Density functional theory has long struggled to obtain the exact exchange-correlational functional. Numerous approximations have been designed in the hope of achieving chemical accuracy. However, designing a functional involves numerous methodologies, which have a greater possibility for error accumulation if the functionals are poorly formulated. This study aims to investigate the performance and limitations of second-order correlation functionals within the framework of density functional theory. Specifically, we focus on three major classes of density functional approximations that incorporate second-order energy expressions: ab initio (primarily Görling–Levy) functionals, adiabatic connection models, and double-hybrid functionals. The principal objectives of this research are to evaluate the accuracy of second-order correlation functionals, to understand how the choice of reference orbitals and eigenvalues …