4. Direct methods

4.1 Theoretical introduction

The considerations that led to direct methods are in fact based on the obvious. The structure factor is the Fourier transform of the electron density: while a structure factor can be a complex or real quantity, positive or negative, the electron density is always real and positive.

However, an intensity measurement cannot give the phase. It is therefore not possible to calculate electron density directly. The problem lies in determining the phase of each structure factor.

Let's consider a mesh in which all atoms, at rest, are identical and distinct from each other in such a way that their electron densities do not overlap. We'll evaluate the expression : $G($