Science: How to choose a molecule in a million: Some liquid crystals act like football crowds, others line up like soldiers. Christine Hewitt looks at research that could create a better calculator

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The Independent Online
Chemists brewing new chemicals are like children playing with Lego bricks - they often learn by trial and error which structures fall apart and which can be built into exciting and useful playthings. But drudgery in the laboratory will soon be reduced by using computer simulation as a way of predicting which patterns of molecules will produce useful industrial materials.

According to Dr Mike Allen, of Bristol University, computer simulations of complex liquid crystal molecules will help companies to design better display and switching devices such as calculator displays. 'The UK is right up there in the world rankings, with several successful simulation research teams', he says.

'There is a great need to understand the relation between the properties of important materials like liquid crystals and the structure of the molecules of which they are made: their sizes, shapes, the forces between them. The tried and trusted approach of actually synthesising a range of different molecules and measuring their liquid crystalline properties can be expensive and time- consuming.' Using the computer first allows chemists to concentrate experimental work on promising areas.

Dr Allen feeds the computer with basic information about a molecule and its constituent atoms. The computer, using basic laws of physics, works out how large groups of these molecules will behave together over a period of time and displays a picture of what is happening.

Liquid crystal molecules are long and thin, so Dr Allen looks at whether they jostle together like a football crowd, with lots of friction, or line up in military fashion. Will they move easily up and down or jiggle from side to side against their neighbours? If he pushes them closer, what happens?

Liquid crystal molecules move about like the molecules in ordinary liquids such as water, but at certain temperatures they are more 'ordered', like those in a crystalline solid such as ice. Dr Allen's computer can show whether the molecules line up only in one dimension - in a row - or in two, with rows neatly piled on top of each other like the layered people in the Halifax wedding cake advertisement. The behaviour of the molecules in relation to their neighbours predicts the bulk properties, which he then compares either with experimental results, if available, or with data suggested by scientific theories of what is going on.

'Simulation results have provided theoreticians with a playground in which to try out their latest approximations,' he says. 'Ninety per cent of theories are wrong - but if we manage to get the 10 per cent right, we have succeeded; that's what theoreticians are paid for.'

Dr Allen's technique can study a million molecules at a time, but he usually looks at only a few hundred to keep costs down. To save computer time, he often uses very simple model molecules to try to answer basic questions: for example, how does overall molecular shape direct key properties? How important is flexibility or rigidity? Are long-range attractive forces between molecules important?

Industry is less interested in theory than in the applications. The makers of digital watches use liquid crystal structures which look like spiral staircases. When an electric or magnetic field is applied the molecules line up, the twist of the staircase changes, and the substance becomes visible. Firms are looking to improve performance, response times and reliability. Displays must also work well within normal household temperature ranges and have good optical properties.

'Manufacturers would like to get a specific degree of twist or a particular level of resistance to the current,' Dr Allen says. His research should help them to find or make molecules that have the right properties - and thus save many hours' slaving at the laboratory bench.

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