Efficient implementations of quantum chemistry methods for strongly correlated electrons Open Access

Hannon, Kevin Patrick (2017)

Permanent URL: https://etd.library.emory.edu/concern/etds/h415p970k?locale=en
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Abstract

Strong correlation presents a big challenge in modern day quantum chemistry.

In many chemical systems such as open-shell species, excited states, and transition metal complexes, one electron configuration is not flexible enough to describe these systems. This thesis is concerned with two topics: developing a parallel algorithm for a zeroth order description of strong correlation and developing a fast multireference perturbation theory for acheving quantitative accuracy. We aim to develop theories that are computationally efficient, widely applicable to various areas of chemistry, and acheive quantitative accuracy. In chapter II, we report an implementation of the atomic orbital complete active space self consistent field (AO-CASSCF) method on a massively parallel computer using a combination of distributed and multicore computing. We demonstrate the scalability of the AO-CASSCF algorithm with a benchmark set of systems. In chapter III, we report an efficient implementation of a second-order multireference perturbation theory based on the driven similarity renormalization group (DSRG-MRPT2) [C. Li and F.A. Evangelista, J. Chem. Theory Comput.11, 2097 (2015)]. Our implementation employs factorized two-electron integrals to avoid storage of large four-index intermediates. It also exploits the block structure of the reference density matrices to reduce the computational cost to that of second-order MÖller--Plesset perturbation theory. Our new DSRG-MRPT2 implementation is benchmarked on ten naphthyne isomers using basis sets up to 5z quality. We find that the singlet-triplet splittings of the naphthyne isomers strongly depend on the equilibrium structures. For a consistent set of geometries, the singlet-triplet values predicted by the DSRG-MRPT2 are in good agreements with those computed by the reduced multireference coupled cluster theory with singles, doubles, and perturbative triples.

Table of Contents

1. Introduction

2. A MPI/OpenMP implementation of Atomic-Orbital Complete Active Space Self Consistent Field Method

3. An integral-factorized implementation of the driven similarity renormalization group second-order multireference perturbation theory

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