Treatment of accidentally absent reflexions in least-squares refinement of crystal structures

An improved method of including accidentally absent reflexions in the least-squares refinement of crystal structures, so as to accelerate the initial convergence of the procedure, is discussed. It is shown that when the calculated structure factors of these reflexions are less than the observed threshold value they determine nothing more about the structure, and should not be included in expressions for either the standard deviations or the shifts during refinement of atomic parameters.