In this chapter, we will describe how to describe open-shell systems as well as systems with static (or strong) correlation when a single determinant does not even provide a qualitative description of the system.
Figure 1:Molecular orbital illustration of restricted closed-shell Hartree–Fock (RHF), restricted open-shell Hartree-Fock (ROHF), and unrestricted Hartree-Fock (UHF) states.