On the SHELX, PLATON and IUCr/checkCIF crystal structure validation platform.

With checkCIF the IUCr has created a unique platform for the validation and archiving of the result of a small-molecule crystal structure study. That information, supplied in CIF (crystallographic information file) format, should include not only the numerical results of that study but also much of the supporting experimental data and details of the procedures used during the structural investigation. This not only allows for a proper validation of a structure report but also provides an option for a future re-evaluation or use of the data for an unrelated study. The experimental data might be difficult to obtain again or even unique. George Sheldrick's SHELX software was an early adopter of CIF and with that has played a major role in the success of the IUCr validation effort. IUCr/checkCIF initially started out with tests that check for the completeness of the supplied data, along with tests that check whether their values are within the expected range. Currently, the PLATON software carries out most of the tests in the IUCr/checkCIF software framework. Validation issues are reported as a set of ALERTS along with a short clarification. All ALERT messages should be inspected, addressed with a correction where applicable or a short explanation of a special situation. Some ALERT messages are informative but do not necessarily need a comment. More detailed information about a reported issue, such as warnings about missed twinning, missed additional symmetry or voids, might be obtained with a locally installed version of the PLATON software using the same tools on which those ALERTS are based. As an example of this, the use of the PLATON/TwinRotMat tool for the detection of twinning is discussed along with some details about the underlying algorithm. Three twinning examples are presented, covering merohedral, pseudomerohedral and non-merohedral twins, to illustrate the additional twinning information that can be obtained beyond what is shown as an ALERT in the IUCr/checkCIF report.