  The complete active space self-consistent field (CASSCF) method is the
principal approach employed for studying strongly correlated systems. However,
exact CASSCF can only be performed on small active spaces of ~20 electrons in
~20 orbitals due to exponential growth in the computational cost. We show that
employing the Adaptive Sampling Configuration Interaction (ASCI) method as an
approximate Full CI solver in the active space allows CASSCF-like calculations
within chemical accuracy (<1 kcal/mol for relative energies) in active spaces
with more than ~50 active electrons in ~50 active orbitals, significantly
increasing the sizes of systems amenable to accurate multiconfigurational
treatment. The main challenge with using any selected CI-based approximate
CASSCF is the orbital optimization problem; they tend to exhibit large numbers
of local minima in orbital space due to their lack of invariance to
active-active rotations (in addition to the local minima that exist in exact
CASSCF). We highlight methods that can avoid spurious local extrema as a
practical solution to the orbital optimization problem. We employ ASCI-SCF to
demonstrate lack of polyradical character in moderately sized periacenes with
up to 52 correlated electrons and compare against heat-bath CI on an iron
porphyrin system with more than 40 correlated electrons.
