The potential, charge density, and interface states have been calculated for the ideal interface between intrinsic GaAs, terminated on a (100) Ga plane, and intrinsic Ge. The conduction band is found to be nearly continuous across the interface and only a small interface dipole moment is found. Even though the two crystals, Ge and GaAs, have the same crystal structure and lattice parameters (a situation which might be thought to produce an interface containing no states in the forbidden gap at the Fermi level), a simple quantum-mechanical counting argument shows that there must be interface states at the Fermi energy for the unreconstructed interface. These states are the band-picture equivalent of unsaturated bonds. We find that fractional occupancy of the interface bonds (each of which contains 1.75 rather than two electrons) arises via a single partially occupied band of interface states. Consideration of the electronic energy suggests that the actual interface will be reconstructed.