Much of electronic structure theory developments focus on bridging the gap between real physical systems and computationally feasible non-interacting models. In wavefunction-based post-Hartree-Fock (HF) and Kohn-Sham (KS) density functional theory 1 (DFT), this challenge reduces to approximating correlation energy. While correlation energy is crucial in both frameworks, it represents distinct objects in each (see, e.g., Ref. 2). Here, we use E c to denote the wavefunction-based definition (i.e., the difference between the exact total energy and the HF energy) and E DFT c for its KS DFT counterpart (i.e., the difference between the exact total energy and the expectation value of the true Hamiltonian on the KS non-interacting wavefunction). Exact evaluation of E c and E DFT c is infeasible for all but the smallest systems , requiring practical approximations. These approximations have traditionally been developed through two fundamentally different approaches. Post-HF wavefunctionbased methods recover E c by systematically expanding correlated wavefunctions space beyond the HF single Slater determinant . In contrast, KS DFT avoids constructing an explicitly correlated wavefunction by using density functional approximations (DFAs), which map the electron density and related quantities to E DFT c (or, more broadly, to the exchange-correlation energy).