Double hybrid density functional approximations

In this chapter, double hybrid density functional approximations are introduced and reviewed. These approximations combine concepts from (standard) hybrid density functionals with correlation corrections from wave function theory. The many approaches to the general idea are compiled and put into context. Latest developments are presented and critically discussed. Overall, it is shown how the field has matured since the seminal paper by Grimme a decade ago and that it is ready to be incorporated into the standard toolbox for computational chemistry.