Finally, the Thomas–Fermi model assumes that the electrons are in equilibrium, meaning that the total chemical potential is the same at all points. (In electrochemistry terminology, "the electrochemical potential of electrons is the same at all points". In semiconductor physics terminology, "the Fermi level is flat".) This balance requires that the variations in internal chemical potential are matched by equal and opposite variations in the electric potential energy. This gives rise to the "basic equation of nonlinear Thomas–Fermi theory": ρ ρ induced ( r ) = − − e [ n ( μ μ 0 + e ϕ ϕ ( r ) ) − − n ( μ μ 0 ) ] where n (μ) is the function discussed above (electron density as a function of internal chemical potential), e is the elementary charge, r is the position, and ρ ρ induced ( r ) is the induced charge at r. The electric potential ϕ ϕ is defined in such a way that ϕ ϕ ( r ) = 0 at the points where the material is charge-neutral (the number of electrons is exactly equal to the number of ions), and similarly μ 0 is defined as the internal chemical potential at the points where the material is charge-neutral.