Brueckner calculations of the binding energy of finite nuclei are usually performed with harmonic oscillator single-particle wave functions. Therefore it is computationally more convenient to keep the oscillator potential in intermediate states and then subtract out its effects by calculating higher-order diagrams involving potential insertions in particle lines. It is found that the lowest-order diagram involving a single potential insertion in a particle line can be approximated by — κ Ūc.m., where κ is the wound integral and Ūc.m. is the average over the c.m. potential energy of the pairs of particles in the nucleus which occupy relative S-states.