Bader. He used the density gradient, (r), to partition atoms in molecules with the zeroflux criterion. He also applied the density Laplacian to categorize different types of chemical bonding. The second work was the NCI (noncovalent interaction) index by Johnson et al. NCI  |(r)| 2 /  4/3 (r). (7) where both the electron density and its gradient are combined to form a dimensionless index in coordinate scaling. This index can identify noncovalent interactions in molecular complexes. Very recently, with the electron density and Laplacian, we proposed a USI (ultrastrong interaction) index 14