in which |22...2⟩ is a single Slater determinant consisting of doubly occupied orbitals (called inactive orbitals in the CASCI context), the C n 1 n 2 ..n L are coefficients, and the determinants |n 1 n 2 ...n L ⟩ span the space of all possible configurations obtained by distributing a fixed number of active electrons N elec among L active orbitals. Each determinant |n 1 n 2 ...n L ⟩ is defined by its orbital occupation numbers n i ∈ {0, ↑, ↓, 2} of the active electrons in the active orbitals, and diagonalization of the Hamiltonian in this space (and in the mean field of |22...2⟩) is known as CASCI. However, because the size of the space scales exponentially with the number of orbitals L, this approach is only feasible up to active space sizes of about 20 electrons in 20 orbitals. Many methods exist to approximate the solution for the coefficients C n 1 n 2 ..n L in equation 1. Among the most successful approaches in this regard is the density matrix renormalization group (DMRG), in which the coefficients of equation 1 are approximated by the matrix product C n 1 n 2 ..n L = ij...(L-1)