MOLECULAR COLLISIONS .6. DIAGRAMMATIC METHODS

The expressions, developed in previous papers of this series, for the coefficients in spherical harmonic expansions of the degeneracy-averaged cross sections for collisions between diatomic molecules are reformulated. The reformulation is based on the introduction of a new basis set of functions in the expansion of the wavefunctions. In the new basis set, the three basic angular momenta of the problem (the angular momenta of each of the molecules and that associated with the relative motion) are treated symmetrically. The manipulation of the vector coupling coefficients is facilitated by the use of diagrammatic methods.