Equation-of-Motion Coupled-Cluster Method with Double Electron-Attaching Operators: Theory, Implementation, and Benchmarks

We report a production-level implementation of equation-of-motion coupled-cluster method with double electron- attaching EOM operators of 2p and 3p1h types, EOM-DEA-CCSD. This ansatz, suitable for treating electronic structure patterns that can be described as two-electrons-in-many orbitals, represents a useful addition to EOM-CC family of methods. We analyze the performance of EOM-DEA-CCSD for energy differences and molecular properties. By considering reduced quantities, such as state and transition one-particle density matrices, we can compare EOM-DEA- CCSD wave-functions with wave-functions computed by other EOM-CCSD methods. The benchmarks illustrate that EOM-DEA-CCSD capable of treating diradicals, bond-breaking, and some types of conical intersection.