The connection between crystallography and quantum chemistry has always been very tight, after X-ray diffraction techniques became available in crystallography. In fact, the scattering of radiation enables mapping the one-electron distribution or the elements of a density matrix. The kind of radiation and scattering determines the quantity which is represented (electron charge or spin) and the space in which it is represented (position or momentum space). Although the wave function is typically assumed not to be directly measurable, recent advances enable also to compute wave functions that are restrained to some experimentally measurable observable (like the scattering of a radiation).