Multi-component molecular orbital study on positron attachment to alkali-metal hydride molecules: nature of chemical bonding and dissociation limits of [LiH; e+]

In order to study positronium compounds we have developed the multi-component molecular orbital (MC MO) method, in which a positron in addition to the electrons are treated as 'quantum particles' based on the molecular orbital (MO) approaches. We have chosen to study [LiH; e+] because the experimental dipole moment of LiH is more than 3 times larger than the critical value. We have performed multi-component full-configuration interaction calculations to investigate the nature of chemical bonding of [LiH; e+] at the small and large internuclear distance R. We discuss the importance of geometrical changes in positronium compounds induced by a positron attachment in terms of the virial theorem, with a comparison of the adiabatic- and vertical-positron affinity (PA). The systematic improvement of the PA values achieved by optimisation of (i) the molecular geometry and (ii) the positronic basis centre is also discussed. The stable dissociation channel of [LiH; e+] is compared with the ionic- and neutral-dissociation channels of its parent molecule LiH through the analysis of the potential energy curve and the electronic and positronic densities. The vertical PA as a function of R is also presented, which is the difference between the potential energy curve of the parent molecule (LiH → Li+ + H-) and its positronic compound ([LiH; e+] → Li+ + [H-; e+]). Unlike the preceding study of [M. Mella et al., J. Chem. Phys. 113, 6154 (2000)], it took more than R = 50 Bohr to converge the vertical PA due to the long-range ionic bonding interaction