No abstract

The crystal structure of 2,2-dihydroxymethylbutanoic acid (C 6 H 12 O 4 ) in monoclinic form has been determined ab initio from synchrotron radiation powder diffraction data. Two O and ®ve C atoms were ®rst derived by direct methods. Two missing O atoms and one C atom were found by the Monte Carlo method without applying constraint to their relative positions. Positional and isotropic displacement parameters of these non-H atoms were re®ned by the Rietveld method. Molecules are linked by hydrogen bonds and they make sheetlike networks running parallel to the (010) plane. The Monte Carlo method is demonstrated to be a powerful tool for ®nding missing atoms in partially solved structure.

The Monte Carlo method is applied to finding missing atoms in solving inorganic crystal structures without applying a rigid-body approximation. Whole powder patterns of α-SiO2 and Mg2SiO4 were used for testing a procedure. Four atoms among the six in the asymmetric unit of Mg2SiO4 could be found in the present analysis. The use of well-refined profile parameters enhanced the frequency of correct structure configurations in the Monte Carlo search. Utilizing structural information available for constructing a trial configuration is also considered to be important for efficiently searching the structure solution. A procedure for assignment of equivalent positions to respective atoms is presented. The method can be used as a powerful tool for finding missing atoms in a partially solved structure. A histogram of weighted reliability index in Monte Carlo calculations is very informative for evaluating the performance of the method. ©