A graphical unitary group approach-based hybrid density functional theory multireference configuration interaction method

A hybrid density functional theory multireference configuration interaction hybrid model (DFT/MRCI) method for computing electronic excitation energies in heavy-element atomic and molecular systems has been developed within a graphical unitary group approach (GUGA) based configuration interaction framework. Implemented in the COLUMBUS suite of programs, the method advances several new capabilities for evaluating systems involving several electronic spin states, relativistic effects and spin-orbit coupling. The method has been evaluated on several types of systems including carbon monoxide, bromine, bromine fluoride, and uranium +4, uranium +5 and uranyl 2+ ions. The method delivers a high level of experimental consistency with absolute accuracies ranging from 11% to 22%. A reduction in relative error ranging from 11% to 42% is observed for the new method when compared with time dependent density functional theory (TDDFT). The new approach provides a significant improvement in computational effort for comparable MRCI calculations with an observed reduction of expansion size by factors up to 64.