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.