The coupled cluster approach with a hybrid treatment of connected triple excitations: Spectroscopic constants in open-shell diatomic molecules, and bond-breaking or twisting potential energy surfaces

The twisting potential energy surfaces (PESs) of C2H4 are calculated using various CC methods with the cc-pVDZ basis set. The PES of CCSD(T)-h is smooth at torsional angles around 90° and is very close to that of CCSDT.