[en] Highlights: • We present a CASSCF implementation for large active spaces and atomic orbital bases. • This algorithm may easily be combined with modern active space solvers. • We demonstrate its capabilities using a CAS(22, 27) active space and a 3000 AO basis. We present a new second order complete active space self-consistent field implementation to converge wavefunctions for both large active spaces and large atomic orbital (AO) bases. Our algorithm decouples the active space wavefunction solver from the orbital optimization in the microiterations, and thus may be easily combined with various modern active space solvers. We also introduce efficient approximate orbital gradient and Hessian updates, and step size determination. We demonstrate its capabilities by calculating the low-lying states of the Fe(II)-porphine complex with modest resources using a density matrix renormalization group solver in a CAS(22, 27) active space and a 3000 AO basis.