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In these experiments, two beams of particles are crossed and the collision cross section determined by measuring the angular and energy distributions of the products. These measure the cross-sections directly.
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This data could come from atomic and molecular experiments, such as those mentioned above, or from molecular modelling which takes configurational information about how the atoms are arranged within the molecule, and derives the forces acting between the molecules.
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P

may   be defined by considering the infinitesimal force 6#6 exerted by the fluid across a surface element 7#7. This force need not be aligned with the normal to the area element, so the most general relationship is given as 8#8, where P is a rank two tensor.
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In microscopic terms [Evans and Standish

[1990]] , the pressure tensor can be written as:

26#26

where 27#27 is the force acting between particles i and j, and 28#28 is their relative displacement.

Russell Standish
Thu May 18 11:43:52 EST 1995