7.8 Coupled-Cluster Excited-State and Open-Shell Methods


Double-ionization potential (DIP) is another non-electron-conserving variant of EOM-CCSD.998, 507, 508 In DIP, target states are reached by detaching two electrons from the reference state:

Ψk=RN-2Ψ0(N+2), (7.48)

and the excitation operator R has the following form:

R = R1+R2, (7.49)
R1 = 1/2ijrijji, (7.50)
R2 = 1/6ijkarijkaakji. (7.51)

As a reference state in the EOM-DIP calculations one usually takes a well-behaved closed-shell state. EOM-DIP is a useful tool for describing molecules with electronic degeneracies of the type “2n-2 electrons on n degenerate orbitals”. The simplest examples of such systems are diradicals with two-electrons-on-two-orbitals pattern. Moreover, DIP is a preferred method for four-electrons-on-three-orbitals wave functions.

Accuracy of the EOM-DIP-CCSD method is similar to accuracy of other EOM-CCSD models, i.e., 0.1–0.3 eV. The scaling of EOM-DIP-CCSD is 𝒪(N6), analogous to that of other EOM-CCSD methods. However, its computational cost is less compared to, e.g., EOM-EE-CCSD, and it increases more slowly with the basis set size. An EOM-DIP calculation is invoked by using DIP_STATES, or DIP_SINGLETS and DIP_TRIPLETS.