Hello,

There is no analytical formula for polyatomic systems. You need to do calculations with fix impact parameters.   You should increase the impact parameter and check the reactivity or other physical quantity what you want to calculate.  The obtained function, the reaction (or other) probability as a function of the impact parameter is the opacity function. Where the opacity function has a falloff and it matches the axis there will be bmax. If you want to study the role of the rotational and/or the vibrational effects then you need to check again the maximum value of bmax for the maximal vibrational and rotational quantum states.