The phase problem in crystallography is the inability to reconstruct an electron density map due to the lack of phase information of the diffracted rays.

Recall that the modulus of a complex function is . Since the structure factor *F _{hkl}* is a complex function of the form

*z*=

*re*, where

^{iΦ}*r*= I

*z*I ,

Hence, *F _{hkl}* is composed of two factors, the modulus I

*F*I and the phase . If we replace

_{hkl}*F*in eq48 with just the modulus factor, information with regard to the phase is lost. Without phase information, it is impossible to reconstruct an electron density map. This is known as the

_{hkl}**, which fortunately can be overcome by a few methods, one of which is the Patterson method.**

*phase problem*