We describe a method for the calculation of the leading term of a previously proposed low-density expansion for the self-energy of nucleons in nuclear matter. We compute the single-particle complex potential energy, the average binding energy per nucleon, the complex symmetry potential, and the symmetry energy. We use Reid's hard core nucleon-nucleon interaction and take a Fermi momentum kF=1.4 fm-1. The calculated single-particle potential energy is compared with the phenomenological values of the optical-model potential in the inner region of a nucleus. The real part of our theoretical value is given by 56-0.3E (MeV) below E=150 MeV, and changes sign at 200 MeV. The imaginary part rises from 2 MeV at low energy to about 20 MeV at E=200 MeV. These features are in good agreement with experimental evidence. The average binding energy B per nucleon calculated with a self-consistent potential energy for the particle states above kF is equal to -11 MeV. In the standard approach, with no potential energy for intermediate particle states above kF, one finds -8.65 MeV. We also calculate the symmetry potential. At low energy, its real part is equal to 14 (N-Z)A (MeV); it changes sign at 110 MeV. Its imaginary part is equal to 3.5 (N-Z)A (MeV) at low energy, and rises to 8.5 (N-Z)A (MeV) at 200 MeV. The symmetry energy is equal to 27.8 MeV. NUCLEAR REACTIONS Calculated complex optical-model potential, symmetry potential, average binding energy and symmetry energy for nucleons in nuclear matter, for a Fermi momentum equal to 1.4 fm-1, from Reid's hard core nucleon-nucleon interaction, in the frame of Brueckner's theory.