Seth Olsen has a really nice paper that just appeared ASAP online in the Journal of Chemical Theory and Computation.
A real challenge of high-level quantum chemistry is to connect it to chemical concepts, chemical trends and semi-empirical theories. This seems to rarely happen. There is a tendency to just throw ones arms up in the air and say, "It is too hard! One just has to calculate the properties of every molecule from scratch."
However, Seth has shown how correlated quantum chemical calculations for an important class of methine dyes can illuminate, clarify, and provide justification for resonance theories of dye colour that go back to the forties, including from John R. Platt.
The figure below compares excitation energies calculated ab initio with those from a parametrisation of Platt's model.
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