An isotopic mass effect on the intermolecular potential

The impact of isotopic variation on the electronic energy and intermolecular potentials is often suppressed when calculating isotopologue thermodynamics. Intramolecular potential energy surfaces for distinct isotopologues are in fact equivalent under the Born–Oppenheimer approximation, which is sometimes used to imply that the intermolecular interactions are independent of isotopic mass. In this communication, the intermolecular dipole–dipole interaction between hetero-nuclear diatomic molecules is considered. It is shown that the intermolecular potential contains mass-dependent terms even though each nucleus moves on a Born–Oppenheimer surface. The analysis suggests that mass dependent variations in intermolecular potentials should be included in comprehensive descriptions of isotopologue thermodynamics.