A dynamic-correlation doubles correction to CASSCF linear response excitation energies

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The goal of the project is the introduction of a doubles (D) correction to CASSCF linear response (MCRPA) excitation energies. This excitation-energy correction introduces dynamical correlation in a perturbative manner and will significantly improve the accuracy of MCRPA. With the combined MCRPA(D) method, the calculation of accurate core and valence excitation energies of medium-sized open-shell transition metal complexes will be possible for both local and charge-transfer excitations. With existing methods, these calculations are currently not yet possible in a satisfactory fashion. Moreover, I will work on a formulation and implementation of a (D) correction for spin-flip transitions that occur likely in open-shell orbital-near degenerate TMC. To enable larger-active space (D) calculations, an efficient reformulation of so-called Koopmans matrices will be pursued.