Frenkel defects show that the number n of interstitial atoms in equilibrium with n lattice vacancies in a crystal having N lattice points and N' possible interstitial positions is given by the equation E1 = kBT In [(N – n) (N' – n)/n²], whence, for n << N, N', we have n = (NN')^1/2 exp (– E₁/2k_BT). Here E₁ is the energy necessary to remove an atom from a lattice site to an interstitial position.