where ∇ρ is the first derivative of ρ. Here, we investigated the relationship between s(ρ) and sgn(λ 2 )ρ to understand non-covalent interactions, where λ 2 is the second eigenvalue of the Hessian matrix of the electron density. While ρ itself provides information regarding the strength of interactions, sgn(λ 2 ) can be used to distinguish between bonded (λ 2 < 0) and non-bonded (λ 2 > 0) interactions, which is why sgn(λ 2 )ρ was used in lieu of ρ.