Failure of the DFT Ladder for the Fulminic Acid Challenge

Properties of the historically pivotal fulminic acid (HCNO) molecule have been computed with a panoply of 473 density functionals of all varieties, providing a snapshot of the performance of contemporary density functional theory (DFT) for a challenging chemical system. Exhaustive tabulations and statistical analyses have been carried out for geometric parameters, vibrational frequencies, barriers to linearity, and the HCN–O dissociation energy. As the DFT ladder is climbed, only befuddlement ensues as to whether HCNO has a linear or bent equilibrium structure. The distinctive, extremely flat H–C–N bending potential of fulminic acid is almost universally misrepresented by the DFT functionals, which are thus incapable of solving quasilinear/quasibent issues of this molecule. High-ranking DFT functionals produce catastrophic errors in the HCN + O(³P) → HCNO reaction energy, and lower rungs emerge as the best performers for many of the bond distances and harmonic vibrational frequencies. This research shows that the current DFT zoo of approximations is incapable of yielding any consensus or convergence on the equilibrium structure, H–C–N bending frequencies, or enthalpy of formation of HCNO. Additional analyses are performed on the side effects of popular dispersion corrections on the covalently bonded properties and thermochemistry of HCNO.