Form the reduced density matrix ρ ρ for the first two block system, the block and the left-site. By definition it is the ( d ∗ ∗ 3 ) × × ( d ∗ ∗ 3 ) matrix: ρ ρ i , j ; i ′ , j ′ ≡ ≡ ∑ ∑ k , w Ψ Ψ i , j , k , w Ψ Ψ i ′ , j ′ , k , w ∗ ∗