Multireference coupled cluster study of the oxyallyl diradical

Adiabatic singlet–triplet gap of the oxyallyl diradical has been studied by multireference Brillouin–Wigner and Mukherjee’s coupled cluster methods (BWCC and MkCC). The results demonstrate the necessity to include connected triples excitations, as well as employing a size-extensive MRCC method. The best result, calculated at the MkCCSD(Tu)/cc-pVQZ level, is in an excellent agreement with both experiment and recent spin-flip EOMCCSD(dT) calculations.