|t t⟩, |uū⟩ and |tū + u t⟩. In general, identifying and indexing excitations from a multireference configuration can be very difficult and tedious. Howevever, for CASSCF(2,2) refrence, this task is not terrible. If we define the reference state configuration to be |t t⟩, then there are four relevant classes of excited state slater determinants composed of molecular orbitals i, t, u, a: