Based on first principles calculations, this study reveals that magnetism in otherwise non-magnetic materials can originate from the partial occupation of antibonding states. Since the antibonding wavefunctions are spatially antisymmetric, the spin wavefunctions should be symmteric according to the exchange antisymmetric principle of quantum mechanics. We demonstrate that this phenomenon can be observed in a graphitic carbon nitride material, $g$-C$_4$N$_3$, which can be experimentally synthesized and seen as a honeycomb structure with a vacancy. Three dangling bonds of N atoms pointing to the vacancy site interact with each other to form one bonding and two antibonding states. As the two antibonding states are near the Fermi level, and electrons should partially occupy the antibonding states in spin polarization, this leads to 1~$mu_B$ magnetic moment.
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