On the X-ray or neutron diffraction determination of atomic position parameters

It is shown that, unless the atomic position parameters in a crystal are completely determined by symmetry, the position parameters corresponding to the minimum of the potential in the thermally strained lattice differ from those corresponding to thermodynamic equilibrium. By appropriate refinement of X-ray or neutron diffraction data, it is possible to obtain both sets of position parameters as well as the linear and higher-order coefficients of one-particle potentials expanded about the equilibrium positions.