The accurate description of electron correlation is a central challenge in quantum chemistry. Correlated wave function methods systematically converge to the exact solution and are broadly divided into single- and multi-reference approaches, each suited for different applications. In particular, this talk will focus on multi-reference methods, which are used in
the presence of strong electron correlation as found in open-shell transition metal complexes and near-degenerate electronic states (e.g. in excited-state applications). I will begin by discussing the language and framework of quantum chemistry for describing electron correlation, followed by an introduction to multiconfigurational (MC) wave function methods [1]. These approaches address the so-called "static electron correlation" by constructing wave functions as linear combinations of multiple electronic configurations and optimizing both the many-body expansion coefficients and the molecular orbitals simultaneously. I will then discuss advanced multireference methods [2], focusing on second-order multireference perturbation theory and its quasi-degenerate extensions [3,4]. These methods combine MC reference states with a perturbative treatment of the "dynamic electron correlation" and effective Hamiltonian theory, enabling accurate descriptions of systems where single reference methods, such as coupled cluster theory, fail. The talk aims to provide an overview of these foundational methods and their applications, providing a bridge between quantum chemistry and condensed matter physics in the study of correlated electronic systems.