How does the formula search work in WebCSD?

In WebCSD you can search for an entry in the Cambridge Structural Database (CSD) by its molecular formula by entering search criteria in the Molecular Formula box on the Formula Search tab. The formula search is a per molecule search so will search each individual molecule or ion in the structure for the specified formula. A tick box allows you to specify a more exact search by allowing/disallowing other elements within the formula search. 

• Elements should be followed by a whole number, a range of numbers or greater than or less than. Ranges should be specified by a dash and less than or greater than with < or >
• You can search for molecular formulas that do not contain a given element by entering a zero after the element
• You can search for charged molecules by adding the overall charge of the molecule onto the end of the formula e.g. 2+ (If no charge is specified, it will find both neutral and charged molecules)
• Finally, formula search also allows you to search using multipliers for structures which have more than one molecule with the same partial formula. If no multipliers are searched for, then it will still find structures with molecules that have a multiplier greater than one.

Examples:

• N1 C0 will find all structures with molecules that have one nitrogen and don't contain any carbon atoms
• N1 C1-3 will find all structures with molecules that have one nitrogen and between one and three carbon atoms
• C>2 N1 will find all structures with molecules that have more than two carbon atoms and exactly one nitrogen atom
• C1 N1 2+ will find molecules that have one C, one N and a +2 charge (note the multiplier occurs before the + or - in the formula query)
• 2(N1) will find structures that have two molecules with one nitrogen atom in the formula

Formulas in the CSD are curated formulas based on information from the deposited CIF file. These formulas may sometimes differ from the formula provided in the deposited CIF file, but they should correspond to the contents of the structure and our curation guidelines. Our guidelines ensure that we have a standard way of representing data in the CSD and there is consistency between entries. For example if part of the structure has been SQUEEZED to remove the contribution from a disordered part of the structure, where possible we will include the squeezed part in the formula to provide a more chemically meaningful formula.