If you are interested in multireference methods and/or relativistic effects, here is a new paper for you: "Perturbational treatment of spinorbit coupling for generally applicable highlevel multireference methods" in J. Chem. Phys. What we did is taking the existing spinorbit CI code in Columbus and extended it for quasidegenerate perturbation theory, which is in fact just a fancy way of saying that we stop the MRCI after the first iteration (using the nonrelativistic solutions as initial guesses). Besides that we needed an interface translating the CI vectors between the nonrelativistic and relativistic representations.
With this tool we could compare the perturbational treatment with the full SOCI. The agreement of the relative energies was quite good. But there was a significant difference in the total energies, since spin polarization was missing in the perturbational model space. But this was a systematic error affecting all states more or less the same.
The main reason why we wanted the perturbational approach is that it allows for the computation of gradients and nonadiabatic interactions (assuming that the spinorbit couplings are slowly varying). And then we can do nonadiabatic dynamics with it. So far the methodology is implemented in SHARC with an application in this paper.
And finally, since they always look cool, a representation of the Shavitt graph coding the SOCI configurations:
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