Localized active-space self-consistent field method: a size-extensive, linear-scaling MC-SCF approach for strongly-correlated materials*

The complete active space (CAS) SCF method and its perturbative corrections are the standard computational strategy in the field of quantum chemistry for computing accurate wave functions of strongly-correlated molecular systems. They are, however, not applicable to materials in a condensed phase because they are not size extensive and/or have exponential cost scaling, and common cost-controlling approximations such as restricted or generalized active space (RAS, GAS) do not resolve this difficulty. However, our recently-developed localized active space (LAS) SCF method, which is based on a union of density matrix embedding theory (DMET) and MC-SCF concepts, generates a wave function which, unlike RAS or GAS, is multiplicatively separable between disjoint, real-space-localized active subspaces. LASSCF is therefore both size consistent and size extensive, and in principle, its computational cost is linear scaling with respect to system size. We test this method on various realistic chemical models of strongly-correlated systems and show that LASSCF gives CASSCF-quality results, implying an attractive possibility for computing wave functions of strongly-correlated condensed systems.