The macroscopic diffusivity is then obtained from the barrier of the single diffusion event by transition state theory (rate theory). Assuming hν k B T , the effective frequency ν * = ν 0 exp(S migr /k B T ) in the Arrhenius expression is obtained from the ratio of the product of the 3N normal frequencies of the entire crystal at the initial configuration of a transition to the product of the 3N -1 normal frequencies of the crystal when it is constrained in a saddle point configuration: