On the emergence of molecular structure

The structure of a^±,a^±,b^∓-type Coulombic systems is characterized by the effective ground-state density of the a-type particles, computed via nonrelativistic quantum mechanics without introduction of the Born-Oppenheimer approximation. A structural transition is observed when varying the relative mass of the a- and b-type particles, e.g., between atomic H^- and molecular H₂{}⁺$. The particle-density profile indicates a molecular-type behavior for the positronium ion, Ps${}^{{-}}$.