Effectively, the dynamical correlation energy is a functional of Π(r, r) and ρ(r) and the method works because X(r) locally probes the effect of nondynamical correlation exerted on the dynamical correlation. More precisely, the region where X(r) ≤ 1 represents suppression of dynamical correlation by nondynamical correlation, while the region where X(r) > 1 indicates enhancement of dynamical correlation in excited states of ionic type. The CASΠDFT and ELS+ methods reproduce well, in a relatively small orbital basis, the accurate potential-energy curves in the complete basis-set (CBS) limit; also, the CASΠDFT method yields good-quality vertical excitation energies for prototypical molecules. This development shows a promising new direction of combining ab initio methods with DFAs via the on-top pair density correlation functional.