Multireference coupled cluster study of the oxyallyl diradical

Graphical abstract: Adiabatic singlet-triplet gap of the oxyallyl diradical has been studied by multireference coupled cluster methods. The results demonstrate the necessity to account for connected triexcitations and to employ a size-extensive method. The best result obtained at the MkCCSD(Tu)/cc-pVQZ level is in an excellent agreement both with experiment and with recent spin-flip EOMCCSD(dT) calculations. Highlights: ► 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. - 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.