Develop multireference quantum embedding theories for chemical applications Open Access

He, Nan (Summer 2022)

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Multireference computations of large-scale chemical systems are typically limited by the computational cost of quantum chemistry methods. Quantum embedding schemes are a promising way to extend multireference computations to large molecules with strong correlation effects localized on a small number of atoms. In this dissertation, we develop a series of quantum embedding schemes named active space embedding theory (ASET) for multireference computations. The schemes include mean-field active space embedding theory [ASET(mf)], a simple and automatic approach for embedding any multireference dynamical correlation method based on a frozen-orbital treatment of the environment; and second-order active-space embedding theory [ASET(2)] which improves upon mean-field frozen embedding by treating fragment–environment interactions via an approximate canonical transformation. We benchmark ASET(mf) and ASET(2) on various systems, including the N=N bond dissociation in pentyldiazene, the S0 to S1 excitation in 1-octene, and the interaction energy of the O2–benzene complex. In addition, the ASET schemes are used to study the singlet-triplet gap of p-benzyne and 9,10-anthracyne diradicals adsorbed on a NaCl surface; the inversion of CO on NaCl(100) surface. Despite their simplicity, our results show that ASET schemes are robust and sufficiently accurate, applicable when the coupling between the fragment and the environment is in the weak to medium regime. The ASET(2) explicit treatment of fragment–environment interactions beyond the mean-field level generally improves the accuracy of embedded computations. However, it becomes necessary to achieve an accurate description in several cases. This dissertation also discussed the potential future developments of ASET.

Table of Contents

1 Introduction

2 Mean-field Active Space Embedding Theory: ASET(mf)

3 Second-order active space embedding theory: ASET(2)

4 Studying CO inversion on NaCl surface using ASET

5 Improve orbital partition using modified projectors

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