Snakes on the Rungs of Jacob's Ladder: Anomalous Vibrational Spectra from Double-Hybrid DFT Methods.

The computation by some double-hybrid density functionals of the vibrational modes of a number of CHNO species, including the radicals of carbonic and carbamic acids and of dimethyl carbonate, gives rise to unphysical and anomalous IR spectra with errors well in excess of 1000 cm-1. The effect is not immediately obvious since calculated entropies are largely unaffected, but by contrast, the zero point energies are significantly increased-this has not previously been documented in the literature.