Adiabatic process

Physics

Ab initio quantum chemistry methods

Mathematics

Mathematical optimization

Diabatic

Hamiltonian (control theory)

Computational chemistry

Vibronic coupling

Chemistry

Scattering

Triatomic molecule

Quantum chemistry

Ab initio

Molecule

Born–Oppenheimer approximation

Quantum mechanics

Jahn–Teller effect

Potential energy

Supramolecular chemistry

International Reviews in Physical Chemistry

We review our development on beyond Born–Oppenheimer (BBO) theory and its implementation on various models and realistic molecular processes as carried out over the last 15 years. The theoretical formulation leading to the BBO equations are thoroughly discussed with ab initio calculations. We have employed first principle based BBO theory not only to formulate single surface extended Born–Oppenheimer equation to understand the nature of nonadiabatic effect but also to construct accurate diabatic potential energy surfaces (PESs) for important spectroscopic systems, namely, NO2 radical, Na3 and K3 clusters, NO3 radical, benzene and 1,3,5-trifluorobenzene radical cations (C6H6+ and C6H3F3+) as well as triatomic reactive scattering systems like H++H2 and F+H2. The nonadiabatic phenomena like Jahn–Teller (JT), Renner–Teller, pseudo Jahn–Teller effects and the accidental conical intersections are the key players in dictating spectroscopic and reactive scattering profiles. The nature of diabatic coupling elements derived from ab initio data with BBO theory for molecular processes in Franck-Condon region has been analysed in the context of linearly and bilinearly coupled JT model Hamiltonian. The results obtained from quantum dynamical calculations on BBO based diabatic PESs of the above molecular systems are found to be in accord with available experimental outcomes.

Beyond Born–Oppenheimer theory for spectroscopic and scattering processes