A quantitative method to judge how alike molecular crystal structures are could aid crystal design and understanding.

It is easy to look at two crystal structures and see how similar they are, but how can this comparison be quantified to allow a quick analysis of to what extent molecules look like each other in their solid forms? Scientists from the UK and Australia have designed a way to give crystal structures what they call a similarity index, which does just that.

Andrew Parkin at the University of Glasgow and colleagues have developed a method called structural genetic fingerprinting. They use Hirshfeld surfaces - a way of describing the shape of a molecule in terms of its crystal surroundings - to compare molecules in different crystal structures.

Structural fingerprinting could become a valuable tool for materials scientists

Using this technique the scientists say they can show quantitatively that, for instance, naphthalene is more similar to anthracene than it is to benzene. 

The ability to compare structures and interactions quantitatively will, they say, mean that crystal engineers can develop their ideas and designs more quickly and efficiently.

Conventional structure comparison is a time-consuming exercise, explained Parkin. 'A general method has been needed for years,' he added.

Identifying similarities in solid-state forms of a material quickly and reliably is vital in the pharmaceutical industry, and will have implications across all areas of materials science, said Parkin, because the physical properties of materials depend on their solid-state structure.

Parkin hopes to continue refining the method so that it is very broadly applicable. 'We expect it to become a routine tool for anyone involved in comparing structures or designing new materials,' he said.

Caroline Moore