This chapter outlines the basic principles of the density functional theory (DFT). The introduction of electron density to develop the Kohn-Sham approach has been systematically presented. The various approximations such as LDA, GGA, and hybrid functional for the exchange-correlation energy have been discussed. A separate discussion on the basis sets has also been included. The advantages and shortcomings of DFT based techniques are also revealed. The formulation of time-dependent DFT has been presented in a concise manner. This chapter is intended to provide an overview of the theoretical background of the methods adopted in the succeeding chapters.
Keywords: Basis sets, DFT, Electron density, Exchange-correlation energy, Gaussian, Generalized-gradient approximation, Gradient-corrected functional, Hohenberg-Kohn theorem, Hybrid functional, Kohn-Sham approach, Local density approximation, TDDFT.