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

We studied the positron (e(+)) interaction with the hydrogen molecular dianion H-2(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. H-2(2-) itself is unstable, but it was shown that such an unbound H-2(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 [H-2; Ps(-)]-like configuration with the positronium negative ion Ps(-) at the bond distance approximately equal to the equilibrium H-2 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 [H-2; 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(-)A(2)-like structure for binding a positron to the non-polar negatively charged dihydrogen.