**1. Introduction**

Density functional theory (DFT) is a low-cost, time-saving quantum mechanical (QM) theory, used to compute many physical characteristics of solids with high precision. The research in this field ranges from the development of novel analytical approaches focused on the design of precise exchange-correlation functionals to the use of this technique to predict the molecular and electronic configuration of atoms, molecules, complexes, and solids in both gas and solution phases. The history to DFT's success is the quest for the exchange-correlation functional, which utilizes density to represent advanced many-body phenomena inside one element formalism. If a precise exchange-correlation functional is applied, it may correctly

describe the quantum nature of matter. The estimated character of the exchangecorrelation functional is the basis for DFT implementation success or failure. DFT's early breakthroughs concentrated on the most fundamental issues in chemistry, such as the opportunity to generate functionals that could describe both molecular geometries as well as dissociation energy. The fact that every feature of a system in ground state is a unique ground state density functional was demonstrated by Hohenberg-Kohn, laying the foundation for DFT, which is now used to explore novelty of materials. This chapter is aimed to present an overview of DFT by describing the theoretical foundations, widely used approximations, current advances, and issues addressed, as well as future horizons.
