A perturbative approximation to DFT/MRCI: DFT/MRCI(2)

Abstract

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.

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• Heat-bath Configuration Interaction: An efficient selected CI algorithm inspired by heat-bath sampling
• Semistochastic Heat-bath Configuration Interaction method: selected configuration interaction with semistochastic perturbation theory
• Excited states using semistochastic heat-bath configuration interaction