Publication Date

2024

Document Type

Dissertation/Thesis

First Advisor

Wheeler, Ralph A.

Degree Name

Ph.D. (Doctor of Philosophy)

Legacy Department

Department of Chemistry and Biochemistry

Abstract

The art of quantum chemistry is grounded to the discovery of a time efficient computational method that can capture the essential physics. Historically, the Hartree-Fock method, where an electron moves in the mean field of the other electrons, has provided the orbitals used to construct a single-reference Slater determinant and thus formed the basis for molecular orbital calculations. Kohn-Sham density functionals have become overwhelmingly popular among the chemistry community due to their low computational cost. However, their prediction of thermodynamic properties important in free radical polymerization has shown difficulty due to an artificial delocalization of electron density and spin contamination where the ground state is "contaminated" by states of higher spin. These computational artifacts can generate enthalpy errors on the order of 40 kJ/mol. To mediate these errors, configuration interaction and composite methods have been implemented to capture the necessary dynamic correlation. However, the scaling of these methods is typically on the order of N5−8, where N is the number of atoms. These correlated methods are also highly dependent on the selection of the basis set which makes scaling to large systems difficult. A stochastic approach has the potential of removing these limitations of the mean-field and composite method approaches. Instead of solving for the ground state energy numerically in a Hilbert space of basis functions, fixed-node diffusion Monte Carlo (FN-DMC) samples the electrons’ positions in real space and then imposes an exponential projection of the time dependent Schrödinger equation to approximate the true ground state wavefunction. FN-DMC has been frequently shown to be accurate in predicting solid state chemistry with excellent scaling factors, but its presence in molecular computations is more rare. In this dissertation, FN-DMC will be used to benchmark reaction enthalpies and activation barriers of free radical addition and Cu-mediated atom transfer radical polymerization (ATRP) to reveal its potential in predicting thermodynamic properties of larger, more highly correlated systems.

Extent

150 pages

Language

en

Publisher

Northern Illinois University

Rights Statement

In Copyright

Rights Statement 2

NIU theses are protected by copyright. They may be viewed from Huskie Commons for any purpose, but reproduction or distribution in any format is prohibited without the written permission of the authors.

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Text

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