Using wavefunctions to get more information out of diffraction experiments

Abstract

There is considerable scope for using quantum mechanical wavefunctions to extract more information from diffraction experiments, particularly X-ray diffraction experiments on molecular crystals. This chapter examines the ideas behind this approach, and reviews some recent developments. For example, the possibility to optimise atomic positional information using ab initiowavefunction is discussed. This important issue is a prerequisite to obtaining accurate electronic properties from the X-ray diffraction experiments because the two sets of information are often highly correlated. The constrained wavefunction approach for obtaining electronic properties is described. The method is validated by comparing deformation density plots, dipole moments, and some response properties (such as the in-crystal polarisability and the bulk refractive indices) with independent experimental data or higher level calculations which do not incorporate experimental data as input. Future developments and directions are mentioned throughout the article and in the conclusion.