Second-order correction to the positronium ground-state energy in ionic crystals

Upper and lower bounds on the second-order correction to the positronium ground-state energy due to the influence of the crystalline field in an ionic crystal are obtained, and the corresponding formulas are derived. The approximation adopted in the numerical calculations is the model of a point-ion potential. It is shown that neglecting the contribution of states in the continuous energy spectrum of the electron—positron pair to the correction yields an unsatisfactory upper bound. V. I. Lenin Moscow State Pedagogical University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 37–41, April, 1998.