Beyond Born-Oppenheimer Treatment for Multi-State Photoelectron Spectra, Phase Transitions of Solids and Scattering Processes

Abstract

The "completeness" and "accuracy" of first principle based Beyond Born-Oppenheimer (BBO) theory is reviewed with its theoretical developments and wide applications on various realistic spectroscopic systems, scattering processes and pervoskite molecules exhibiting phase transition phenomena. Over the last two decades, our group has formulated as well as implemented the BBO treatment to construct diabatic Hamiltonian for several spectroscopically interesting molecules (NO ₂ radical, Na₃ and K₃ clusters, NO₃, C₆H₆⁺,1,3,5−C₆H₃F₃⁺ and C₄N₂H₄), octahedral units of perovskites (MnO₆^9- and TiO₆^8-) involving solid state phenomenon as well as triatomic reactive scattering processes (H₃⁺, HeH₂⁺ and F+H₂) to depict the effects of electron-nuclear couplings. Such diabatic Hamiltonians have been further used to perform quantum dynamical calculations to obtain observables (photoelectron spectra for spectroscopic/solid state systems and cross-sections/rate constants for scattering processes), which show excellent agreement with the recent experimental findings. In summary, this article provides the current perspective of BBO approach as well as Jahn-Teller (JT) theory with various applications on molecular systems/chemical processes.