Tricking hard collisions into soft interactions(#AIP_JCP #justpublished)

One of the simplest ways to describe atoms or molecules is to pretend that they are balls with some effective diameter (roughly on the order of nanometers.) If you ignore the fact that they can collide, you recover the ideal gas law you might remember from college chemistry classes. On the other hand, because they occupy space, they must collide. These collisions can be "hard" if the balls are treated like billiard balls that hit each other without changing their shape. But actual molecules are more like tennis balls in that they deform during the collision and the nearest approach depends on their velocities. That is, molecular-scale particles undergo soft interactions. Unfortunately, it takes a lot more computational effort to simulate soft particles and we'd like to avoid the extra expense. The question my coworkers and I asked is whether we could mimic soft particles using a model in which the particles moved like hard particles some of the time and like ideal particles the rest of the time. It's a little tricky because the ideal gas particles don't occupy space so we had to calculate the effective density of a given system as a function of the degree to which the particles collided with each other as hard or soft. Meanwhile, the comparison between our model and corresponding soft particle systems with the same density gave us similar structure! We intend to use this model to understand more complicated reactions, and the degree to which we can understand the behavior of complex solvents that interact with the reacting system at comparable time scales.

The title of the article is "Stochastic dynamics of penetrable rods in one dimension: Occupied volume and spatial order" and the work was funded by the National Science Foundation. It was released just this week at J. Chem. Phys. 138, 244901 (2013). (doi:10.1063/1.4810807).