X-Ray Constrained Wave Functions: Fundamentals and Effects of the Molecular Orbitals Localization

The determination of wave functions from experimental data is an appealing objective that has stimulated many researchers over the years. Among the various strategies developed to accomplish this task, the X-ray constrained wave function fitting proposed by Jayatilaka is probably the most promising and it is currently used to determine experimental electron distributions in crystals. To directly introduce an easy chemical interpretation in terms of the traditional Lewis molecular picture without resorting to a posteriori techniques, this approach has recently been extended in order to obtain molecular orbitals strictly localized on small molecular fragments (e.g., atoms, bonds, or functional groups). In this chapter, after reviewing in detail both the historical development of the "experimental" wave function methods and the theoretical foundations of the X-ray constrained wave function techniques, we analyze for the first time the effect of introducing an a priori localization of molecular orbitals in the framework of Jayatilaka's approach. Our results mainly show that, when large and flexible basis-sets are used in the calculations, the strong initial assumption represented by the predefined localization has limited influence due to information provided by the experimental data. © 2016 Elsevier Inc.