Bao, Jie2021-04-202021-04-202021-01https://hdl.handle.net/11299/219412University of Minnesota Ph.D. dissertation.January 2021. Major: Chemistry. Advisor: Donald Truhlar. 1 computer file (PDF); xii, 155 pages.Photochemical reaction, which starts by exciting a system into an electronically excited state, is ubiquitous, for example, in the atmosphere. This has made photochemical reactions a very interesting topic. Multiconfigurational pair-density functional theory (MC-PDFT) is a powerful and efficient method for studying photochemical processes. This method has proved very efficient compared with other wave function methods, such as multi-state complete active space second order perturbation theory (MS-CASPT2), especially for large systems. Successful as MC-PDFT is, there are some limitations that stop MC-PDFT from being applied to studying photochemistry problems. The first limitation is that, like other multireference methods, the performance of MC-PDFT depends on the quality of the reference wave function, which by convention is optimized by an active-space method, such as complete active space self-consistent field (CASSCF). The second limitation is that MC-PDFT is a single-state method that does not include state interaction between reference states. This means that MC-PDFT gives wrong topologies of potential energy surfaces, which are important in studying photochemical reactions. My work is focused on resolving these two limitations. We proposed the ABC scheme and the ABC2 scheme to automatically generate the active space that gives good-quality reference wave functions thus successfully reproducing vertical excitation energies obtained from experiments or high-level calculations. We proposed the extended multi-state PDFT (XMS-PDFT) and compressed-state multi-state PDFT (CMS-PDFT) as two options to introduce state-interaction in pair-density functional theory. Among two methods, XMS-PDFT is more efficient, while CMS-PDFT is more robust. Both methods proved successful in providing correct topologies of potential energy surfaces for a variety of systems.endensity functional theoryelectronic structure theoryexcited statespotential energy surfacestheoretical chemistryThesis or Dissertation