Doubly electron-attached and doubly ionized equation-of-motion coupled-cluster methods with 4-particle-2-hole and 4-hole-2-particle excitations and their active-space extensions.

The full and active-space doubly electron-attached (DEA) and doubly ionized (DIP) equation-of-motion coupled-cluster (EOMCC) methods with up to 4-particle-2-hole (4p-2h) and 4-hole-2-particle (4h-2p) excitations are developed. By examining bond breaking in F2 and low-lying singlet and triplet states in the methylene, (HFH)(-), and trimethylenemethane biradicals, we demonstrate that the DEA- and DIP-EOMCC methods with an active-space treatment of 4p-2h and 4h-2p excitations reproduce the results of the analogous full calculations at the small fraction of the computer effort, while improving the DEA/DIP-EOMCC theories truncated at 3p-1h/3h-1p excitations.