Hi every one

I worked on molecular collision between two molecules in forcite,and I have successfully to finish it under guidence of  Reinier and Simon.But there is another problem.I make two phenanthrene molecules move towards each other and they will be mainly bond by   van der  waals force when they are very close to each other,resulting in a dimer.And I need to roughly estimate the mean effective diameters(collision effective diameters) of the dimer under the assumption of central collision, since I want to calculate the collision frequency of  the dimer and other molecules.My question is how can I roughly estimate or calculate mean effective diameters of the dimer in ms?

Sincerely

Li