Electron Affinities from Equation-of-Motion Frozen Pair-Type Coupled Cluster Methods and Their Dependence on Single Excitations, Molecular Orbitals, and Basis Set Sizes.

We introduce a series of alternative electron affinity equation-of-motion frozen-pair coupled cluster (EA-EOM-fpCC) methods for computing electron affinities and open-shell electronic structures. These methods are systematically benchmarked against the reference Δ-CCSD(T) approach and experimental data for a representative molecular data set using natural pair coupled cluster doubles (pCCD) orbitals and various basis set sizes. A comparison to canonical CC methods is also discussed. Additionally, EA-EOM-fpCC results are compared with those derived from the difference between double and single ionization potentials (DIP-EOM-CC and IP-EOM-CC) of dicationic species within the same ground-state fpCC reference framework. Our results demonstrate that frozen-pair approaches significantly reduce computational costs while maintaining high accuracy, offering an efficient strategy for studying electron affinities and open-shell systems in large molecules. The IP/DIP-EOM-fp(L)CCSD model stood out as the best post-pCCD flavor to predict EAs, achieving a mean error of 0.09 eV compared to experimental results, while EA-EOM-fpCCD is closest to Δ-CCSD(T) reference data. Finally, diffuse functions are not recommended for EA calculations using the IP/DIP-EOM-fpCC recipe and are not required for the EA-EOM-fpCC variants if sufficiently large basis sets are employed.