The effective Hamiltonian for the linear EâŠ-e Jahn-Teller model describes the coupling between two electronic states and two vibrational modes in molecules or bulk crystal impurities. While in the Born-Oppenheimer approximation the Berry curvature has a delta function singularity at the conical intersection of the potential energy surfaces, the exact Berry curvature is a smooth peaked function. Numerical calculations revealed that the characteristic width of the peak is K1/2/gM1/2, where M is the mass associated with the relevant nuclear coordinates, K is the effective internuclear spring constant, and g is the electronic-vibrational coupling. This result is confirmed here by an asymptotic analysis of the M→ limit, an interesting outcome of which is the emergence of a separation of length scales. Being based on the exact electron-nuclear factorization, our analysis does not make any reference to adiabatic potential energy surfaces or nonadiabatic couplings. It is also shown that the Ham reduction factors for the model can be derived from the exact geometric phase.
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