Exploring the Electronic Structure of Strongly Correlated Molecular Systems using Tensor Product Selected Configuration Interaction

The field of theoretical chemistry has provided undeniably useful insights about molecular systems that otherwise, through experiment, would not be obtainable. We are constantly developing new and improved methods to fill in the gaps about how various factors including the electronic structure can affect the chemistry seen experimentally. The goal of most quantum chemistry methods is to develop a method that is widely applicable, has low computational costs, but with as much accuracy as possible. Some of the most challenging systems in our field include those that are considered strongly correlated. Strong correlation is usually referring to the need for a large number of configurations to properly model the chemistry. These systems can not be solved exactly, thus various approximations must be made. A set of methods that take advantage of truncating only the unimportant configurations to solve these challenging systems are selected configuration interaction methods. Even though these selected CI methods can often provide accurate results, their general application is limited by memory bottlenecks. In 2020, our group developed the Tensor Product Selected Configuration Interaction (TPSCI) method to overcome these memory bottlenecks. We take advantage of the local character of these strongly correlated systems by doing a change of basis into tensor products, then do a selected CI algorithm in that basis. In this dissertation, we discuss how we have extended TPSCI to compute excited states. We first test on a set of polycyclic aromatic hydrocarbons that were previously studied with TPSCI. We find very high accuracy and dimension reduction as compared to state of the art selected CI approaches. We then validate TPSCI's ability to study the electronic structure involved in the singlet fission process in tetracene tetramer with extending analysis using a Bloch effective Hamiltonian. This effective Hamiltonian allows for intuitive analysis of the singlet fission process. We also show how accurate and interpretable TPSCI can be on an open-shell biradical transition bimetallic complex, in addition to, hexabenzocoronene that is not straightforward clustering due to the conjugated benzene rings. To alleviate the previous system size limitations, we recently implemented a Restricted Active Space Configuration Interaction as a local solver for clusters. We present novel results of using this new solver on a tetracene dimer. We comment on specific coupling strengths and show the electronic dynamics of our TPSCI effective Hamiltonian which support a CT-mediated mechanism for the tetracene dimer singlet fission.

Name thesis:degree_name

Doctor of Philosophy

Level thesis:degree_level

doctoral

Discipline thesis:degree_discipline

Chemistry

Department dc:contributor.department

Chemistry

Grantor dc:publisher

Virginia Tech

Year dc:date.issued

2024

Author dc:creator

• Braunscheidel, Nicole Mary

Chair dc:contributor.committeechair

• Mayhall, Nicholas

Committee members dc:contributor.committeemember

• Economou, Sophia Eleftherios
• Crawford, Daniel
• Valeyev, Eduard Faritovich

• Tensor product
• Electronic Structure Theory
• Strong Correlation
• Excited States

Statement dc:rights

• In Copyright

Licence dc:rights.uri

• http://rightsstatements.org/vocab/InC/1.0/

Language dc:language.iso

en

Dc Identifier Other

vt_gsexam:41570

OAI identifier oai:identifier

oai:vtechworks.lib.vt.edu:10919/121340