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While high quality spectroscopic data of Van der Waals complexes remain the best single source of information about intermolecular potential energy surfaces, those data are most sensitive to the region near the intermolecular potential energy minima. Thus, while they provide detailed information regarding the dependence of an atommolecule intermolecular potential on the intramolecular vibrational coordinate, this behaviour is most accurately determined for intermolecular distances significantly larger than those associated with vibrational inelasticity or vibrational predissociation. A better probe of the intramolecular stretching dependence of the potential at those relatively small intermolecular distances is provided by infrared or Raman pressure shifting $coefficients.1-2$ For example, Green's recent close-coupling calculations of the Raman Q-branch lineshape parameters for the H2-Ar $system3$ uncovered inadequacies of the TT3 surface of Hutson and $LeRoy.4$ Unfortunately, the cost of performing such accurate calculations makes their use in an iterative potential fitting procedure $prohibitive.3$ However, they still provide a reliable benchmark with which to compare approximate schemes for the calculation of this property. This paper presents an approximate and computationally inexpensive method for calculating pressure-shifting coefficients which permits their use in iterative fits to determine intramolecular-stretching dependent potential energy surfaces.