Low-Scaling Equation-of-Motion Coupled Cluster Method for Open-Shell Systems: I. IP-EOM-DLPNO-CCSD

We present the formulation and implementation of an unrestricted/open-shell domain-based local pair natural orbital equation-of-motion coupled-cluster method with single and double excitations for vertical ionization potentials (UHF IP-EOM-DLPNO-CCSD). The method extends the open-shell DLPNO–CCSD reference calculation to the (N – 1)-electron sector by reusing the localized occupied orbitals, pair domains, pair lists, and pair natural orbital spaces generated for the correlated ground state. Removal of an α or β electron is treated in separate spin sectors. The one-hole sector is retained in the fully localized occupied space, whereas the two-hole–one-particle sector is represented in compact, spin-resolved PNO spaces. The accuracy is controlled by the standard LoosePNO, NormalPNO, and TightPNO hierarchy. For ionized states dominated by one-hole character, canonical and local calculations performed with the same quasi-restricted-orbital (QRO) reference give typical RMSD differences of a few hundredths of an eV for the main-group radical and transition-metal benchmark sets. A representative TightPNO comparison for naphthalene further indicates that the additional local IP-EOM approximation contributes less than 0.10 eV for the ten matched states examined. Larger delocalized systems show less regular threshold dependence, and even predominantly one-hole states can exhibit changes greater than 0.1 eV between NormalPNO and TightPNO. Timing calculations for anionic acenes up to heptacene, with 1038 basis functions, show low-order effective scaling for the IP-EOM step. Applications to C70– and to the Pheophytin radical anion, the latter containing 3020 basis functions, demonstrate that the method can treat large open-shell molecular systems beyond the size range routinely accessible to canonical IP-EOM-CCSD. Finally, comparisons with available experimental data for the N-carbazolyl radical and the ovalene radical cation provide additional context for the calculated vertical ionization energies.