Development of many-body electronic structure methods for ground- and excited-state properties of strongly correlated molecules and periodic solids Open Access

Abstract

This dissertation develops many-body electronic structure methods for strongly correlated molecules and periodic solids, with a focus on ground- and excited-state properties that are difficult to describe with single-reference theories. The molecular part of the work builds on the driven similarity renormalization group (DSRG) and internally contracted multireference coupled-cluster formalisms. First, cumulant-truncated DSRG-MRPT2 approximations are introduced to reduce the memory cost associated with high-order active-space density cumulants. These approximations preserve the intruder-free character of the DSRG and give smooth potential energy curves and accurate energy differences for challenging systems. Second, an equation-of-motion extension of internally contracted multireference unitary coupled-cluster theory is formulated. This approach gives size-intensive excitation energies and accurate potential energy curves, while also clarifying the formal and computational challenges that arise in internally contracted multireference excited-state theories. The dissertation then develops equation-of-motion DSRG methods for ionization and X-ray spectroscopy. The IP-EOM-DSRG formalism provides accurate valence ionization potentials and spectroscopic constants for molecules with strongly correlated ground states. Its core-valence-separated extension, CVS-IP-EOM-DSRG, enables the computation of core-ionization energies and the simulation of vibrationally resolved X-ray photoelectron spectra. Benchmarks show that perturbative third-order DSRG and linearized DSRG truncations offer a practical balance of accuracy and computational cost. Finally, this dissertation addresses correlated calculations in periodic solids. An optimized parallel implementation of periodic CCSD and EOM-CCSD is used to compute cohesive energies and band gaps for simple semiconductors and insulators near the thermodynamic limit. These calculations quantify basis-set and finite-size errors and provide converged coupled-cluster benchmarks for periodic electronic structure methods.

Table of Contents

1) Introduction

2) Many-body electronic structure theory

3) Intruder-free cumulant-truncated DSRG-MRPT2 theory

4) The EOM-ic-MRUCC theory

5) The IP-EOM-DSRG formalism for ionization potentials

6) The CVS-IP-EOM-DSRG formalism for simulating X-ray photoelectron spectra

7) Reaching the thermodynamic limit of periodic CCSD cohesive energies and band gaps with denser Brillouin zone sampling

8) Conclusions and Outlook

About this Dissertation

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School	Laney Graduate School
Department	Chemistry
Degree	Ph.D.
Submission	Dissertation
Language	English
Research Field	Chemistry, Physical
Keyword	Theoretical Chemistry Electronic Structure Theory Quantum Chemistry
Committee Chair / Thesis Advisor	Evangelista, Francesco A., Emory University
Committee Members	Liu, Fang, Emory University Ribeiro, Raphael F., Emory University

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