CASSCF with Extremely Large Active Spaces Using the Adaptive Sampling Configuration Interaction Method

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 a 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.