The Spin Hamiltonian Effective Approach to the Vibronic Effects–Selected Cases

Here we propose and apply the use of spin Hamiltonian for the effective analytic description and fit to the computed spectrum of correspondingly selected states as a strategy to describe the potential energy surfaces and related vibronic effects in terms of physically meaningful valence bond picture. In order to achieve numerical precision and a physically consistent parameterization we introduced here new terms to the spin Hamiltonian, equated in the intercentric biquadratic form, invested with the meaning of interbond interaction, avoiding the artifact of large next nearest neighbor direct spin coupling constants in the numerical fit. This model was applied for prototypic vibronic cases: (i) the Jahn–Teller effect H3 molecule regarded as a spin-frustration topology and (ii) the bond length alternation vs. homogeneity in conjugated hydrocarbons (C4H4 and C6H6) as manifestation vs. silence, respectively, of a pseudo-Jahn–Teller effect involving states effectively described by the spin Hamiltonian.