Doubly electron-attached and doubly ionised equation-of-motion coupled-cluster methods with full and active-space treatments of 4-particle–2-hole and 4-hole–2-particle excitations: the role of orbital choices

Abstract

We have recently developed the full and active-space doubly electron-attached (DEA) and doubly ionised (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. By examining the low-lying singlet and triplet states of the methylene, (HFH)−, and trimethylenemethane biradicals, and a few different types of molecular orbitals (MOs) to describe the corresponding wave functions, including the restricted or restricted open-shell Hartree–Fock orbitals of the target N-electron species and their counterparts associated with the (N ∓ 2)-electron reference systems, we demonstrate that the DEA- and DIP-EOMCC approaches with the full and active-space treatments of 4p-2h and 4h-2p excitations provide high-accuracy results which are not only in perfect agreement with one another, but also practically insensitive to the choice of the underlying MO basis. This should be contrasted with the DEA- and DIP-EOMCC methods truncated at 3p-1h/3h-1p excitations, which are generally less accurate and more sensitive to the types of orbitals used in the calculations.