Nondynamical pair-correlation energies have been calculated for various configurations in seven members of the Mg I isoelectronic sequence. The energies were calculated using a configuration interaction approach within the framework of the ``Many Electron Theory'' of Sinanoğlu. The influence of the 3d2 configuration was investigated, and the dependence on Z, the nuclear charge, was examined. It is found that the nondynamical correlation energy of the 3s2 configuration, ε(3s2), is almost perfectly linear in Z.