  A theoretically consistent approach to the calculation of the Coulomb
correlation corrections to the single--particle energies is presented. New
contributions to the single--particle energies previously overlooked in the
literature are now identified and taken into account. We show that the
interplay between the Coulomb interaction and the strong interaction, which is
enhanced in the nuclear surface, leads to an upward shift of the proton
single--particle levels. This shift affects the position of the calculated
proton drip line, a shift towards decreasing $Z$. We describe briefly a similar
mechanism which is at work for neutron levels. The same mechanism is
responsible for significant corrections to the mass difference of the mirror
nuclei (Nolen--Schiffer anomaly) and to the effective proton mass.
