Why does the number of hits from a search change when I add a measurement to my substructure query?

When drawing a substructure query in ConQuest, two potential sources of information from the CSD may be referenced. The 2D diagram contains the chemical information (e.g. bond types) assigned to the crystal structure, and the 3D atomic coordinates are used to calculate geometric parameters (e.g. bond lengths or angles).

To search for a measurement in a substructure query, it is necessary for the CSD entry to have a match between the 2D diagram and the 3D atomic coordinates to provide results. This is not always the case – for example, if a disordered solvent molecule is present in the structure, but the crystallographer has decided not to model the coordinates from the diffraction data. In this scenario, the 2D diagram will contain the solvent molecule, but no atomic coordinates will be seen in the 3D view. A substructure search for the solvent in this case would hit the CSD entry, but a search for the solvent with a measured bond length would not, as the geometric data is not available. This would lead to a different number of hits from apparently identical substructure searches.