By virtue of Eqs. 21 and 36, e ′ c (r) integrates to, e ′ c (r) dr = 2E MP2 c . In the context of DFT, the initial (λ → 0) slope of e DFT,λ c (r) of Eq. 19 is modeled by an MP2-like form for correlation energy densities (evaluated though on KS orbitals),