According to the two basic DFT theorems by Hohenberg and Kohn, the electron density of a molecular or other system in the ground state should be adequate to determine all electronic properties of the system including bonding, stability, function, reactivity, etc. To solve the Euler-Lagrange equation alone, only the energy density functional, its components, and their functional derivatives are needed. However, to establish a chemical reactivity theory in DFT, which may deal with not only energetics but also many other functionalities, more forms of density functions and density functionals are required. These forms can be the ones containing density associated functions such as gradient and Laplacian, or density functionals and functional derivatives of different formulas. In the current literature, following four avenues to derive these forms are available.