Positronium Atoms Solvated in Liquid Alcohols: A Multicomponent Quantum Mechanics/Molecular Mechanics Approach

Positronium (Ps) atoms are highly sensitive probes of condensed-phase structure and dynamics, yet their theoretical description in complex molecular environments remains challenging. We present an extension of our QM/MM protocol to investigate Ps in methanol and ethanol, building on our earlier study in water. Classical Monte Carlo simulations employing newly parametrized Ps-solvent force fields reveal well-defined cavity structures, whose sizes are consistent with hydrated Ps and systematically smaller than those predicted by empirical bubble models. Multicomponent quantum calculations employing the Any Particle Molecular Orbital (APMO) method have identified physically meaningful cavity states, characterized by substantial electron-positron overlap, and have ruled out spurious surface states as artifacts of the QM region truncation. While vertical detachment energies are insensitive primarily to solvent structure, computed pick-off annihilation lifetimes showed a marked dependence on cavity size. Employing orbital-dependent enhancement factors, our results achieve good agreement with experimental PALS data for both solvents. This study demonstrates the transferability and predictive power of our QM/MM protocol for Ps, providing a framework that can be systematically extended to more complex solvents and biological environments, thereby advancing the theoretical interpretation of Ps annihilation in complex systems.