Tests of the quasiclassical trajectory cross-correlation moment method against accurate quantum dynamics for V-V energy transfer in HF-HF collisions

V-V energy transfer probabilities for breathing-sphere HF-HF collisions are calculated both by a quasiclassical trajectory moment analysis and by converged quantal close coupling calculations. The two methods agree within factors of 1.1-1.5 for transition probabilities greater than or equal to 0.17 and within factors of 1.9-9.5 for transition probabilities in the range 0.04-0.065, but only within factors of 5.1-82 for those in the range 0.0008-0.004 and within factors of 20-150 for those in the range 0.0002-0.0004.