Abstract
In this review article, we report on the recent progresses in the field of quantum crystallography that has witnessed a massive increase of production coupled with a broadening of the scope in the last decade. It is shown that the early thoughts about extracting quantum mechanical information from crystallographic experiments are becoming reality, although a century after prediction. While in the past the focus was mainly on electron density and related quantities, the attention is now shifting toward determination of wavefunction from experiments, which enables an exhaustive determination of the quantum mechanical functions and properties of a system. Nonetheless, methods based on electron density modelling have evolved and are nowadays able to reconstruct tiny polarizations of core electrons, coupling charge and spin models, or determining the quantum behaviour at extreme conditions. Far from being routine, these experimental and computational results should be regarded with special attention by scientists for the wealth of information on a system that they actually contain.
Highlights
The interplay between crystallography and quantum mechanics is very tight and long-standing.It started in the early 20th century, when both the modern (X-ray based) crystallography and Bohr’s first quantum physics began
The first example in this direction is the investigation conducted by Jayatilaka and Grimwood, who computed electron localization functions (ELFs) for X-ray restrained Hartree–Fock wavefunctions corresponding to different molecular crystals and clearly pointed out the differences compared to the corresponding gas phase results [57]
In this short review article, we have summarized the main last-decade-achievements and improvements concerning the “electronic structure methods” of quantum crystallography, which has witnessed a lively debate on new approaches, new techniques and new applications
Summary
The interplay between crystallography and quantum mechanics is very tight and long-standing. Crystals 2020, 10, 473 and the implications for theoretical chemistry, especially those concerning the development of chemical bonding theories, are well known This strengthened the correlation between quantum chemistry and crystallography, without using experimental crystallography (and diffraction techniques) for a direct determination of quantum mechanical properties or functions. It is mainly within this stream that the term quantum crystallography was coined, the name quantum crystal was introduced much earlier by Nosanow [31] to define nonclassical behaviour of atoms in crystalline matrices at very low temperatures This definition has not found much application in the literature and it is no longer in use. We will consider only those studies devoted to directly extract electron densities, density matrices or wave functions from experimental data rather than on other techniques that exploit electron densities resulting from standard quantum mechanical calculations to refine crystal structures. A prominent example of these latter techniques is HAR, which, in the last years emerged as a very promising quantum crystallographic X-ray refinement method able to provide hydrogen bond lengths in excellent agreement with those resulting from neutron diffraction measurements [32,33,34,35,36]
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