DFT/MRCI Hamiltonian for odd and even numbers of electrons

Abstract

DFT/MRCI is a well-established method of Grimme and Waletzke [J. Chem. Phys. 111, 5645 (1999)] combining density functional theory and multireference configuration interaction. It was later redesigned by Lyskov, Kleinschmidt, and Marian [J. Chem. Phys. 144, 034104 (2016)] to provide a better treatment of bi-chromophores while treating all other systems as well as Grimme's version did by computing individual energy shifts for each state function of a configuration. But all previous operators lack the ability to compute states with an odd number of electrons (doublet and quartet states). Here we present a general Hamiltonian based on Lyskov's redesign which calculates excited singlet, doublet, triplet, and quartet states of systems that have up to one open shell in the parent determinant. The multiplicity-independent correction parameters provide an extra correction for the open shell in the parent determinant. The Hamiltonian in combination with two parameter sets for different selection thresholds has been tested and compared to experimental vertical excitation and ionization energies yielding similar statistics for all multiplicities with a root mean square deviation smaller than 0.2 eV while maintaining the good computational performance of the Hamiltonians of Grimme and Lyskov.