Molecular form factors in X-ray crystallography

The calculation of molecular form factors from ab initio molecular electronic wavefunctions is discussed, and a scheme for application to X-ray diffraction structure analysis is given. The method is used to calculate the form factor of the NH⁺₄ molecular ion from three accurate molecular wavefunctions, as well as for the corresponding isolated-atoms arrangement. The merits of the different form factors obtained are tested on experimental single-crystal X-ray diffraction data for NH₄F. It appears that the quality of the form factor is determined by the number of centres in the wavefunction basis rather than by the wavefunction energy.