Files and links (1)
HRP_thesis_8august2.41 MB
Embargoed Access, Embargo ends: 08/28/2026
Dissertation
Finite temperature electronic structure methods and applications to astronomically relevant molecules
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Summer 2025
DOI: 10.25820/etd.008083
Abstract
The work presented in this thesis uses quantum Monte Carlo (QMC) methods, which solve the electronic Schrodinger equation via an exact-on-average, stochastic methodology. QMC methods are a specific niche in electronic structure theory, and can provide highly accurate energies at the ground state (T=0) and at elevated electronic temperatures. The goal of the work presented here is to expand the scope of density matrix quantum Monte Carlo (DMQMC), a finite temperature electronic structure method, for applications that involve modeling the bond dissociation at elevated temperatures, such as astrochemical spectroscopy.
DMQMC uses the thermal density matrix to calculate properties of systems at finite temperature. Prior to the work performed in this thesis, it had not yet been applied to molecular \emph{ab initio} Hamiltonians. First, I show that DMQMC has the same success with molecules representing different chemical regimes, as it did with model Hamiltonians. I then demonstrate how electronic temperature effects the dissociation of small molecules through an analysis of the free energy dissociation curve. Using the thermal density matrix, I analyze how the electronic structure of diatomic molecules changes as the temperature rises and the bond is stretched.
After demonstrating that DMQMC can be used to elucidate effects of finite temperature across the dissociation curve, I then move towards applications. I first show that by using density functional theory embedding with the ground state version of DMQMC, full configuration interaction quantum Monte Carlo (FCIQMC), the computational cost needed to perform high accuracy calculations can be decreased. I show that the FCIQMC-in-DFT embedding method is successful in calculating the correlation energy contribution to the dissociation energy of diatomic molecules physisorbed on benzene, and performs very well at long bond lengths when applied to the bond breaking of a diatomic molecule. Finally, I show how the finite temperature methods developed in this thesis can be applied to diatomic molecules with astronomic relevance. I calculate the free energy dissociation curves for six molecules, analyze how the dissociation energy changes with temperature, and present a comparison of the electronic partition function to literature values.
Overall, this thesis lays the ground work for DMQMC being a competitive method in the study of the effects of finite electronic temperature on molecular systems.
Details
- Title: Subtitle
- Finite temperature electronic structure methods and applications to astronomically relevant molecules
- Creators
- Hayley R. Petras
- Contributors
- James J. Shepherd (Advisor)Scott R. Daly (Advisor)Renée S. Cole (Committee Member)Michael J. Schnieders (Committee Member)Alexei V. Tivanski (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Chemistry
- Date degree season
- Summer 2025
- DOI
- 10.25820/etd.008083
- Publisher
- University of Iowa
- Number of pages
- xvii, 200 pages
- Copyright
- Copyright 2025 Hayley R. Petras
- Grant note
The work shown in Chapter 2 and Chapter 4 were funded by University of Iowa. The work in Chapter 3 and Appendix A were funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences Early Career Research Program (ECRP) under Award Number DE-SC0021317. The research in Chapter 3 also used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 (computer time for calculations only) using NERSC award BES-ERCAP0019952. The work shown in Chapter 5 was supported in part by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0025900.
(ii)The work of FDM was performed under the auspices of the U.S. Department of Energy (DOE) by LLNL under Contract No. DE-AC52-07NA27344.
(41)This research was supported by the Nanoporous Materials Genome Center, funded by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences under Award DE-FG02- 17ER16362.
(103)The research was also supported in part by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0025900.
(126)- Language
- English
- Date submitted
- 07/29/2025
- Description illustrations
- Illustrations, tables, graphs, charts
- Description bibliographic
- Includes bibliographical references (pages 131-188).
- Public Abstract (ETD)
The study of materials and molecules at extremely high temperature and pressures is referred to as the field of warm dense matter. This type of matter primarily exists outside of our planet, in astronomical bodies such as exoplanets and stars. These conditions are very elusive to us, and as such this area of research has drawn much interest due to its challenging nature to study.
Extreme temperatures are near impossible to study in laboratories, therefore there is a need to model with computers how temperature affects molecules and chemical reactions. This area of research is still new, and there are ongoing developments taking place. In this thesis, I study how electrons in molecules and reactions behave at high temperatures, and show my developments for better understanding these temperature dependent phenomena and measurements.
- Academic Unit
- Chemistry
- Record Identifier
- 9984948738902771
Metrics
18 Record Views