There are also two smaller changes motivated by the goal of choosing large active spaces for DMRG (rather than the previous goal of choosing smaller active spaces that are affordable with conventional solvers for the CASCI equations). First, we no longer limit selection to orbitals close in energy to the HOMO and LUMO; we instead select from a complete manifold of molecular orbitals. Second, instead of dropping orbitals until a size constraint is met based on the number of configurations in the CAS space, we impose a limit on the number of active orbitals. The reason for the latter change is because although the cost of conventional CASCI scales directly with the number of configurations, the cost of DMRG depends more directly on the number of orbitals than on the number of configurations.