VEC can perform automatic indexing for a diffraction pattern, provided three non-colinear reflections have been assigned indices manully. In order to provide sufficient accuracy, these three reflections should be  far appart from the origine of the reciprocal lattice.

To assign indices to a given reflection, click the reflection first. This leaves a green + mark on it. Fine tune the position of the + mark by the arrow keys on your keyboard. Then click the 'hklm' button  . A dialog box will appear as shown on the lower right corner of the above figure. Type in indices and click OK. The assigned indices will then appear adjacent to the + mark. The index M in the dialog box is for use with one-dimensionally modulated structures. For conventional structures please keep M equal to zero.