The most popular multireference method is the complete active space self-consistent field (CASSCF) method, which expresses approximate wave functions in the space of all possible configurations of electrons in an "active space" of orbitals and electrons. These wave functions can then serve as references for perturbation theories such as MC-QDPT, CASPT2, and NEVPT2. Alternatively, quantitative accuracy can be achieved by using a nonclassical-energy functional applied to the the converged wave function in multiconfiguration nonclassical functional theory (MC-NCFT). The total energy is then a sum of the classical portion of the CASSCF energy and nonclassical energy from the functional.