The development of Monte Carlo methods for crystal structure solution from 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 technique to the solution of the crystal structure of p-methoxybenzoic acid from X-ray powder diffraction data collected on a conventional laboratory diffractometer. The `structural fragment' used in the Monte Carlo structure solution calculation comprised the C and O atoms of the benzoate group (i.e. C<sub>6</sub>CO<sub>2</sub>) plus the O atom of the methoxy group in the para position. The structural fragment was translated and rotated through the unit cell under a Monte Carlo algorithm, with trial structures accepted 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<sub>wp</sub> to assess the agreement between experimental and calculated X-ray powder diffractograms) from `wrong' positions 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 successful report of the use of simultaneous translation and rotation of the structural fragment in crystal structure solution from experimental powder diffraction data using the Monte Carlo method.