I'm reading a book entitled "Electrons in Metals and Semiconductors" at the moment, and have reached a part where it is describing the potential of the atomic lattice. I understand that as you bring atoms closer together, their potentials overlap, creating the possibility that electrons can occupy states which belong to no particular atoms, and hence their wavefunctions overlap. Now, the book says that there are different ways to combine these wavefunctions which result in different energies. I don't understand how the wavefunctions combine in different ways. Can somebody enlighten me here?