THESE days, many chemists sit down in front of a computer before they move to the lab bench. That鈥檚 because programs running the equations of quantum chemistry have the power to predict what will happen in a reaction, analyse the peaks in a spectrum, or explain the physical properties of a substance. For this we can thank Walter Kohn and John Pople, who share this year鈥檚 Nobel Prize in Chemistry.
The father of quantum theory, Erwin Schr枚dinger, showed that the motion of an electron can be described by an equation called its wave function. That鈥檚 fine for hydrogen, which has one electron, but what about bigger atoms and molecules?
鈥淎n electron, like everything else, exists in three-dimensional space,鈥� explains Kohn, a physicist at the University of California, Santa Barbara. Many molecules have thousands of electrons. This creates an equation with thousands of dimensions, three for each electron. 鈥淓verybody can visualise three dimensions,鈥� says Kohn. 鈥淪ome people, like Einstein, can visualise four. But nobody can visualise hundreds of thousands of dimensions.鈥�
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Kohn鈥檚 insight, gained from studies on metal alloys in the mid-1960s, was to realise that simply describing the density of electrons in an atom or molecule in three dimensions was equivalent to calculating each and every electron鈥檚 wave function. That brought the problem back down to three dimensions. 鈥淚t was a revolutionary suggestion,鈥� says William Goddard, a computational chemist at the California Institute of Technology in Pasadena.
Using Kohn鈥檚 density functional theory, chemists can calculate not only the average number of electrons at any point in space around atomic nuclei but also the overall energy of a molecule鈥攅ven huge molecules that are beyond the scope of conventional methods based on wave functions. 鈥淲ith traditional methods, accurate calculations in chemistry stop at molecules that contain about ten atoms,鈥� says Kohn. 鈥淲ith density functional theory, people have now obtained good results for molecules containing up to about a thousand atoms.鈥�
The contribution of Pople, now at Northwestern University in Evanston, Illinois, was to make quantum methods a practical tool that rank-and-file chemists could use. 鈥淚n the early days, people thought you could only solve very simple problems, where you already knew the answers,鈥� he says. He refined calculations based on wave function methods and density functional theory, and in 1970 released the Gaussian computer program which made them easily accessible to non-experts and not too demanding of computer time. 鈥淗e had the vision that this stuff would have an impact if you could make it accurate and available to everyone,鈥� says Goddard. 鈥淚t鈥檚 really brought quantum chemistry into the mainstream,鈥� says Goddard.
Today, researchers use quantum calculations in many different areas鈥攖o predict which molecules exist in deep space, for example, or to design new drugs. Tom Zeigler at the University of Calgary in Canada uses density functional theory to predict which catalysts will be efficient at making polymers. 鈥淚t is very costly to do this by experimentation and luck, so design is very important,鈥� he says. Pople points out: 鈥淭hese are really all the same problem, insofar as the fundamental equations of quantum mechanics are universal.鈥�
No one in chemistry will question the Nobel committee鈥檚 decision to honour Pople and Kohn, says Goddard. 鈥淭hey are the two people who have had the most impact in the area of quantum chemistry. I couldn鈥檛 imagine a better choice.鈥�