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

This article focuses on the development, implementation, and benchmarking of advanced double ionization potential equation-of-motion coupled-cluster (DIP-EOMCC) methods, specifically the DIP-EOMCCSDT(4h–2p) approach with full treatment of 4-hole–2-particle (4h–2p) correlations and triply excited clusters, and its perturbative approximation DIP-EOMCCSD(T)(a)(4h–2p). These methods have been incorporated into the open-source CCpy software package and applied to calculate vertical double ionization potentials (DIPs) of small molecules such as H₂O, CH₄, BN, Cl₂, Br₂, and HBr, using both nonrelativistic and spin-free scalar-relativistic Hamiltonians. The study demonstrates that DIP-EOMCCSDT(4h–2p) significantly improves accuracy over previous DIP-EOMCC methods truncated at lower excitation levels, addressing imbalances observed in earlier approaches, while the approximate DIP-EOMCCSD(T)(a)(4h–2p) method achieves comparable accuracy with reduced computational cost. Results are validated against high-level theoretical benchmarks and experimental data, showing that these new methods provide reliable and computationally feasible tools for studying double ionization phenomena in molecular systems.