A failure of double hybrid density functional method: Out‐of‐plane bending vibrations for the carbon‐carbon double‐bonded molecules

Abstract

AbstractIn this work, we report anomalous descriptions of bending modes of the carbon–carbon double bonded molecules with double hybrid density functional theories. The harmonic frequencies of the out‐of‐plane modes are found to be extremely low, and the anharmonicities are found to be very large. The unphysical large anharmonities of these modes increase significantly with a basis set having diffuse function. Increasing basis set size does not necessarily reduce or eliminate the problem. We observe that the problem with the double hybrid functional is in‐built due to the presence of the part of the second‐order perturbation terms in the functional.