  We introduce a perturbative approximation to the combined density functional
theory and multireference configuration interaction (DFT/MRCI) method. The
method, termed DFT/MRCI(2), results from the application of quasi-degenerate
perturbation theory and the Epstein-Nesbet partitioning of the DFT/MRCI
Hamiltonian matrix. This results in the replacement of the diagonalization of
the large DFT/MRCI Hamiltonian with that of a small effective Hamiltonian, and
affords orders of magnitude savings in terms of computational cost. Moreover,
the DFT/MRCI(2) approximation is found to be of excellent accuracy, furnishing
excitation energies with a root mean squared deviation from the DFT/MRCI values
of less than 0.03 eV for an extensive test set of organic molecules.

Chemical Physics