Double-half-H2: A two-electron model system for benchmarking quantum chemical methods across multiple bond types and molecular conformations

We introduce the double-half-H2 model system comprising two electrons and four possibly fused Z = ½ nuclei to evaluate electronic structure methods across covalent, ionic, and multi-center bonding regimes. Our results confirm that ionic bond dissociation is well described by single-reference wavefunction methods, while covalent dissociation demands proper treatment of static and dynamic correlation effects. Density functional approximations exhibit systematic under- or over-binding, especially at transition states, revealing consistent trends tied to electron delocalization. This model system highlights well-known density functional theory limitations, self-interaction errors, inadequate treatment of dispersion, and spin-symmetry breaking, while offering a practical platform for benchmarking and refining new functionals.