A b i n i t i o and Gordon–Kim intermolecular potentials for two nitrogen molecules

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Abstract
Both ab initio MO–LCAO–SCF and the electron-gas (or Gordon–Kim) methods have been used to compute the intermolecular potential (Φ) of N2 molecules for seven different N2–N2 orientations. The ab initio calculations were carried out using a [4s3p] contracted Gaussian basis set with and without 3d polarization functions. The larger basis set provides adequate results for Φ≳0.002 hartree or intermolecular separations less than 6.5–7 bohr. We use a convenient analytic expression to represent the ab initio data in terms of the intermolecular distance and three angles defining the orientations of the two N2 molecules. The Gordon–Kim method with Rae’s self-exchange correction yields Φ, which agrees reasonably well over a large repulsive range. However, a detailed comparison of the electron kinetic energy contributions shows a large difference between the ab initio and the Gordon–Kim calculations. Using the ab initio data we derive an atom–atom potential of the two N2 molecules. Although this expression does not accurately fit the data at some orientations, its spherical average agrees with the corresponding average of the ab initio Φ remarkably well. The spherically averaged ab initio Φ is also compared with the corresponding quantities derived from experimental considerations. The approach of the ab initio Φ to the classical quadrupole–quadrupole interaction at large intermolecular separation is also discussed.