Spin-adapted theoretical methods for ground and excited states of antiferromagnetic chemical systems, with spectroscopic applications

This work deals with the development of theoretical methods capable of calculating the ground and excited states of molecular systems in which one or more local spin centers are antiferromagnetically coupled. This enables their application to various types of spectroscopy, such as magnetic circular dichroism, X-ray absorption spectroscopy, and magnetic X-ray dichroism. After a brief introduction to the topic in Chapter 1, Chapter 2 provides an overview of Configuration State Functions (CSFs), which are of central importance for the methods developed in the following chapters. A definition of a CSF is then given, along with an explanation of how it can be constructed from a linear combination of Slater determinants. Furthermore, a description is provided of the methods required to calculate the one- and two-body coupling coefficients (for the construction of Born-Oppenheimer Hamiltonians) and the spin-dependent reduced matrix elements (for the treatment of spin-orbit coupling). Chapter 3 describes a general spin-restricted open-shell Hartree-Fock method, called CSF-ROHF, capable of generating wavefunctions for a given Configuration State Function with arbitrary spin coupling situations. Chapter 4 is dedicated to extending the ROCIS (Restricted Open-Shell Configuration Interaction Singles) method, which previously could only be applied to systems in which all unpaired electrons are coupled in parallel, and which is now capable of handling reference CSFs in arbitrary spin coupling situations. Chapter 5 extends the applicability of the method to the calculation of spectroscopic properties, such as magnetic circular dichroism, L-edge X-ray absorption, and magnetic X-ray circular dichroism, by treating spin-orbit coupling and the Zeeman effect using quasi-degenerate perturbation theory.