No, it's not really different from a hard-sphere type interaction. It's the same thing. But that doesn't matter in this context, because in the system we built,

No, it's not really different from a hard-sphere type interaction. It's the same thing. But that doesn't matter in this context, because in the system we built, our problem was harder/more-general because we were simulating 'guys', meaning you can program an arbitrary interaction into your guys (is guy type X near guy type Y, if so what happens?), and those 'guys' also interacted with a 2D height field terrain. Keep in mind this was all on computers from the year 2002, which were pretty damn slow compared to computers today. Today you could do a lot of guys.