(4.1.9) Xu : Despite being highly successful for main-group chemistry, the PT2-based double-hybrid approximations inherit the intrinsic deficiency of the PT2 correlation model for nondynamical correlation, which hinders their application to some of the challenging problems of DFT, such as stretched H 2 and other molecules (see contribution (3.4.1)) without symmetry breaking (see Section 3.6) and transition-metal complexes (see contributions (3.2.2) and ( .1.11)).