Many-body theory of positron binding and annihilation in polyatomic molecules*

Positrons bind to molecules leading to rapid annihilation [1]. Whilst binding energies have been measured for around 90 molecules over past two decades, an accurate \emph{ab initio} theoretical approach has remained elusive [1]. The theoretical difficulty is due to the need to accurately account for strong many-body correlations that characterise the positron-molecule system. These include electronic polarisation, electron screening of the positron-molecule Coulomb interaction, and the unique process of virtual-positronium formation (where a molecular electron temporarily tunnels to the positron) [1]. Although assumed to be important, especially in non-polar molecules, their specific role in positron-molecule binding is not well understood. Standard quantum chemistry approaches, which neglect or treat the correlations perturbatively, have proved deficient, agreeing with experiment to at best 25\% accuracy for polar molecules (see [1] and references therein, and [2]).

In this talk I will discuss how many-body theory insightfully delineates the effects of correlations and gives the most accurate \emph{ab initio} calculations of positron-molecule binding energies in polyatomic molecules (agreeing with experiment to ~10% in most cases). Notably, we find that the non-perturbative process of virtual-positronium formation is essential to support binding in non-polar molecules including CS$_2$, CSe$_2$ and benzene etc, and that it significantly enhances binding in organic polar molecules. We also elucidate the contribution of individual occupied electronic molecular orbitals to binding and positron annihilation from the bound state. 

[1] G. F. Gribakin, J. A. Young and C. M. Surko, Rev. Mod. Phys. 82, 2557 (2010).

[2] J. Romero et al. J. Chem. Phys. 141, 114103 (2014);

[3] J. Horfierka, B. J. Cunningham, C. M. Rawlins, C. P. Patterson and D. G. Green, arXiv2105.06959 (2021).