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Development and applications of Quasi-Variational Coupled-Cluster theory

Black, Joshua 2017. Development and applications of Quasi-Variational Coupled-Cluster theory. PhD Thesis, Cardiff University.
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Abstract

The Quasi-Variational (QV) family of methods are a set of single-reference algorithms that can be used to investigate multireference systems with large nondynamic correlation effects. Within this current work, the Quasi-Variational Coupled Cluster Doubles (QVCCD) equations are derived and implemented into Molpro’s Integrated Tensor Framework (ITF), to produce fast and efficient code. This code, coupled with a new orbital optimisation implementation, is used to calculate potential energy curves for third-row diatomic molecules. In contrast to Traditional Coupled-Cluster methods, the QV methods are able to correctly describe the dissociation of these molecules. QV and several other single-reference methods are also applied to 5 chemical databases comprising of 88 unique reactions. From this, the activation and reaction energies are determined and contrasted. The QV methods produce larger activation energies that may correct the shortcomings of the perturbative triples correction. These results also include a new QV method with n ‘asymmetricrenormalised’ triples correction. The numerical results show there is little difference between this procedure and ‘symmetric-renormalised’ triples. Currently, only closed-shell QVCCD programs exist. Unrestricted QVCCD equations are derived and presented in the hope that this will facilitate the realisation of an open-shell QVCCD program. Finally, calculating the rate of a chemical reaction is of fundamental importance to chemistry. Knowledge of how quickly a reaction proceeds allows for an understanding of macroscopic chemical change. Rate constants are calculated with the on-the-fly Instanton method. In contrast to semi-classical Transition State Theory, the Instanton method incorporates quantum effects like atomic tunnelling into its rate constants. The effects of hydrogen tunnelling are examined for a reaction involving a Criegee intermediate. It is discovered that tunnelling does play a role in the reaction rate and may increase it by a factor of 1000. Combination of the Instanton calculations with the QV methods are discussed.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Chemistry
Subjects: Q Science > QD Chemistry
Date of First Compliant Deposit: 10 October 2017
Last Modified: 20 Apr 2021 11:06
URI: http://orca.cardiff.ac.uk/id/eprint/105353

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