Double Electron Attachment and Double Ionization Potential Equation-of-Motion Coupled-Cluster Approaches with Full and Active-Space Treatments of 4-Particle-2-Hole and 4-Hole-2-Particle Excitations and Three-Body Clusters

The double electron attachment (DEA) and double ionization potential (DIP) equation-of-motion coupled-cluster (EOMCC) methods including up to 4-particle-2-hole (4$p$-2$h$) and 4-hole-2-particle (4$h$-2$p$) excitations on top of coupled-cluster singles, doubles, and triples (CCSDT), denoted DEA-EOMCCSDT(4$p$-2$h$) and DIP-EOMCCSDT(4$h$-2$p$), have been efficiently implemented in full and active-space forms. The resulting methods are applied to determine the ground and low-lying excited states of methylene, the singlet-triplet gap of trimethylenemethane, and the lowest singlet and triplet DIPs of 23 atoms and molecules. In all cases considered, the active-space DEA/DIP-EOMCC approaches recover the highly accurate parent DEA-EOMCCSDT(4$p$-2$h$)/DIP-EOMCCSDT(4$h$-2$p$) data at small fractions of the computational costs.