As GGA is the analytical functional method, therefore, with plotting F XC (𝑟 𝑠 , 𝑠) with respect to s for various value of r s , we can get an effective way of examining and comparing different GGAs. GGA has shown great importance for various chemical systems, especially covalent bond energies and distance, although it has a shortcoming in describing van der Waals bonding. There are a few GGA functional, e.g., PW91, PBE which are generally used in the DFT calculation or embedded in hybrid functional (discussed in next section) and have been discussed below.