Exploring Chemistry includes a detailed introduction to the CASSCF features in Gaussian. The graphic on the left above illustrates the active space for a RASSCF calculation involving 12 electrons in 13 orbitals: 8 electrons in 4 occupied orbitals in the RAS1 space, 4 electrons in 4 orbitals in the RAS2 space, and 5 virtual orbitals in the RAS3 space. The diagram on the right illustrates the main features of reactions that span both the ground state and the excited state potential energy surfaces.