Triangulene (TRI) is the smallest polybenzenoid known with a triplet-ground state. Recently, the realization of triangulene and its derivatives has led to the observation of many exotic phenomena including the Dirac cone and Haldane phase. Compared to metal-organic complexes and diradicaloids with localized electronic structures, the delocalized and symmetric spin densities with extended π-conjugation make it a distinctive platform to explore electron correlation in metal-free magnets. The magnetic coupling of triangulene has been studied using DFT (B3LYP functional) and CAS (4, 4), giving 160 meV and approximately 500 meV, respectively. B3LYP is the by far most often applied density-functional for describing organic molecules and widely used as standard method. However, while it gives the correct sign of the magnetic coupling, it is a factor 3 off in its quantitative description. Additionally, as dynamic electron correlation has rarely been considered in the reported multi-reference calculations of triangulenes, the available predictions for magnetic coupling may not be accurate. Therefore, it is of fundamental importance to thoroughly investigate the metal-free magnetism of triangulenes using advanced ab initio methods that take both static and dynamic electron correlations into account, and to evaluate the performance of available DFT functionals in predicting key parameters such as magnetic coupling and spin-polarization energy to open the door for periodic calculations, for example in two-dimensional (2D) polymers and covalent-organic frameworks (COFs).