The APDFT calculations of electronic EDS of CO were performed using reference N2 (Supporting Information). Although CCSD-based APDFT EDM for CO has already been calculated, we focus on the investigation of the dependence of EDS on the reference electronic structure theory. We adopted CCSD, Hartree-Fock, and PBE0 theories as the reference electronic structure theory. Regardless of the reference theory and perturbation expansion order, APDFT EDS is reasonably accurate (Table ). As the perturbation order of APDFT increases, the error from the reference theory decreases. In particular, the first-order perturbation expansion considerably improves EDS.