Browsing by Subject "excited states"

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    Developing a Model Chemistry for Multiconfiguration Pair-Density Functional Theory to Study Photochemistry and Molecular Interactions
    (2021-01) Bao, Jie
    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.
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    Projection-based Quantum Embedding for Excited States in Molecules and Solids
    (2020-12) Wen, Xuelan
    Kohn-Sham density functional theory (KS-DFT, hereafter referred to by DFT) has been widely used to study the electronic structure of molecules and solids due to its affordable computational cost and satisfactory accuracy when functional is carefully chosen. However, DFT can be deficient in treating systems with multireference characters, such as broken bonds, conjugated bonds, and transition metals. In solids states, it is also well-known that pure DFT severely underestimates the band gaps for materials. On the other hand, correlated wavefunction (WF) methods are systematically improvable and provide consistent accuracy for various systems, but at the cost of a steep increase in computational scaling.Projection-based quantum embedding methodologies provide a framework for performing DFT-in-DFT and WF-in-DFT calculations. A total system is divided into two or more subsystems, and each subsystem is solved with the others’ embedding potential. The WF-in-DFT embedding calculations enable us to treat large complex systems at the WF level of accuracy and the DFT level of computational cost. This thesis discusses the development and application of projection-based quantum embedding to study the excited states in molecules and solids. Except for the theoretical works on quantum embedding, this thesis also includes several computational works collaborated with experimentalists from different chemistry fields. Computational modelings help us understand the reaction mechanisms and experimental observable.