A two-chain path integral model of positronium

We have used a path integral Monte Carlo technique to simulate positronium (Ps) in a cavity. The primitive propagator is used, with a pair of interact ing chains representing the positron and electron. We calculate the energy and radial distribution function for Ps enclosed in a hard, spherical cavit y, and the polarizability of the model Ps in the presence of an electrostat ic field. We find that the positron distribution near the hard wall differs significantly from that for a single particle in a hard cavity. This leads to systematic deviations from predictions of free-volume models which trea t Ps as an effective, single particle. A virial-type estimator is used to c alculate the kinetic energy of the particle in the presence of hard walls. This estimator is found to be superior to a kinetic-type estimator given th e interaction potentials, cavity sizes, and chain lengths considered in the current study. (C) 2000 American Institute of Physics. [S0021-9606(00)5044 7-4].