Double ionization potential equation-of-motion coupled-cluster approach with full inclusion of 4-hole-2-particle excitations and three-body clusters

The double ionization potential (DIP) equation-of-motion (EOM) coupled-cluster (CC) method with a full treatment of 4-hole-2-particle (4h-2p) correlations and triply excited clusters, abbreviated as DIP-EOMCCSDT(4h-2p), and its approximate form called DIP-EOMCCSD(T)(a)(4h-2p) have been formulated and implemented in the open-source CCpy package available on GitHub. The resulting codes work with both nonrelativistic and spin-free scalar-relativistic Hamiltonians. By examining the DIPs of a few small molecules, for which accurate reference data are available, we demonstrate that the DIP-EOMCCSDT(4h-2p) and DIP-EOMCCSD(T)(a)(4h-2p) approaches improve the results obtained using the DIP-EOMCC methods truncated at 3h-1p or 4h-2p excitations on top of the CC calculations with singles and doubles.