The noncovalent interaction analysis method proposed by Yang and coworkers in 2010, commonly referred to as the NCI or NCIplot method, is widely used to graphically reveal the regions and types of noncovalent interactions. Since the ideas and details of this method have been thoroughly introduced in the author's recent review, only the key points of the NCI method are summarized in this section. The reduced density gradient (RDG) function is defined as follows where  is the electron density, || is the gradient norm of the electron density, and r represents the position vector in three-dimensional real space. RDG is the dimensionless form of || and appears in the definition of many exchangecorrelation functionals. In the NCI method, RDG is used to display the regions where noncovalent interactions occur. Specifically, in low-density areas (usually  < 0.05 a.u.), the isosurface of RDG with an isovalue around 0.5 can satisfactorily exhibit the regions where noncovalent interactions predominantly occur in the system.