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.
Elements should be followed by a whole number, a range of numbers or greater than or less than. Any elements not followed by any number will default to 1. Ranges should be specified by a dash and less than or greater than with < or >. Charges may also be specified. A tick box allows you to specify a more exact search by allowing/disallowing other elements within the formula search.
You are able to search for molecular formulas that do not contain a given element by entering a zero after the element. For example searching for N1 C0 will find all structures with molecules that have one nitrogen and don't contain any carbon atoms. You are also able to search for ranges by using a dash, for example searching for N1 C1-3 will find all structures with molecules that have one nitrogen and between one and three carbon atoms. Similarly, you can perform greater or less than searches by using > or <. For example C>2 N1 will find all structures with molecules that have more than two carbon atoms and one nitrogen atom.
The molecular formula search also allows you to search for charges, the charge corresponds to the overall charge of the molecule and not the individual atom. For example searching for 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). If no charge is specified, it will find both neutral and charged molecules.
Finally, formula search also allows you to search using multipliers. For example 2(N1) will find structures that have two molecules with one nitrogen atom in the formula. Again, if no multipliers are searched for, then it will still find structures with molecules that have a multiplier greater than one.
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 where possible we will include the squeezed part of the structure in the formula so that it is a more chemically meaningful formula.