Furthermore, to distinguish the types of weak interactions in different regions, the NCI method defines a function, sign(2), where 2 is the second-largest eigenvalue of the electron density Hessian matrix, and sign(2) represents the sign of 2. The sign(2) is mapped onto the RDG isosurfaces that describe noncovalent interactions, with different colors assigned according to the value. A larger  implies a stronger interaction. Positive and negative sign(2) indicate attractive and repulsive interactions, respectively. The introduction of sign(2) was inspired by the classification of bond critical points (BCPs) and ring critical points (RCPs) in the QTAIM theory: BCPs often appear between atoms with attractive interactions, while RCPs typically appear within sterically hindered ring regions. The difference between BCPs and RCPs just depends on the sign of 2 at the corresponding position.