Jahn–Teller Effect in Orthorhombic Manganites: Ab InitioHamiltonian and Roto-vibrational Spectrum

For the first time, using three different electronic structure methodologies, namely, CASSCF, RS2c, and MRCI(SD), we construct ab initioadiabatic potential energy surfaces (APESs) and nonadiabatic coupling term (NACT) of two electronic states (5Eg) of MnO69–unit, where eight such units share one La atom in LaMnO3crystal. While fitting those APESs with analytic functions of normal modes (Qx, Qy), an empirical scaling factor is introduced considering the mass ratio of eight MnO69–units with and without one La atom to explore the environmental (mass) effect on MnO69–unit. When the roto-vibrational levels of MnO69–Hamiltonian are calculated, peak positions computed from ab initioconstructed excited APESs are found to be enough close with the experimental satellite transitions [J. Exp. Theor. Phys.2016, 122, 890−901] endorsing our earlier model results [J. Chem. Phys.2019, 150, 064703]. In order to explore the electron–nuclear coupling in an alternate way, theoretically “exact” and numerically “accurate” beyond Born–Oppenheimer (BBO) theory based diabatic potential energy surfaces (PESs) of MnO69–are constructed to generate the photoelectron (PE) spectra. The PE spectral band also exhibits good peak by peak correspondence with the higher satellite transitions in the dielectric function spectra of the LaMnO3complex.