Density functional theory (DFT) is a quantum mechanical approach developed by Hohenberg-Kohn (1964) and Kohn-Sham (1965) in the local density approximations by considering many body electronic ground state in terms of single particle equation and an effective potential. The effective potential forms of the ionic potential which are from atomic cores, hartree potential for defining the electrostatic electron-electron interaction and excahange-correlation potential that takes into account the many body effects to get one solution for the equation. Today Density functional theory has applications in Physics, Chemistry, Biology, Pharmaceuticals etc. The DFT also known as (first principle approach) or (ab-initio) method to explain the structural, electronic, vibrational properties of the materials. Physicist, Material Scientist use widely this method to look for electronic structure of the solid state materials. DFT application varies from novel materials to the existing materials whose properties are characterised by the ground state energy calculation of the atoms and molecules. While the chemist looks for the chemical properties like reaction rate of the molecular species, chemical potential of the compound, Gibbs free energy of the reaction, Electron affinity of the compound etc. While the Biologist looks for the ground state configuration of the molecules which are useful in making of drugs and their effects on the diseases. As today, we are looking to integrate the different scales ranging from Astronomical to Atomistic level to the Electronic level (DFT). It becomes very important to study Density Functional Theory as it is the most fundamental scale (Electronic scale) from which we can analyse the results of our calculation performed for the target material then it can be further processed for the upscaling. Therefore the rule of Downscaling first then Upscaling become very true in the words of Prof. Mark Horstemeyer. Density functional theory newly active research application is in defining the electronic excitation in molecules of organic chemistry, photovoltaics, photochemistry, and also in calculating the Nuclear Magnetic Response (NMR), Electron Paramagnetic Resonance (EPR) including relativistic DFT. Reference:  Hohenberg P, Kohn W. 1964. Inhomogeneous electron gas. Phys. Rev. 136, 864.
 Kohn W. 1999. Nobel lecture: electronic structure of matter–wave functions and density functionals. Rev. Mod. Phys. 71, 1253–1266.
[[DFT]] [[Electronic scale]]