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:

$${\mathrm{\Psi}}_{k}={R}_{N-2}{\mathrm{\Psi}}_{0}(N+2),$$ | (7.48) |

and the excitation operator $R$ has the following form:

$R$ | $=$ | ${R}_{1}+{R}_{2},$ | (7.49) | ||

${R}_{1}$ | $=$ | $1/2{\displaystyle \sum _{ij}}{r}_{ij}ji,$ | (7.50) | ||

${R}_{2}$ | $=$ | $1/6{\displaystyle \sum _{ijka}}{r}_{ijk}^{a}{a}^{\u2020}kji.$ | (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 $\mathcal{O}({N}^{6})$,
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.