An essentially exact evaluation of transport cross-sections for a model of the helium-nitrogen interaction

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Abstract
Essentially exact calculations of the transport collision integrals for a realistic model of the intermolecular pair potential of helium and nitrogen are reported. The collision dynamics for the interaction of the atom and molecule have been treated within the framework of the Arthurs and Dalgarno closecoupled formalism and the calculations are free from all approximation except inevitable numerical round-off. The direct calculation of collision integrals covers the temperature range 70 K to 300 K. The range has been extended upwards to 500 K with the aid of complementary classical calculations without loss of accuracy. The results serve as the first set of ‘benchmark’ calculations of transport collision integrals for a heavy molecular system with a realistic anisotropic pair potential. They are used to assess the accuracy of various approximate schemes for the evaluation of the same quantities. It is found that classical trajectory methods are in good agreement with the close-coupled calculations above 200 K in the range studied for both viscosity and diffusion. On the other hand, the IOS/Mason-Monchick approximation, while adequate for viscosity, yields discrepancies of up to 7 per cent for the mass diffusion coefficient at low temperatures which remain at around 2 per cent even at 300 K. Other transport coefficients, more sensitive to the inelastic collisions in the system, are also quite poorly predicted by the IOS/Mason-Monchick approximation. The final, exact results for these quantities, as well as various intermediate data in the calculation, provide a basis for the testing of improved approximate calculation procedures to be developed in the future.

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