What is phase problem in x-ray crystallography?

The phase problem in X-ray crystallography arises because X-ray diffraction data only provide the magnitudes of the scattered waves, not their phases. Since both amplitude and phase are needed to reconstruct the electron density of a crystal, the missing phase information complicates the determination of the crystal structure. Various methods, such as using multiple wavelengths or employing techniques like molecular replacement, are employed to estimate these phases and solve the phase problem. This is crucial for accurately interpreting the molecular arrangement within the crystal.