On the distinguishable cluster approximation for triple excitations

The distinguishable cluster approximation applied to coupled cluster doubles equations greatly improves absolute and relative energies. We apply the same approximation to the triples equations and demonstrate that it can also improve results of the coupled cluster with singles, doubles and triples. The resulting method has a nominal computational scaling of ${\cal O}(N^7)$ in the real-space representation, is orbital invariant, size extensive, and exact for three electrons.