Introduction

Introduction

The DASH Program

DASH solves crystal structures from powder diffraction data. It assumes that the molecular formula of the compound being studied is known, and that the unit cell and space group can be determined by indexing the powder pattern. It constructs a trial crystal structure by placing a 3D model of the compound inside the unit cell. This 3D model will generally consist of rigid units connected by links having unknown torsion angles, i.e. an accurate description of the molecule, but with an unknown molecular conformation. The chances of choosing the correct conformation and positioning the model at the correct point in the cell with the correct orientation are very small. However, DASH checks how close a trial solution is to the correct structure by calculating diffraction data and comparing it with the measured diffraction data. DASH uses simulated annealing to adjust the trial structure until it agrees well with the measured data, thereby solving the crystal structure directly from the powder diffraction data.

Basic Steps for Structure Solution

The basic steps involved in solving structures from powder data are:

• Data collection and data treatment (see Data Collection and Data Treatment).

• Cell indexing (see Indexing) and space group determination (see Space Group Determination).

• Extraction of reflection intensities by Pawley fitting (see Pawley Fitting).

• Building and constraining the molecules (see Building and Constraining Molecules).

• Use of simulated annealing to solve the structure (see Structure Solution).