6.12 Coupled-Cluster Methods 6.12.3 Coupled Cluster Singles, Doubles and Triples (CCSDT)6.12.5 Stochastic CC2 and CCSD

6.12.4 Second-Order Approximate Coupled Cluster Singles and Doubles (CC2)

(July 4, 2026)

The equations for the second-order approximate coupled cluster singles and doubles model (CC2) ²⁴⁹ Christiansen O., Koch H., Jørgensen P.
Chem. Phys. Lett.
(1995), 243, pp. 409. Link are similar to the CCSD equations with the doubles amplitude equations approximated as:

E_{\mathrm{CC2}}=\langle\Phi_{0}\left|\hat{H}\exp\left(\hat{T}_{1}+\hat{T}_{2}% \right)\right|\Phi_{0}\rangle

(6.70)

0=\left\langle{\Phi_{i}^{a}\left|{\bar{H}}+\left[{\bar{H}},\hat{T}_{2}\right]% \right|\Phi_{0}}\right\rangle

(6.71)

0=\left\langle{\Phi_{ij}^{ab}\left|{\bar{H}}+\left[\hat{F},\hat{T}_{2}\right]% \right|\Phi_{0}}\right\rangle

(6.72)

where the similarity-transformed Hamiltonian with the exponential function of the single excitation cluster operator is given by:

\bar{H}=\exp\left(-\hat{T}_{1}\right)\hat{H}\exp\left(\hat{T}_{1}\right)

(6.73)

CC2 energies are available in Q-Chem, and are requested by setting the keyword METHOD to CC2 or EOM-CC2. CC2 and MP2 ground state energies and properties are of similar quality, but CC2 can be efficiently combined with the EOM formalism to treat excited/electron attached/ionized/… states. When used with resolution-of-identity (RI; see Section 6.12.10), RI-CC2 can be applied to large systems. RI-CC2 can be used either via CCMAN2 or via libgmbpt; Section 7.9.4 provides a guide for setting up CC2 and EOM-CC2 calculations.

Search Results

6.12.4 Second-Order Approximate Coupled Cluster Singles and Doubles (CC2)