Diffraction patterns may contain a zero point shift, e.g. due to a misaligned detector. You can check and (if necessary) correct for this error using the "Correct zero point" command in the "Pattern" menu, or by pressing the corresponding button in the main toolbar.
Match! will analyze the d-value quotients of all pairs of peaks. The selection of pairs is based on the fact that e.g. d(100) = 2.0 * d(200). If the quotient d(peak 1) / d(peak 2) is close to 2.0, Match! will add the corresponding 2theta-correction to a histogram. When all pairs have been checked, the histogram will be displayed in order to let the user select the actual 2theta correction value. Match! will propose a value for the correction based on the analysis of the histogram, indicated by a red line.
The higher a bar in the histogram, the more peak pairs have been found for the corresponding 2theta correction value. Hence, the user should generally select the 2theta correction according to the highest bar as close to 0.0 as possible. A value is either selected by double-clicking at the corresponding position in the histogram or by entering the value manually in the input field. Finally, please press "OK" to perform the actual correction. All data views (diffraction pattern, candidate list, peak list etc.) will instantly reflect the change of the peak positions. If the candidate list contains entries, their FoM-values will also be updated immediately.