Multipole Polarizabilites of Positronium and Its Interaction with Atoms and Molecules

The positron, besides all its intriguing fundamental properties as the antimatter counterpart to the electron, may form metastable bound states with atomic/molecular systems before its annihilation [Rev. Mod. Phys. 82, 2557 (2010)]. One possible mechanism behind the formation of bound positron-electron states is the polarization of the electronic cloud towards the positron. Furthermore, the positron can take away an electron forming a positronium (Ps) atom, which then can have non-covalent interaction with regular matter. These interactions require an accurate description of electron-positron correlation effects. Here, we develop a highly-correlated quantum Monte-Carlo (QMC) approach by including electron-positron geminal orbitals and we apply it to study the polarization properties of Ps and its dispersion interaction with other atoms and molecules through the computation of polarizabilities and dispersion coefficients. The calculated multipole polarizabilites of Ps are compared to our analytical derivations based on the direct transition-matrix approach [Ann. Phys. 355, 153 (2015)]. The even-order polarizabilities are found vanishing suggesting that the dipole-dipole approximation should be enough to describe the interactions between Ps and matter.