A DFT/MRCI Hamiltonian Parameterized Using Only Ab Initio Data: I. Valence Excited States

A new combined density functional theory and multi-reference configuration interaction (DFT/MRCI) Hamiltonian parameterized solely using benchmark ab initio electronic structure obtained from the QUEST databases is presented. This new formulation differs from all previous versions of the method in that the choice of the underlying exchange cor- relation (XC) functional employed to construct the one-particle (orbital) basis is considered, and a new XC functional, QTP17, is chosen for its ability to generate a balanced description of core and valence vertical excitation energies. The ability of the new DFT/MRCI Hamiltonian, termed QE8, to furnish accurate excitation energies is confirmed using benchmark quantum chemistry computations, and a mean absolute error of 0.16 eV is determined for the wide range of electronic excitations included the validation data set. In particular, the QE8 Hamiltonian dramatically improves the performance of DFT/MRCI for doubly-excited states. The performance of fast approximate DFT/MRCI methods, p-DFT/MRCI and DFT/MRCI(2), are also evaluated using the QE8 Hamiltonian and are found to yield excitation ener- gies in quantitative agreement with the parent DFT/MRCI method, with the two methods exhibiting a mean difference of 0.01 eV with respect to DFT/MRCI over the entire benchmark set.