Coupled cluster-based methods for linear and nonlinear spectroscopies in different frequency regions

Josefine Hvarregaard Andersen

Research output: Book/ReportPh.D. thesis

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Abstract

Linear and nonlinear spectroscopies have been implemented and applied. Most of the efforts revolve around the phenomenon of circular dichroism (CD), i. e., the differential absorption of left and right circularly polarized light.

The simplest manifestation of CD is electronic circular dichroism (ECD). ECD is a linear property only occurring for chiral molecules. In this work, it has been applied to a variety of small to medium-sized molecules, to investigate electronic excitations in the UV-Vis and X-ray regimes, the latter also exploring the effects of spin–orbit coupling in the latter. In relation to the implementation of ECD, optical rotation has also been made available along with its damped formulation from which the ECD cross section can be obtained.

Magnetic circular dichroism (MCD) and nuclear spin-induced circular dichroism (NSCD) are the two nonlinear dichroisms treated in this work. They are perturbation-induced dichroisms; MCD originates from the interaction of the electronic system with an external magnetic field, while NSCD is induced by the magnetic field arising from the spin of the nuclei. 

Furthermore, the core–valence separation and uncoupled valence singles approximation has been derived for computing X-ray two-photon absorption (X2PA) and investigated for a set of small benchmark molecules.

The developments rely on the ab initio coupled cluster (CC) method within which the coupled cluster singles and doubles (CCSD) and the second-order coupled cluster singles and doubles (CC2) models have been applied. ECD, optical rotation, and X2PA have been implemented for CCSD in Q-Chem, while the CC2 framework was employed for the implementations of MCD and NSCD in TURBOMOLE. An in–house python code for prototyping implementations of CC-based molecular properties, pyCCRSP, has been extended and applied in connection with the CCSD work.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages209
Publication statusPublished - 2023

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