  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(\Roman{2})-porphine complex with modest resources
using a density matrix renormalization group solver in a CAS(22,27) active
space and a 3000 AO basis.
