TEST OF THE ASYMPTOTIC METHOD AS APPLIED TO ATOM DIATOM INTERACTION POTENTIALS

The accuracy of the asymptotic method in calculating long-range atom-d iatom exchange interactions is tested for two sample systems. In both cases high-quality ab initio results are available for comparison. As a first example, the energy splitting between the A' and A '' surfaces arising from OH(X(2) Pi) + Ar were expressed analytically in terms of asymptotic parameters of the O and Ar atoms. The agreement with the c alculated ab initio quantity is very good for a range of interatomic d istances and different angles of approach. Secondly, for the system CN + He, the diabatic off-diagonal matrix element V-Sigma A' between two diabatic A' potential energy surfaces arising from CN(X(2) Sigma(+)) + He and CN(A(2) Pi)+He is considered. Using the asymptotic approach, this element can be expressed in terms of the three diagonal matrix el ements for CN(X and A) + He which are known ab initio. The resulting v alues of V-Sigma A' are found to be in very good agreement with the di rectly ab initio calculated coupling matrix element over a wide range of CN-He geometries.