Two-Positron-bonded Dihalides: Ps₂XY (X, Y=F, Cl, Br)

This study explores the energetic stability and physical properties of $$ complexes formed by two halide anions (X^-, Y^-=F^-, X^-, Br^-), and two positrons (Ps: positron-electron pair). We combine electronic coupled cluster (CCSD(T)) calculations with positronic multicomponent renormalized partial third-order propagator (MC-REN-PP3) calculations to effectively recover correlation energies. Analysis of potential energy curves confirms the energetic stability of these positronic molecules, with optimized structures identified as global minima. Further investigation of electron and positron densities reveals stabilization owing to the formation of two-positron bonds. The global stability of the $$ complexes contrasts with the metastable two-positron-bonded (PsH)₂, which energetically favors the emission of Ps₂. Comparative analysis of one- and two-positron dihalides indicates that the addition of a positron to PsXY^- generally results in shorter bond distances, higher force constants, and lower dissociation energies, with exceptions due to differences in positron affinities of PsXY^- and Y^-. We explore the analogy between two-positron-bonded dihalide systems $$ and two-electron-bonded dialkali molecules AB, (A, B=Na, K, Rb). The bonding properties in two-positron dihalides and their electronic dialkali analogs are comparable, displaying identical periodic trends. However, compared to their isoelectronic AB counterparts, the positron bonds in $$ have shorter bond lengths, higher force constants, and higher bond energies.