Multireference coupled cluster study of the oxyallyl diradical

► This paper reports a MRCC study of the adiabatic singlet-triplet gap of the oxyallyl diradical.B. ► To get correct results, connected triples and size-extensive MRCC method are needed. ► The best result obtained at the MkCCSD(Tu)/cc-pVQZ level is in an excellent agreement with experiment.