CRYSTAL-STRUCTURE SOLUTION FROM NEUTRON POWDER DIFFRACTION DATA BY A NEW MONTE-CARLO APPROACH INCORPORATING RESTRAINED RELAXATION OF THE MOLECULAR-GEOMETRY

We report the development of a new aspect of the Monte Carlo method fo r crystal structure solution from powder diffraction data and demonstr ate the advantages of this new approach by applying it to solve the lo w-temperature crystal structure (phase II) of perdeuterated pyrene fro m neutron powder diffraction data. In previous applications of our Mon te Carlo technique, the structural fragment was constrained to have a standard geometry throughout the calculation, whereas in the new appro ach, restrained relaxation of the structural fragment from standard ge ometry is considered for each structure sampled during the calculation , The new approach gives rise to a substantially improved discriminati on (in terms of the weighted profile R factor R-wp) between the correc t structure solution and incorrect structures sampled during the Monte Carlo calculation [for the calculation with standard geometry of the structural fragment, R-wp ranged from circa 27.5-30% for 'wrong' struc tures to 21.2% for the best structure solution (i.e. discrimination si milar to 6.3%); for the calculation with relaxation of the geometry of the structural fragment, R-wp ranged from circa 24-26% for 'wrong' st ructures to 11.6% for the best structure solution (i.e. discrimination similar to 12.4%)]. The work reported here represents the first appli cation of the Monte Carlo method for structure solution using neutron powder diffraction data; the new methodology (applicable to both neutr on and X-ray powder diffraction data) represents a substantial enhance ment of the scope and potential of the method, particularly for cases in which the discrimination in R-wp between the correct structure solu tion and incorrect structure solutions is expected to be inherently sm all.