THE DEVELOPMENT OF MONTE-CARLO METHODS FOR CRYSTAL-STRUCTURE SOLUTIONFROM POWDER DIFFRACTION DATA - SIMULTANEOUS TRANSLATION AND ROTATION OF A STRUCTURAL FRAGMENT WITHIN THE UNIT-CELL

This paper reports the successful application of a Monte Carlo techniq ue to the solution of the crystal structure of p-methoxybenzoic acid f rom X-ray powder diffraction data collected on a conventional laborato ry diffractometer. The 'structural fragment' used in the Monte Carlo s tructure solution calculation comprised the C and O atoms of the benzo ate group (i.e. C6CO2) plus the O atom of the methoxy group in the par a position. The structural fragment was translated and rotated through the unit cell under a Monte Carlo algorithm, with trial structures ac cepted or rejected on the basis of Metropolis importance sampling. The best position of this structural fragment was discriminated readily ( using the profile R factor R(wp) to assess the agreement between exper imental and calculated X-ray powder diffractograms) from 'wrong' posit ions sampled during the Monte Carlo calculation, and the best position was then used as the initial structural model in Rietveld refinement and difference-Fourier calculations. This represents the first success ful report of the use of simultaneous translation and rotation of the structural fragment in crystal structure solution from experimental po wder diffraction data using the Monte Carlo method.