Seth Olsen and I have just finished a paper An Effective Hamiltonian for Symmetric Diarylmethanes from a Series of Analogous Quantum Chemical Models.
Finding simple effective Hamiltonians for classes of complex chemical systems is not easy. Justifying them from quantum chemistry is even harder. We consider a family of organic dye molecules related to Michler's hydrol blue, including auramine-O and malachite green. These molecules are increasingly being used as sensors of the local environment in biomolecules.
The three lowest singlet states can be described by a 3x3 matrix Hamiltonian whose parameters vary across the family of the dyes. This variation appears to be correlated with an empirical parameter [Brown-Okamoto] used to characterise the effect of substituents. There is also a subtle and interesting variation in the character of the diabatic states as one traverses the family of dyes.