were performed on both bent and linear HCNO structures using 473 density functional theory (DFT) methods, implemented in the Psi4 software package. The large C, N, O [7s 4p 3d 2f 1g] and H [4s 3p 2d 1f] def2-QZVP basis set was utilized to eliminate any concerns over basis set deficiencies. Selection of this basis set was informed by recent and comprehensive DFT assessments by Goerigk and coworkers. All DFT methods employed are specified in the Supplementary Information (SI), where in each case references are provided and proper functional names are matched with keywords in the Psi4 manual. Electron repulsion integrals were approximated using density fitting, which is the default for DFT computations in Psi4. Preliminary testing was performed on 26 functionals (denoted with an asterisk in SI) that were selected based on performance recommendations or popularity. This preliminary set of functionals was assessed at three different (radial, spherical) integration grid settings: (75, 302), (99, 590), and (175, 974). The sensitivity of HCNO optimum geometries and harmonic vibrational frequencies to both functional selection and computational parameters dictated use of the finest available grid size (175, 974), as well as an energy convergence for geometry optimizations of 10 -8 Eh and an even tighter underlying SCF convergence of at least 2 x 10 -10 Eh. Structures and frequencies were also computed for HCN with all functionals to yield reference data. Finally, the DFT methods were rigorously tested by application to the reaction energy of HCN + O( 3 P) ® HCNO, which is a convenient route for determining the enthalpy of formation of fulminic acid.