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In this paper we investigate the role of electron correlation in predictingthe S$_1$-S$_0$ and T$_1$-S$_0$ excitation energies and hence, thesinglet-triplet gap ($\Delta$E$_{ST}$) in a set of cyclazines which act astemplates for potential candidates for 5th generation Organic Light EmittingDiode (OLED) materials. This issue has recently garnered much interest with thefocus being on the inversion of the $\Delta$E$_{ST}$, although experiments haveindicated near degenerate levels with both positive and negative being withinthe experimental error bar (J. Am. Chem. Soc., 102: 6068 , J. Am. Chem. Soc.,108: 17 ). We have carried out a systematic and exhaustive study of variousexcited state electronic structure methodologies and identified the strengthsand shortcomings of the various approaches and approximations in view of thischallenging case. We have found that near degeneracy can be achieved eitherwith a proper balance of static and dynamic correlation in multireferencetheories or with state-specific orbital corrections including its coupling withcorrelation. The role of spin contamination is also discussed. Eventually, thispaper seeks to produce benchmark numbers for establishing cheaper theorieswhich can then be used for screening derivatives of these templates withdesirable optical and structural properties. Additionally we would like topoint out that the use of DLPNO-STEOM-CCSD as the benchmark for$\Delta$E$_{ST}$ (as used in J. Phys. Chem. A, 126: 8: 1378, Chem. Phys. Lett.,779: 138827) is not a suitable benchmark for this class of molecules.

The nature of static and dynamic correlation in nearly degenerate singlet and triplet states of N-heterocyclic chromophores