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

• Published in: International journal of quantum chemistry 2024-01, Vol.124 (1), p.n/a
• Main Author: Ravichandran, Lalitha
• Format: Article
• Language: English
• Subjects: anharmonic; double hybrid DFT; harmonic; vibrations
• ISSN: 0020-7608; 1097-461X
• DOI: 10.1002/qua.27303

In 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. Double hybrid functionals produce abnormally large anharmonicities for the out‐of‐plane bending (OOPB) vibrations of C‐C double‐bonded molecules. The OOPB description with double hybrid functionals follows a similar pattern of wavefunction based methods like MP2 and CCSD(T). The presence of non‐local correlation from KSMP2 introduces an error in the anharmonic calculation with double hybrid functionals.