Physical interpretation of the SM function of equation (9). (a) Calculated modulus function M expressed in terms of the true Φ (line) and corresponding observed origin-free modulus function M′ (dots). (b) The M′M(Φ) product function. Since M′ has no origin peak, the peaks of the product function are evenly distributed along the unit cell (all interatomic peaks contribute). If the origin peak were present, it would play a dominant role in the product function, so that a solution of Φ giving rise to a single very strong peak in the E map would be a positive maximum of equation (23).