ECAMP 15

Positrons are unique probes of matter, with applications in materials science (ultra-sensitive diagnostic studies of surfaces, defects and porosity), medical imaging (positron emission tomography), astrophysics, molecular spectroscopy, and are central to the formation of more complicated antimatter, including positronium and antihydrogen, used for studies of fundamental physics.

Low-energy positron interactions with atoms and molecules are characterized by strong many-body correlations, including positron-induced polarization of the molecular electron cloud,, screening of the electron-positron Coulomb interaction, and the unique process of virtual-positronium formation (where a molecular electron temporarily tunnels to the positron). They enhance annihilation rates by orders of magnitudes, modify scattering cross sections and annihilation $\gamma$ spectra, and can enable positron binding. They also make the description of positron-atom/molecule interactions --required to properly interpret fundamental experiments and materials science techniques, and develop antimatter-based technologies-- a challenging theoretical and computational problem.

I will present our recently developed ab initio many-body theory description of positron binding, scattering and annihilation in polyatomic molecules, and its state-of-the-art computational implementation in our open-source EXCITON+ code [1]. Since early 2000's, positron binding energies had been measured for around ~100 molecules. However, accurate ab initio calculations had proved elusive, with previous sophisticated quantum chemistry calculations severely deficient, giving agreement to at best 25% error (see [1] and references therein). By properly accounting for the correlations, our many-body theory approach has provided the first ab initio description of positron binding to molecules in agreement with experiment (see e.g., [1-3]). It has also provided fundamental insight into the role of correlations and molecular symmetry and predicted binding energies in other molecules including nucleobases [1-3]. It's predictive capability has also been demonstrated via joint theory-experimental work in which binding energies for ringed hydrocarbons were calculated in agreement with new measurements [3]. In addition, it has predicted new classes of positronically-bonded molecules (where two otherwise repulsive anionic species are stabilized by a positronic bond) [4]. I will also discuss recent developments extending the method and EXCITON+ code to enable ab initio calculation of positron scattering [5] and annihilation $\gamma$-ray spectra on polyatomic molecules [6].

This work was/is supported by the European Research Council grants 804383 & 101170577. It was performed with J. Hofierka, B. Cunningham, C. Rawlins, J. Cassidy, S. Gregg and A. Swann, and in collaboration with Charles Patterson (TCD) and the UCSD experimental positron group of Cliff Surko & James Danielson and colleagues.

[1] J. Hofierka, ..., D.G. Green, Nature 606, 688 (2022).
[2] J. Cassidy, ..., D.G. Green, PRA Letter 109, L040801 (2024).
[3] A. Baidoo, ..., D.G. Green, PRA 109, 062801 (2024).
[4] J. Cassidy, ..., D.G. Green, JCP (Spec.~Collec.) 160, 084304 (2024).
[5] C. M. Rawlins, ..., D.G. Green, PRL 130, 263001 (2023), & in prep.~(2025).
[6] S. K. Gregg, ..., D.G. Green, arXiv:2502.12364 (2025).