Twisting fluorescent proteins

Seth Olsen and I just finished a paper A two-state model of twisted intramolecular charge transfer in monomethine dyes.
It presents a simple characterisation the ground and excited state potential energy surfaces associated with bond twisting of the chromophore for the green fluorescent protein. This is based on a 2 x 2 matrix Hamiltonian acting on diabatic states which have a simple valence bond interpretation.