Crystal structure prediction defined

Further improvements in our model potentials and simulation methods will therefore undoubtedly increase the detailed accuracy of molecular crystal structure predictions and will be required for crystal structures that correspond to weakly defined minima. However, for a routine transferable scheme, the addition of a realistic ab initio based electrostatic model clearly improves the range of molecules where a minimum in the lattice energy is close to the observed structure. The use of a theoretically derived, rather than an empirical potential, also increases confidence in the extrapolation of the potential to regions sampled in hypothetical crystal structures. 

Thus, crystal structure prediction is a forinidable problem and a major test of our quantitative ability to model the details of supramolecular recognition. The answer to the question as to whether crystal structures are predictable has only recently advanced from an emphatic "no" to a maybe .Experience in this field is still relatively limited.Although there have been cases where the crystal structure of a molecule was genuinely predicted just from the chemical diagram, it is certainly not yet possible to predict ab initio the crystal structures of any organic molecule. Indeed, it is not yet possible to define the molecules for which current methods are likely to be adequate. 

The general emphasis in force field development is towards transferrable force fields, where the functional form and the values of associated parameters can be used in a wide variety of molecules and crystals. As the parameters are developed empirically, transferability implies a degree of reliability and confidence that the parameters will work for crystals for which they were not specifically parameterised. In a recent development of the so-called tailor-made force field, it was pointed out that for the specific case of crystal structure prediction, the force field does not need to be transferable and that in fact there are some important advantages to having a force field derived specifically for the molecule of interest. Given sufficiently accurate information from quantum mechanical calculations, the tailor-made force field can be obtained by fitting to the quantum mechanical potential energy surface. Neumann defined a number of quantum mechanical data sets which represented both the non-bonded and bonded interactions in the crystal. The parameters of the force field were then optimised to fit these data sets. The quantum mechanical method chosen for the calculations was the DFT(d) method which will be described below. 

Put this way, the problem of crystal structure prediction is poorly defined - in the worst sense, because any computer-generated crystal structure would be a legitimate successful prediction. The case must be reduced to a subset that, while preserving the integrity of first principles, may be meaningful and tractable in the real world and produce some results that can be comfortably checked against experiment [14]. The desirable features of a computational machine for crystal structure prediction will be now analyzed, with their thermodynamic underpinnings and practical aspects. 

One can distinguish two phases in a crystal structure prediction, both quite difficult. The first phase is to make a list of structures that are not entirely unreasonable. This list was in older work rather short and obtained by hand, by comparing the molecule with related ones and guessing what kind of interactions could be possible and what kind of structures looked probable. A review of that type of work can be found in the book by Pertsin and Kitaigorodsky. In recent work this subjective method has been replaced by more or less well-defined algorithms, always based on energies calculated by an empirical force field. It turns out that lists obtained in that way can contain hundreds of possible structures. Two strategies are used random search versus systematic search. The... 

Mineral series Minerals that have an identical basic chemical and structural unit in which small amounts of chemical substitution of similar elements (cations of similar size, stereochemical, and bonding character) in the same site in the crystal structure are usual and predictable. Mineral series are usually defined by the end member species, that is, those compounds that contain only one of the possible cations. Intermediate members may have specific names or be identified by the ratio of the cations (see chapter 2). 

At this point mechanistic studies have reached an impasse. All of the observable intermediates have been characterized in solution, and enamide complexes derived from diphos and chiraphos have been defined by X-ray structure analysis. Based on limited NMR and X-ray evidence it appears that the preferred configuration of an enamide complex has the olefin face bonded to rhodium that is opposite to the one to which hydrogen is transferred. There are now four crystal structures of chiral biphosphine rhodium diolefin complexes, and consideration of these leads to a prediction of the direction of hydrogenation. The crux of the argument is that nonbonded interactions between pairs of prochiral phenyl rings and the substrate determine the optical yield and that X-ray structures reveal a systematic relationship between P-phenyl orientation and product configuration. 

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