Giles W. Turner

Implementation of Lamarckian concepts in a Genetic Algorithm for structure solution from powder diffraction data

Abstract Previous implementations of Genetic Algorithms in direct-space strategies for structure solution from powder diffraction data have employed the operations of mating, mutation and natural selection, with the fitness of each structure based on comparison between calculated and experimental powder diffraction patterns (we define fitness as a function of weighted-profile R -factor R wp ). We report an extension to this method, in which each structure generated in the Genetic Algorithm is subjected to local minimization of R wp with respect to structural variables. This approach represents an implementation of Lamarckian concepts of evolution, and is found to give significant improvements in efficiency and reliability.

Investigation of geometric shell aluminum clusters using the Gupta many-body potential

A many-body Gupta-type potential, derived by fitting bulk experimental parameters for aluminum, has been used to study geometric shell clusters of aluminum with icosahedral, decahedral, fcc, and bcc packing. The stabilities of fcc-like octahedral clusters are compared with cuboctahedral and regularly truncated octahedral clusters. A stepwise pattern of truncation is observed and truncated octahedra are predicted to remain the preferred fcc-like structure until complete transition to the bulk fcc lattice.

Gaining insights into the evolutionary behaviour in genetic algorithm calculations, with applications in structure solution from powder diffraction data

A general strategy is introduced for analysing the evolutionary events that occur during a genetic algorithm calculation, and the application of this approach is illustrated for the specific case of understanding the evolutionary trajectory leading to the correct structure solution in structure determination from powder diffraction data. Methods providing visual representations of the evolution of the population during a genetic algorithm calculation are also introduced. It is demonstrated that our strategy can provide much more detailed insights concerning the evolutionary behaviour than was previously accessible, and can thus be exploited to further develop our genetic algorithm methodology.

Recent advances in opportunities for solving molecular crystal structures directly from powder diffraction data: new insights in crystal engineering contexts

Crystalline solids that can be prepared only as microcrystalline powders are not suitable for structural investigation using single crystal diffraction techniques, and it is necessary instead to carry out structure determination using powder diffraction data. In this paper we highlight recent advances in the opportunities for solving molecular crystal structures directly from powder diffraction data, focusing on a direct-space strategy in which a hypersurface based on the powder profile R-factor Rwp is searched using a Genetic Algorithm. Some recent fundamental developments are described, and the application of the method is illustrated by the structure determination of a range of materials, giving particular focus to the opportunities to exploit these techniques within the context of structural rationalization towards crystal engineering strategies.

A method for understanding characteristics of multi-dimensional hypersurfaces, illustrated by energy and powder profile R-factor hypersurfaces for molecular crystals

Abstract A method is described for determining general sections through multi-dimensional hypersurfaces, in which the values of all the variables that define the hypersurface are allowed to vary simultaneously. The method is general, in the sense that any trajectory through any specified point on the hypersurface may be readily defined. The application of this method is illustrated through a comparison of the energy and powder profile R-factor hypersurfaces for the β phase of L-glutamic acid.

Ab initio structure determination of a peptideβ-turn from powder X-ray diffraction data

Ab initio crystal structure determination of the peptide Piv-Pro-Gly-NHMe directly from powder X-ray diffraction data, using the genetic algorithm technique for structure solution, has allowed the complete structural characterization of the Type II β-turn conformation and the intermolecular interactions in this structure, and highlights the opportunities that now exist for structure determination of peptide systems when single crystals appropriate for single crystal X-ray diffraction experiments cannot be prepared.

Recent advances in the opportunities for solving molecular crystal structures directly from powder diffraction data

For those solids that can be prepared only as microcrystalline powders, and are not suitable for investigation by single crystal diffraction methods, it is necessary to carry out structure determination using powder diffraction data. Here we highlight recent developments in the opportunities for solving molecular crystal structures from powder diffraction data, focusing on a direct-space strategy in which a hypersurface based on the powder profile R-factor R wp is searched using a Genetic Algorithm. Recent fundamental developments are described, and the application of the method is illustrated by the structure determination of two oligopeptide materials.

Systematic computational study of the geometrical dependence of deuterium quadrupole interaction parameters in an O–2H⋯OC hydrogen bonded system

Abstract A systematic theoretical study of the geometrical dependence of deuterium ( 2 H) quadrupole interaction parameters (quadrupole coupling constant and asymmetry parameter) in O– 2 H⋯OC hydrogen bonds is reported, using the water–formaldehyde complex as a model system. From ab initio HF–SCF calculations (using 6-31G** basis set), the principal components of the electric field gradient tensor at the 2 H nucleus, and hence the quadrupole interaction parameters of the 2 H nucleus, have been determined as a function of the intermolecular geometry of the complex. The results provide a systematic understanding of the dependence of quadrupole interaction parameters on hydrogen bonding geometry, and may be applied in order to guide the interpretation of experimental results, for example in the case of solid state 2 H-NMR of hydrogen bonded systems.

Fundamental Developments in Direct-Space Techniques for Structure Solution from Powder Diffraction Data

Solids that can be prepared only as microcrystalline powders are not suitable for structural investigation using single crystal diffraction techniques, and it is necessary instead to carry out structure determination using powder diffraction data. In this paper, we focus on a direct-space strategy for solving crystal structures directly from powder diffraction data in which a hypersurface based on the powder profile R-factor Rwp is searched using a Genetic Algorithm, and we highlight some recent fundamental developments relating to this methodology.