These total energy partition schemes under OF-DFT have been successfully applied to numerous chemical processes and transformations. For example, a universal view has been obtained about the nature and origin of bond rotational barrier heights. A reactivity descriptor based on the kinetic energy density functional to quantify local temperature of molecules was recently proposed. In addition, the ELF (electron localization function) index proposed by Becke and Edgecombe 12 made use of the Pauli energy in Eq. ( ) to identify lone pairs in molecules. Very recently, we also applied the Pauli energy to identify strong covalent interactions as well as noncovalent interactions. Conceptual DFT (CDFT). As the first attempt to apply DFT to quantify chemical reactivity, CDFT was pioneered by the late Robert G. Parr and his coworkers. Its main idea is to treat chemical processes of a molecular as perturbations with respect to two variables, the total where N and (r) are the changes in N and , (E/𝜕N) and [E/(r)]N are the first-order partial derivatives of the total energy with respect to N and (r) with (r) and N fixed, respectively, and ( 2 E/  N 2 ) and [ 2 E/ 2 (r)]N are the corresponding second-order terms, whereby [/N (E/)N] is the second-order cross term. The major achievement of CDFT was to make sense of these derivatives in chemical language. For example, (E/ 𝜕 N) is electronegativity (chemical potential ), (𝜕 2 E/𝜕N 2 ) chemical hardness , [𝜕/𝜕N (E/)N] = ( 𝜕 (r)/ 𝜕 N) Fukui function f(r) to identify electrophilic and nucleophilic attacks, and [ 2 E/ 2 (r)]N the response function. Recent developments include the quantification of electrophilicity with    2 /2 and dual descriptor f 2 (r) = (𝜕f(r)/𝜕N). These descriptors have been applied to rationalize the HSAB (hard and soft acid and base) principle, MHP (maximum hardness principle), etc. This framework was also applied to quantify molecular acidity and basicity, 17 rationalize metal specificity, and predict the PCET (proton-coupled electron transfer) mechanism. Density Associated Quantities (DAQ). In this framework, quantities associated with electron density, gradient, or Laplacian are directly employed to appreciate physiochemical properties such as bonding, nonbonding, electron pair localization, etc. The first example of this category is the seminal work of Atoms in Molecules (AIM) theory by the late Richard F.W.