Molecular, crystallographic and algorithmic factors in structure determination from powder diffraction data by simulated annealing

The crystal structure of famotidine form B has been solved directly from powder diffraction data by the application of simulated annealing. The molecule crystallizes in the monoclinic space group P2(1)/c with refined unit-cell dimensions a = 17.6547 (4), b = 5.2932 (1), c = 18.2590 (3) Angstrom and beta = 123.558 (1)degrees at T = 130 K. The core of this work is a systematic investigation of the influence of algorithmic, crystallographic and molecular factors on the structure solution process. With an appropriate choice of annealing schedule, molecular description and diffraction data range, the overall number of successes in solving the crystal structure is close to 100%. Other factors, including crystallographic search space restrictions and parameter sampling method, have little effect on the structure solution process. The basic principles elucidated here have been factored into the design of the DASH structure solution program.