Equation-of-motion coupled-cluster methods for doubly ionized and doubly electron-attached states with single, double, and triple substitutions: Theory, implementation, and benchmarks.

We present new and computationally efficient implementation of the equation-of-motion (EOM) coupled-cluster methods for doubly ionized (DIP) and doubly electron-attached (DEA) states including single, double, and triple substitutions. In particular, EOM operators include up to 4-hole-2-particle substitutions in EOM-DIP-CCSDT and up to 4-particle-2-holes substitutions in EOM-DEA-CCSDT; both treatments include up to 3-hole-3-particle substitutions in the CC part. These methods have been implemented in the Q-CHEM package using effective and open-mp parallel libtensor and libxm backends. The implementation takes advantage of permutational and point-group symmetries (Abelian subgroups only) and is partially spin-adapted in the case of closed-shell references; it enables both the conventional double-precision and single-precision execution. This paper reports the programmable expressions and details of the implementation. The capabilities of the full EOM-DIP/DEA-CCSDT are demonstrated by application to methylene diradical (CH2), water (H2O), ammonia (NH3), cyclobutadiene (C4H4), acetylene (C2H2), ethylene (C2H4), and formaldehyde (CH2O), highlighting the utility of these methods for treating diradicals, excited states, and Auger decay.