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

TL;DR

The paper presents DFT/MRCI(2), a perturbative approximation that significantly reduces computational cost while maintaining high accuracy in excitation energy calculations for organic molecules.

Contribution

It introduces DFT/MRCI(2), a novel perturbative approach that simplifies DFT/MRCI calculations using quasi-degenerate perturbation theory, enhancing efficiency.

Findings

DFT/MRCI(2) achieves less than 0.03 eV RMS deviation from full DFT/MRCI.

The method offers orders of magnitude computational savings.

It maintains high accuracy across an extensive test set of organic molecules.

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.