Molecular Formula Information Items

The standard molecular formula (QH followed by other element/count combinations in alphabetical order) may be searched for specified elements or for element/count combinations in two basic ways (a) within the discrete bonded residues or ions of the crystal chemical unit, or (b) within a formula that has been summed over all such residues and ions. The test  

Even with the advent of interactive menu-driven query construction (see Section 3.5.8 below), it is still advisable to draw the required fragment carefully on paper and to consider the query definition under the following four subheadings. 

At its simplest level, this consists of the definition of the atomic nodes for the fragment, in terms of element type only, and of the simple connections that must exist 

User definition of element-group symbol and group composition (see text) 

Number of occurrences of fragments required Locate fragments in same bonded residue Reset default bond type from single to [nval] 

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If organic chemists had to choose a single item of information above all others that are usually available from spectra or from chemical manipulations, they would certainly choose the molecular formula. 

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