Matter-antimatter molecules form bonds that defy easy classification

Positronium hydride (PsH) pairs two hydrogen anions with two positrons — the antimatter counterpart of electrons — forming a molecule whose bonding type has been disputed for decades. Accurate quantum Monte Carlo calculations reveal that the two positrons occupy a single delocalized molecular orbital stretching across both hydrogen anions, behaving like a covalent bond built from negatively charged pseudo-nuclei. Despite this covalent geometry, the bond energy falls squarely in the van der Waals range, resolving the classification dispute by showing both descriptions are simultaneously correct. The result hints that delocalized proto-bonds may arise in a wider range of particle-antiparticle and quasi-particle systems.