But perfectly rigid atoms are compatible with an atomic model of matter. And in such a model, the indeterminism comes right back. Yes, the idea of perfectly ri

But perfectly rigid atoms are compatible with an atomic model of matter. And in such a model, the indeterminism comes right back. Yes, the idea of perfectly rigid atoms is incompatible with experiment. But so is classical mechanics on the scale where that fact becomes obvious. And so it isn't classical mechanics that is deterministic. You are right that n-body classical mechanics combined with the assumption of a continuous response does become deterministic by the standard existence and uniqueness theorems of ordinary differential equations. But the point still remains, classical mechanics winds up not being deterministic without that extra assumption. And that extra assumption is often omitted in real models of things like elastic collisions.