First-principles quantum Monte Carlo studies for prediction of double minima for positronic hydrogen molecular...

We studied the positron (e⁺) interaction with the hydrogen molecular dianion H₂ ^2- to form the positronic bound state of [H^-; e⁺; H^-] using the first-principles quantum Monte Carlo method combined with the multi-component molecular orbital one. H₂ ^2- itself is unstable, but it was shown that such an unbound H₂ ^2- may become stable by intermediating a positron and forming the positronic covalent bond of the [H^-; e⁺; H^-] system [J. Charry et al., Angew. Chem., Int. Ed. 57, 8859-8864 (2018)]. We newly found that [H^-; e⁺; H^-] has double minima containing another positronic bound state of [H₂; Ps^-]-like configuration with the positronium negative ion Ps^- at the bond distance approximately equal to the equilibrium H₂ molecule. Our multi-component variational Monte Carlo calculation and the multi-component configuration interaction one resulted in the positronic covalent bonded structure being the global minimum, whereas a more sophisticated multi-component diffusion Monte Carlo calculation clearly showed that the [H₂; Ps^-]-like structure at the short bond distance is energetically more stable than the positronic covalent bonded one. The relaxation due to interparticle correlation effects pertinent to Ps^- (or Ps) formation is crucial for the formation of the Ps^-A₂-like structure for binding a positron to the non-polar negatively charged dihydrogen.