Abstract: The energy levels of Σ^- atom are obtained by solving the Dirac equation numerically. The calculated result agrees with the relevant experimental data very well. Our result, along with the previous calculation result concerning K^- atom, indicates the validity of the optical model potential to the strange exotic atoms. Moreover, it can be derived from our result that the strong interaction between the nucleons is a repulsive force.

Key words: Dirac equation, Σ^- atom, optical model potential