High-Multiplicity Natural Orbitals in Multireference Configuration Interaction for Excited State Potential Energy Surfaces

We have previously proposed the use of natural orbitals obtained from a single reference correlated calculation describing a high-multiplicity state (HMNOs), as a substitute for complete active space self-consistent field (CASSCF) molecular orbitals, in the calculation of singlet excited states using the multi-reference configuration interaction (MRCI) method. In our previous work, we found that the MRCI/HMNO method reliably reproduces vertical excitation energies that can be obtained from the standard MRCI/CASSCF procedure for several representative molecules with an average absolute difference of ca. 0.1 eV. In this paper, we test how the MRCI/HMNO method performs when used to generate excited state potential energy surfaces of three sample systems: ethylene, cyclohexadiene, and uracil. We find that the MRCI/HMNO method gives potential energy surfaces for valence excited states that are in good agreement with those generated with the analogous MRCI/CASSCF method, although there are some cases where special care is needed to avoid discrepancies between the two approaches.