Browsing by Subject "potential energy surfaces"

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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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    Electronic structure data for ³A´ and ³A´´ N₂O
    (2020-10-28) Lin, Wei; Varga, Zoltan; Song, Guoliang; Paukku, Yuliya; Truhlar, Donald G; truhlar@umn.edu; Truhlar, Donald G; University of Minnesota Department of Chemistry
    This dataset constitutes the electronic structure data that was fitted to obtain global reactive potential energy surfaces (PESs) for Born-Oppenheimer collisions of oxygen atoms with nitrogen molecules. In particular it contains data for the lowest-energy ³Α´ and ³A´´ PESs for the high-energy reaction N₂(X ¹Σ) + O(³P) → NO(X ²Π) + N(⁴S); these potential energy surfaces can serve to generate forces for dynamics calculations. The data was obtained by multireference configuration interaction (MRCI) calculations that were improved by a dynamically scaled external correlation (DSEC) term. The MRCI calculations are based on wave functions obtained from state-averaged complete active space self-consistent-field calculations for 2280 geometries for the three lowest ³A´´ states and for 2298 geometries for the three lowest ³A´ states. The lowest-energy ³A´ and ³A´´ state at each of these geometries was then improved by applying the DSEC method to all MRCI points.