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You searched for subject:(multireference perturbation theory). Showing records 1 – 3 of 3 total matches.

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1. Timian, Eric. Relativistic Multireference Perturbation Theory With Applications To D And F Block Metal Systems.

Degree: PhD, Chemistry, 2018, University of North Dakota

Electronic structure theory programs strive to be as widely applicable as possible. In order to account for effects exhibited by heavier elements, relativistic considerations must be incorporated into these programs. The methods developed in recent years generally succeed in describing the relativistic nature of systems containing heavier elements with reasonable accuracy, but have limited application due to their complexity and computational demand. Highly correlated systems exhibiting significant relativistic effects remain as a challenge to quantum chemical methods. In this thesis, I present the application of a well-defined relativistic Hamiltonian to a high-level electronic structure theory to generate a relativistic variant of a high-level multireference electronic structure theory capable of obtaining accurate results for highly correlated relativistic systems. This theory applies the exact two-component (X2C) relativistic Hamiltonian and a third-order Douglass-Kroll-Hess (DKH3) transformation for the spin-free and spin-orbit terms, respectively. The spin-orbit integrals are contracted into an effective one-electron Hamiltonian using the atomic mean field (AMFI) approximation, which increases computational efficiency with little loss in accuracy. By applying this scheme to the second-order generalized van Vleck perturbation theory (GVVPT2), which offers appropriate treatment of electron correlation, a theory providing an accurate analysis of chemical systems with strong relativistic effects is obtained. The method developed in this work is used to explore ground and low-lying excited states of the lanthanide dimer systems Gd2 and Dy2. Results from scalar relativistic studies show that GVVPT2 can accurately characterize these systems. The ground electronic states obtained (Gd2: 19Σg- ; Dy2: 11Πg) match literature and theoretical results. The spectroscopic data obtained for the ground state of Gd2 (Re = 2.826 Å; De = 2.48 eV; ωe = 153.0 cm-1) are in excellent agreement with literature values (Re = 2.877 Å; D0 = 2.1 ± 0.7 eV; ωe = 138-149 cm-1). Inclusion of spin-orbit coupling in these studies is expected to improve the results to agree with literature values to within chemical accuracy. Future work is planned to extend this method to transition metal trimers. Advisors/Committee Members: Mark Hoffmann.

Subjects/Keywords: Chemistry; Dimer; Lanthanide; Multireference; Perturbation Theory; Relativistic

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APA · Chicago · MLA · Vancouver · CSE | Export to Zotero / EndNote / Reference Manager

APA (6th Edition):

Timian, E. (2018). Relativistic Multireference Perturbation Theory With Applications To D And F Block Metal Systems. (Doctoral Dissertation). University of North Dakota. Retrieved from https://commons.und.edu/theses/2365

Chicago Manual of Style (16th Edition):

Timian, Eric. “Relativistic Multireference Perturbation Theory With Applications To D And F Block Metal Systems.” 2018. Doctoral Dissertation, University of North Dakota. Accessed September 23, 2019. https://commons.und.edu/theses/2365.

MLA Handbook (7th Edition):

Timian, Eric. “Relativistic Multireference Perturbation Theory With Applications To D And F Block Metal Systems.” 2018. Web. 23 Sep 2019.

Vancouver:

Timian E. Relativistic Multireference Perturbation Theory With Applications To D And F Block Metal Systems. [Internet] [Doctoral dissertation]. University of North Dakota; 2018. [cited 2019 Sep 23]. Available from: https://commons.und.edu/theses/2365.

Council of Science Editors:

Timian E. Relativistic Multireference Perturbation Theory With Applications To D And F Block Metal Systems. [Doctoral Dissertation]. University of North Dakota; 2018. Available from: https://commons.und.edu/theses/2365

2. Hicks, Jason M. Computational Studies Of Oxides Relevant To Clean Energy, Catalytic Processing Of Renewables, And Biological Systems.

Degree: PhD, Chemistry, 2018, University of North Dakota

Computational chemistry has grown into a large field and is continuing to grow every year in both number and variety of applications. This dissertation will give a few such applications relevant to cleaner energy production from coal, catalytic degradation of renewable agricultural and forest waste into valuable chemicals, and extending the reach of electronic structure methods to systems of biological and macromolecular interest. The first two studies presented in this dissertation are concerned with the remediation of trace elements released into the environment through the combustion of coal for power production. In flue gases, arsenic and antimony exists most often as oxides. Despite the prevalence and importance of remediating these oxides, critical information on the thermodynamics of plausible intermediates and transition states in reaction pathways have been missing prior to these studies. Several of the intermediates, and essentially all transition states, were found to be electronically multiconfigurational for the arsenic oxides. In this work, the electronic structures of several oxides of arsenic, AsxOy, where x = 1, 2 and y = 1-5, were investigated using the second-order generalized van Vleck variant of multireference perturbation theory (GVVPT2), using the cc-pVTZ basis set, with comparison to multi-reference configuration interaction (MRCISD) and the linked completely renormalized coupled cluster through perturbative triple excitations (CR-CCSD(T)L or CR-CC(2,3)) when relevant. Calculated oxidation reaction energies for the formation of AsO2 and AsO3 from AsO were predicted to be energetically favorable and formation energies of the lowest energy compounds containing two metalloid atoms, called dimers for brevity, from the monomers were also predicted to be energetically favorable. The energetics of the monomers, five isomers of As2O3 and eleven isomers of As2O5 were characterized using a composite methodology along with the key transition states between the isomers. Geometry optimizations as well as harmonic vibrational frequencies of AsxOy were obtained at the B3LYP/6-311G* level of theory and gave satisfactory agreement with experimental data when available. It was discovered that several isomers of As2O3 and As2O5 have comparable energies and relatively low barrier heights. Therefore, we expect these isomers to be chemically relevant. The antimony oxides were also found to be electronically multiconfigurational. The electronic structures of several antimony oxides, SbxOy, where x = 1, 2 and y = 1-5, were investigated using GVVPT2 and the SBD-aug-cc-pVTZ basis set. The oxidation reaction energies of elemental antimony toward the formation of SbO and SbO2 was found to be energetically favorable, while the further oxidation of those species to SbO3 was found to be unfavorable. It was found that the accretion of the monomers into Sb2O3 was highly energetically favorable at both the B3LYP/SBD-aug-cc-pVTZ and GVVPT2/SBD-aug-cc-pVTZ//B3LYP/SBD-aug-cc-pVTZ levels of theory. However,… Advisors/Committee Members: Mark R. Hoffmann.

Subjects/Keywords: Cu2O2; electronic structure theory; GVVPT2; lignin; metalloid oxides; multireference perturbation theory

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APA · Chicago · MLA · Vancouver · CSE | Export to Zotero / EndNote / Reference Manager

APA (6th Edition):

Hicks, J. M. (2018). Computational Studies Of Oxides Relevant To Clean Energy, Catalytic Processing Of Renewables, And Biological Systems. (Doctoral Dissertation). University of North Dakota. Retrieved from https://commons.und.edu/theses/2232

Chicago Manual of Style (16th Edition):

Hicks, Jason M. “Computational Studies Of Oxides Relevant To Clean Energy, Catalytic Processing Of Renewables, And Biological Systems.” 2018. Doctoral Dissertation, University of North Dakota. Accessed September 23, 2019. https://commons.und.edu/theses/2232.

MLA Handbook (7th Edition):

Hicks, Jason M. “Computational Studies Of Oxides Relevant To Clean Energy, Catalytic Processing Of Renewables, And Biological Systems.” 2018. Web. 23 Sep 2019.

Vancouver:

Hicks JM. Computational Studies Of Oxides Relevant To Clean Energy, Catalytic Processing Of Renewables, And Biological Systems. [Internet] [Doctoral dissertation]. University of North Dakota; 2018. [cited 2019 Sep 23]. Available from: https://commons.und.edu/theses/2232.

Council of Science Editors:

Hicks JM. Computational Studies Of Oxides Relevant To Clean Energy, Catalytic Processing Of Renewables, And Biological Systems. [Doctoral Dissertation]. University of North Dakota; 2018. Available from: https://commons.und.edu/theses/2232

3. Li, Run. Theory And Application Development Of Electronic Structure Methods Involving Heavy Computation.

Degree: PhD, Chemistry, 2017, University of North Dakota

The propargyl radical, the most stable isomer of C3H3, is very important in combustion reactions. However, theoretical calculations have never been able to find a strong absorption around 242 nm as seen in experiments. In this study, we calculated the electronic energy levels of the propargyl radical using highly accurate multireference methods, including multireference configuration interaction singles and doubles method with triples and quadruples treated perturbatively [denoted as MRCISD(TQ)], as well as second and third order generalized Van Vleck perturbation theories (GVVPT2 and GVVPT3). Calculations indicate that this absorption can be solely attributed to a Franck-Condon-allowed transition from the ground B1 state to the Rydberg-like first A1 excited state. Calculations also show that GVVPT2 with a relatively small active space fails to capture enough Rydberg character of this excited state, while it can be recovered by GVVPT3, MRCISD, and MRCISD(TQ). In order to speed up MRCISD(TQ) calculations, the triple and quadruple (TQ) perturbative corrections, the most time-consuming part of MRCISD(TQ) calculations, were parallelized using Message Passing Interface (MPI). The MRCISD(TQ) method is organized in the framework of macroconfigurations, which allows the use of incomplete reference spaces and provides an efficient means of screening large number of non-interacting configuration state functions (CSFs). The test calculations show that the parallel code achieved close to linear speed-up when the number of CSFs in each macroconfiguration is small. The speed-up suffers when large numbers of CSFs exist in only a few macroconfigurations. The computer algorithm for second-order generalized van Vleck multireference perturbation theory (GVVPT2) was similarly parallelized using the MPI protocol, organized in the framework of macroconfigurations. The maximum number of CSFs per macroconfiguration is found to have less influence on the MPI speedup and scaling than in the case of MRCISD(TQ). It was previously found that unrestricted local density approximation (LDA) orbitals can be used in place of MCSCF to provide orbitals for GVVPT2. This inspired us to use the more controllable restricted density functional theory (DFT) to provide unbiased orbitals for GVVPT2 calculations. In this study, the relationship between restricted DFT and unrestricted DFT were explored and the restricted DFT results were obtained by utilizing subroutines from unrestricted DFT calculations. We also found that the DIIS technique drastically sped up the convergence of RDFT calculations. Plane wave DFT methods are commonly used to efficiently evaluate solid state materials. In this work, the electronic properties of pristine graphene and Zn-phthalocyanine tetrasulfonic acid (Zn-PcS) physisorbed on single-layer graphene were calculated using plane wave DFT. The Perdew-Burke-Ernzerhof functional with dispersion correction (PBE-D2) was used. The densities of states were… Advisors/Committee Members: Mark R. Hoffmann.

Subjects/Keywords: electronic structure theory; GVVPT2; MPI; MRCISD(TQ); multireference perturbation theory; parallelization

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APA · Chicago · MLA · Vancouver · CSE | Export to Zotero / EndNote / Reference Manager

APA (6th Edition):

Li, R. (2017). Theory And Application Development Of Electronic Structure Methods Involving Heavy Computation. (Doctoral Dissertation). University of North Dakota. Retrieved from https://commons.und.edu/theses/2269

Chicago Manual of Style (16th Edition):

Li, Run. “Theory And Application Development Of Electronic Structure Methods Involving Heavy Computation.” 2017. Doctoral Dissertation, University of North Dakota. Accessed September 23, 2019. https://commons.und.edu/theses/2269.

MLA Handbook (7th Edition):

Li, Run. “Theory And Application Development Of Electronic Structure Methods Involving Heavy Computation.” 2017. Web. 23 Sep 2019.

Vancouver:

Li R. Theory And Application Development Of Electronic Structure Methods Involving Heavy Computation. [Internet] [Doctoral dissertation]. University of North Dakota; 2017. [cited 2019 Sep 23]. Available from: https://commons.und.edu/theses/2269.

Council of Science Editors:

Li R. Theory And Application Development Of Electronic Structure Methods Involving Heavy Computation. [Doctoral Dissertation]. University of North Dakota; 2017. Available from: https://commons.und.edu/theses/2269

.