How the Amount of Hartree-Fock Exchange Affects the Observation of the Pseudo Jahn-Teller Effect

Abstract

Heavy group 14 analogs of planar carbon molecules are well known to prefer bent or pyramidalized geometries. The pseudo Jahn-Teller effect (pJTE) is a general approach to understanding the origin of such molecular geometries. Computationally intensive multi-reference post Hartree-Fock methods are generally required to obtain values relevant to the pJTE. However, an alternative approach, using more computationally efficient density functional theory (DFT) and fitting a model vibronic Hamiltonian, can account for the pJTE parameters. Previous studies have detailed the effect of both integration grid size and exact exchange on various properties including the nature of stationary points (i.e., transition structure v. minimum) on potential energy surfaces but none have addressed the root cause of such discrepancies. In this work we examine two contentious stationary points belonging to planar disilene and 2Si TCNQ from the perspective of the pseudo Jahn-Teller effect (pJTE) using DFT methods. First the planar stationary points are characterized using a variety of model chemistries and integration grids. The effect of the amount of Hartree-Fock exchange is then studied and the usage of DFT for assessing pJT parameters is explored. It is shown that the DFT approach is a viable alternative for determining values relevant to the pJTE. A complementary cusp catastrophe theory approach to the problem is also introduced. Difficulties associated with the DFT-based approach to the study of the pJTE are discussed and changes in the electronic structure are explored using the Quantum Theory of Atoms in Molecules.