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The focus of this work is OS-CCSD-SPT(2), which is a second-order similaritytransformed perturbation theory correction to opposite spin coupled clustersingles doubles, where in the latter the same-spin amplitudes are removed andthe opposite-spin ones are solved self consistently. We demonstrate that, fornon-multireference molecules, OS-CCSD-SPT(2) produces relative energies thatrival the accuracy of higher-order methods like CCSD(T). For example, usingPBE0 orbitals in the reference, OS-CCSD-SPT(2) exhibits a mean absolutedeviation (MAD) of 0.66 kcal/mol with respect to CCSD(2) benchmark values forthe non-multireference subset of W4-08 atomization energies (c.f. a MAD > 6kcal/mol for CCSD). OS-CCSD-SPT(2) is free of empirical parameters, has aninstrinsic scaling of $O(N^6)$, and makes no use of triples. It is alsonaturally amenable to higher order corrections: the associated third-ordercorrection, OS-CCSD-SPT(3), which does involve connected triples andquadruples, exhibits a MAD of 0.44 kcal/mol for the same benchmark.

The remarkable accuracy of an $O(N^6)$ perturbative correction to  opposite-spin CCSD: are triples necessary for chemical accuracy in coupled  cluster?