PROGRAM HARDPACK: CRYSTAL STRUCTURE DETERMINATION FROM CELL CONSTANTS AND MOLECULAR STRUCTURE ONLY

Rainer Rudert,

Federal Institute for Material Research and Testing, V. 41, Unter den Eichen 44-46, 12203 Berlin, Germany,
E-mail: rudert@chemie.fu-berlin.de

Keywords: Crystal structure determination, crystal energy minimization

Organic crystal structures can not only be determined by X-ray diffraction data, but also by the knowledge of molecular structure and interaction between molecules.

The computer program HARDPACK [1], originally developed for predicting crystal structures, is a useful tool to solve crystal structures with only the cell constants, maybe the space group, and the molecular structure known. The program calculates the crystal energy by summation of the atom-atom potentials and searches the minimum energy for different molecular positions and orientations. It can repeat this search for different space groups and molecular conformations.

The molecules are assumed to be rigid except for a few rotating groups. Hydrogen bonds are modeled by electrostatic interaction and special potential parameters. The potentials used are:

V(r_ij) = A_ij·exp(-B_ij·r_ij)/r_ij^Dij - C_ij/r_ij⁶ + coul·q_i·q_j/r_ij (Interaction between two atoms i and j),

A_ij, B_ij, C_ij, D_ij are parameters depending on the atom types of atom i and j, and on the surface type, q_i and q_j are the partial charges of the atoms i and j, coul is a conversion factor.

The program helps solving crystal structures with poor or few diffraction data. If a powder diffraction pattern exists, the results of HARDPACK may serve as a start structure for a Rietveld refinement.

In his lecture the author will describe the program and present some examples of crystal structure determinations.

Reference

1. Rudert, R. (1996). Acta Cryst. A52 (Supplement), C-94.