If you could perfectly simulate a brain down to every atom, why wouldn't it have feelings? It would be indistinguishable from the real thing...
We actually have no idea how a bunch of atoms interacting create qualia, or even whether they do. There's no math to tell us that a configuration of atoms makes qualia, or what qualia it makes. We have no way of distinguishing between a conscious being who experiences, and a philosophical zombie with the same behavior but no internal experience.
Therefore we can't know whether a simulation of atoms actually does capture what's necessary for qualia.
It does have a sort of abstract mapping to something that looks like me, but since I have no idea what produces qualia, I have no way to know whether something essential is lost in that mapping.