De(HCN-O) proves important in pinpointing the enthalpies of formation of the classic [H,C,N,O] isomers. Finally, the shifts in bond distances (Δre) and vibrational frequencies (Δω) in going from HCN to HCNO are useful probes of the chemical bonding environment in these compounds. Historically, such shifts in properties are understood to be less sensitive to level of theory than the values themselves; hence, an interesting test is whether quick convergence to our benchmark values occurs as the DFT ladder is climbed.