To model the behaviour of humans en masse, treat them as people, not molecules.
The behaviour of crowds is sometimes unpredictable and occasionally deadly, but there is good reason to believe it is governed by simple rules. Presumably, like molecules in a liquid, people in a crowd all behave in more or less the same way. Capture those similarities in a model and it should be possible to predict how a crowd will behave.
Existing models of crowd behaviour do just that. They treat moving masses of humanity as though they were fluids. This works, up to a point. But it often fails to predict the changes that happen as a crowd’s density increases and its movement becomes chaotic. That is why Mehdi Moussaid of Paul Sabatier University, in Toulouse, and his colleagues have made a radical innovation. Instead of treating the individual human beings in a crowd as if they were molecules, they have treated them as if they were human beings. They have, in other words, given them volition.