Browsing by Author "Sharma, Prachi"

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    Advances in Multiconfiguration Pair Density Functional Theory and Applications to Excited State, Singlet-Triplet Gaps and Magnetism
    (2020-11) Sharma, Prachi
    Computational chemistry methods based on quantum mechanics play a fundamental role in explaining and predicting the properties of atoms and molecules such as excitation energies, electronic spectra, magnetism, characterization of chemical bonds, catalysis etc. It is therefore important to be able to correctly describe the interactions between electrons, also known as electron correlation, whose description is a major challenge for modern electronic structure methods. The correct description of electron correlation has been a major challenge in the development of these methods.A recent computational method called multiconfiguration pair-density functional theory (MC-PDFT), developed by Gagliardi and Truhlar groups, can be used to accurately account for electron correlation effects in multireference systems. The advantage of MC- PDFT is that it can accurately describe molecular systems and chemical reactions requiring less computational time and memory than other multireference methods, like, for example, multireference perturbation theory (CASPT2). In this work, MC-PDFT is applied to study excited states and magnetism. MC-PDFT is extended to use density matrix renormalization group wavefunction as reference to study large systems such as singlet-triplet gaps in polyacenes and polyenes. Finally, the challenges in MC-PDFT have been explored and active-space dependence of MC-PDFT energy and other components is investigated.
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    Data for Crystal-Chemical Origins of the Ultrahigh Conductivity of Metallic Delafossites
    (2023-11-09) Zhang, Yi; Tutt, Fred; Evans, Guy N; Sharma, Prachi; Haugstad, Greg; Kaiser, Ben; Ramberger, Justin; Bayliff, Samuel; Tao, Yu; Manno, Mike; Garcia-Barriocanal, Javier; Chaturvedi, Vipul; Fernandes, Rafael M; Birol, Turan; Seyfried Jr, William E; Leighton, Chris; leighton@umn.edu; Leighton, Chris; Leighton Electronic and Magnetic Materials Lab
    Despite their highly anisotropic complex-oxidic nature, certain delafossite compounds (e.g., PdCoO2, PtCoO2) are the most conductive oxides known, for reasons that remain poorly understood. Their room-temperature conductivity can exceed that of Au, while their low-temperature electronic mean-free-paths reach an astonishing 20 um. It is widely accepted that these materials must be ultrapure to achieve this, although the methods for their growth (which produce only small crystals) are not typically capable of such. Here, we first report a new approach to PdCoO2 crystal growth, using chemical vapor transport methods to achieve order-of-magnitude gains in size, the highest structural qualities yet reported, and record residual resistivity ratios (>440). Nevertheless, the first detailed mass spectrometry measurements on these materials reveal that they are not ultrapure, typically harboring 100s-of-parts-per-million impurity levels. Through quantitative crystal-chemical analyses, we resolve this apparent dichotomy, showing that the vast majority of impurities are forced to reside in the Co-O octahedral layers, leaving the conductive Pd sheets highly pure (~1 ppm impurity concentrations). These purities are shown to be in quantitative agreement with measured residual resistivities. We thus conclude that a previously unconsidered “sublattice purification” mechanism is essential to the ultrahigh low-temperature conductivity and mean-free-path of metallic delafossites. This dataset contains all digital data in the published paper of the same name.