The bonding in the X 1Σg+ state of Mg2 is investigated using near-complete valence one-particle Slater and Gaussian basis sets containing up to h functions. Full configuration interaction (FCI) calculations are used to calibrate four-electron correlation treatments. We show that the four-electron complete CI limit can be approached using a sequence of either second-order CI (SOCI) or interacting correlated fragment (ICF) calculations. At the valence level, our best estimate of the dissociation energy (De ) is 464 cm-1. We show that this is a lower limit and probably within 5 cm-1 of the complete basis value. The inclusion of core-valence correlation using a model operator approach decreases De by about 35 cm-1 and increases the bond length by 0.03 ao, thereby yielding spectroscopic constants in good agreement with experiment. Attempts to compute the core-valence effect accurately by expanding the CI treatment were unsuccessful. © 1990 American Institute of Physics.