This work is concerned with the development of theoretical methods capable of computing ground and excited states of molecular systems where one or more local spin centers are antiferromagnetically coupled to each other, allowing for the application to different kinds of spectroscopy, such as magnetic circular dichroism, X-ray absorption spectroscopy and X-ray magnetic circular dichroism. After a brief introduction of the topic in chapter 1, chapter 2 provides an overview of configuration state functions (CSFs), which are central to the methods developed in the following chapters. The definition of a CSF and how it can be constructed from a linear combination of Slater determinants is given, followed by the description of the methods used to calculate the one- and two-body coupling coefficients used in the construction of the Born-Oppenheimer Hamiltonian as well as for the calculation of the spin-dependent reduced matrix elements necessary for the treatment of spin–orbit coupling. Next, in chapter 3, a general spin-restricted open-shell Hartree-Fock method, named CSF-ROHF, capable of generating wavefunctions for a given configuration state function with arbitrary spin-coupling situations is described. Chapter 4 is dedicated to the extension of the restricted open-shell configuration interaction singles (ROCIS) method, which up to this point could only be applied to systems where all unpaired electrons are coupled in parallel to each other, into being capable of dealing with reference CSFs in any spin coupling situation. Chapter 5 extends the applicability of the general-spin restricted open-shell configuration interaction singles to the calculation of magnetic circular dichroism, L-edge X-ray absorption and X-ray magnetic circular dichroism spectroscopies by treating the spin–orbit coupling and the Zeeman effect via quasi-degenerate perturbation theory.