Isotope Effects and Electronic Energy in Molecules

The definition of electronic energy in molecules is analyzed and, in its conventional form, it is shown to be in slight disagreement with experimental experience. By means of a proper treatment of the center-of-mass motion, a modified concept called reduced electronic energy is introduced and shown to give more satisfactory results. In particular, the hydrogen molecule and its isotopic analogues are used for a quantitative comparison of the two concepts with experimental results.