Many-body theory of positron binding in polyatomic molecules

Abstract

Positrons bind to molecules leading to vibrational excitation and spectacularly enhanced annihilation. Whilst positron binding energies have been measured via resonant annihilation spectra for $\sim$90 molecules in the past two decades, an accurate \emph{ab initio} theoretical description has remained elusive. Of the molecules studied experimentally, calculations exist for only 6, and for these, standard quantum chemistry approaches have proved severely deficient, agreeing with experiment to at best 25% accuracy for polar molecules, and failing to predict binding in nonpolar molecules. The mechanisms of binding are not understood. Here, we develop a many-body theory of positron-molecule interactions and uncover the role of strong many-body correlations including polarization of the electron cloud, screening of the positron-electron Coulomb interaction by molecular electrons, and crucially, the unique non-perturbative process of virtual-positronium formation (where a molecular electron temporarily tunnels to the positron): they dramatically enhance binding in polar molecules and enable binding in nonpolars. We also elucidate the role of individual molecular orbitals, highlighting the importance of electronic $\pi$ bonds. Overall, we calculate binding energies in agreement with experiment (to within 1% in cases), and we predict binding in formamide and nucleobases. As well as supporting resonant annihilation experiments and positron-based molecular spectroscopy, the approach can be extended to positron scattering and annihilation $\gamma$ spectra in molecules and condensed matter, to provide fundamental insight and predictive capability required to properly interpret materials science diagnostics, develop antimatter-based technologies (including positron traps, beams and positron emission tomography), and understand positrons in the galaxy.

Significantly updated: improved calculation now gives near-exact agreement with experiment for some molecules (within 1% in some cases); strength of correlation potential has been delineated; contribution of individual molecular orbitals (including pi-bonds) to binding and annihilation has been elucidated etc

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• A Guide to Molecular Properties from the Bethe-Salpeter Equation
• Many-body theory calculations of positronic-bonded molecular dianions
• Positron annihilation and binding in aromatic and other ring molecules
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• Many body theory beyond GW : towards a complete description of 2-body correlated propagation
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• Many-body theory predictions of positron binding energies in five-membered heterocycles involving N, O, S and NH substituents