Crystal structure prediction space groups

A process even more challenging than structure determination is the prediction of a crystal structure without the use of experimental information relevant to that particular structure. This process, called crystal structure prediction CSP), would allow one to announce a crystal structure before any confirmation by chemical synthesis or discovery in nature. The outcome of CSP is the prediction of the atomic coordinates of the structural model, together with the space group and cell constant specifications. The relation of CSP with powder diffraction relies on the fact that a powder pattern can be calculated using this outcome, which could further be used to identify a real compound not yet characterized. 

Fig. 1 A plot of the lattice energy minima found in the crystal structure prediction search for the rigid CHNO molecule in the 2001 blind test (Fig. 4). in the search by Price. This diagram illustrates the plurality of distinct minima in different space groups found in a relatively sparse search, which did not locate the experimental structure.

Fig. 2 The types of molecules studied in the first decade of crystal structure prediction, as derived from a survey of published lattice energy minimisation based studies. Within each category, the molecules are generally the smaller and more rigid molecules whose published crystal structures are in common space groups with Z = l.

Before embarking on a description of the computational methods involved, and how well they perform, we should address the goals of crystal structure prediction. At its most ambitious level, the aim is to start from nothing more than the structural formula of a molecule and to predict, with perfect reliability, the structure of the resulting solid, with no input from experimental observations. (Here, by structure, we mean the space group, unit cell parameters and a fiiU specification of all atomic positions.) This goal is, of course, unrealistic polymorphism in molecular crystals tells us that there is often not just one crystal structure for a molecule and we know that the crystal that is produced in an experiment depends on a variety of factors, from thermodynamic descriptors of the system (temperature and pressure) to the method of crystallization, solvent used and the presence of impurities. Without a detailed description of the crystallization conditions, prediction of the resulting structure cannot be the aim. Furthermore, many of these factors are not sufficiently well understood to be represented in a computational procedure for crystal structure prediction. 

The most intense feature in the Raman microscopy spectrum of the ash specimen is at 475 cm Actinide oxides are known to crystallize at high temperature with a fluorite (CaFs) structure and space group Fm3m(Ol). This structure is predicted to possess a simple vibrational structure with one IR active phonon of T symmetry (302 cm for PUO2) and one Raman active phonon of T2g symmetry (478 cm for PUO2) at = 0 [88]. Therefore, the 475-cm Raman band can be assigned to the T2 mode of PUO2. 

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