It was elaborated upon in other comments, but to drive home the point of extending an atom over the whole universe: The quantum mechanical ground state of an at

It was elaborated upon in other comments, but to drive home the point of extending an atom over the whole universe: The quantum mechanical ground state of an atom is actually determined by the size of the box you put it in. Any finite size box leads to a finite ground state energy and indeed, only by increasing the box size to infinity can you reduce the ground state energy to zero.