2 and H 3 O - 2 by CCSD(T), DFT, DFT-VPT2, and CNEO-DFT. The results are presented in Table . Note that the CCSD(T), DFT, and CNEO-DFT results are harmonic frequencies that are obtained from diagonalizing the mass-weighted Hessian matrices, whereas DFT-VPT2 performs VPT2 calculations at the DFT-optimized geometries, which includes anharmonicity.