Search-Match Basics

The actual core of Match! is the so-called "search-match" routine. It compares the diffraction pattern of an unknown sample to the patterns of known compounds stored in the reference pattern database.

To be exact, Match! compares the peak data found in the experimental diffraction pattern to the peak data in the reference database. This can either be done for all peaks of the diffraction pattern (menu command "Search/Search-Match", shortcut <Ctrl+M>), or for the marked peaks only (menu command "Search/Search-Match (marked peaks only)"; shortcut <Ctrl+Shift+M>). For each reference database pattern, Match! calculates a numerical value describing the quality of the agreement with the pattern of the unknown sample, the so-called "figure-of-merit" (FoM).

If only a limited set of marked peaks shall be used in the search-match calculation, please pay attention to the following hints:

Once the search-match calculation has been finished, the resulting so-called "candidate entries" are ranked according to their FoM values; the entries with the highest FoM are the ones which are most likely to be present in the sample. The ranked list of candidate entries is then presented to the user in the candidate list in the lower part of the Match! window. The user analyzes the entries at the top of the list and selects those which are most likely to be present in the sample ("matching entries/phases").

The user can limit the comparison with the pattern of the unknown sample to certain groups of database entries by applying additional restraints (like chemical composition, color, density, crystallographic data, quality marks, PDF subfiles etc.) in order to keep the number of possible candidate phases as low as possible.

A variety of options and parameters can be adjusted which affect the FoM-calculation. If your search-match calculation does not give a reasonable result, you should "play around" with the parameters a little bit to check how the results could be influenced (e.g. by reducing the effect of intensity differences in order to compensate for textural effects).

You can clear the FoM values and all other results of a search-match calculation by running the Reset search-match results command from the "Search" menu. This might be useful e.g. if you are applying restraints and would like to check which entries would be present without the the minimum figure-of-merit criterion.

The FoM should be the first guide for the user to select the matching entries from the list of candidate entries, however, it should not be the only one. With most "real-live" samples it is definitely necessary to also visually compare the diffraction patterns using the diffraction pattern graphics and the peak list.

Note: If only raw (profile) but no peak data are present when the search-match is executed, it will automatically execute the automatic raw data processing before the actual calculation is started.