Functionals for exchange and correlation
Density functional theory is the most widely used method today for electronic structure calculations because of the success of practical, approximate functionals. The crucial quantity in the Kohn-Sham approach is the exchange-correlation energy which is expressed as a functional of the density Exc[n]. This chapter is devoted to relevant approximate functionals, in particular, the local density approximation (LDA) and examples of generalized-gradient approximations (GGAs). Explicit formulas for certain widely used functionals are given in App. B. Non-local formulations are an active area of research leading to new classes of functionals, in particular, orbital-dependent ...