The Complete Active Space Self Consistent Field (CASSCF) method, is a popular MC-SCF approach. In CASSCF a number of important orbitals, n, usually around the frontier orbitals, and their N electrons are selected to form the active space. Doubly occupied and empty orbitals not included in the active space form the inactive and the virtual spaces, respectively. All possible electronic configurations are generated, compatibly with spin and space symmetry, by distributing the N active electrons among the n active orbitals, a CAS(N, n). While the CI coefficients are obtained via exact or approximated schemes for the CI Hamiltonian diagonalization, the orbitals are variationally optimized via inactiveactive, active-virtual, and inactive-virtual orbital rotations. CAS is conceptually simple because only one active space has to be selected. However, the size of the CAS wave function exponentially grows with the size of the active space, and the computational costs of conventional diagonalization techniques quickly reach their practical limits for CAS (18, 18) wave functions.