The Coulson-Kearsley molecular model has been applied to various forms of the vacancy in diamond with considerable success from the point of view of group theory. Electron-electron interaction and configuration mixing are two important features of the molecular model which is reexamined with the intention of explaining why the model has been so successful to date. Parameters representing both one-electron and electron-electron interactions are deduced within an empirical framework of the molecular approach and are shown to consistently explain several properties of both GR1 (neutral vacancy) and ND1 (negative vacancy) optical features. Conclusions are reached concerning the position of neutral and negative vacancy energy levels relative to the valence-band edges