The extraction of the true Kohn-Sham exchange-correlation potential from near-exact electronic densities is demonstrated, with particular reference to the two-electron systems He and HZ It is shown that if the reference density is obtained from a standard configuration interaction calculation, very large orbital basis sets are required to obtain convergence in the resulting exchange-correlation potential. The correlation contribution to molecular bonding is analysed in terms of contributions from the potential in different regions of real space.