The accurate treatment of excited states is critical for understanding photochemical phenomena, and it has been a long-standing goal of the electronic structure community. Although treating excited states is difficult in general, it is particularly challenging when single-determinant methods such as Hartree-Fock or Kohn-Sham density functional theory provide a poor reference state for predicting excited states. This can occur either because the excited states vary greatly from the ground state (e.g., double excitations ) or because the ground state itself is not well-described (e.g., strongly correlated systems ). One can overcome these deficiencies by using multiple-determinant reference states, and the methods that take this approach are called multireference methods.