Here are a few choice quotes:
A simple analysis shows that significant nonadiabatic corrections to energy levels should occur only when the affected vibrational frequency is large enough to be of comparable magnitude to the energy gap involved in the coupling.
The results provide evidence that nonadiabatic corrections should be given as much weight as issues such as high-level electron correlation, relativistic corrections, etc., in quantum chemical calculations of energy levels for radicals with close-lying and strongly coupled electronic states even in cases where conical intersections are not obviously involved.
One would be tempted to prepare a manuscript based on this result, but such an action would be premature.
Well. [Yes. This is a sentence in the paper!]One thing of particular interest to me is that the adiabatic potential energy curves of both the ground and lowest excited state can be described very accurately by the eigenvalues of a two-state effective Hamiltonian [ascribed here to Koppel, Domcke, and Cederbaum]. This is identical [modulo a 45 degree rotation] to the Hamiltonian considered in a recent paper by Laura McKemmish, Jeff Reimers, Noel Hush, and myself.
The paper makes no mention of the "twin state" concept or how the vibrational frequency is exalted in the excited state relative to the ground state.
This article was one of the 20 most downloaded articles from Journal of Chemical Physics this month. I found it encouraging that people aren't just reading papers about the latest DFT functional recipe book!