Nearly 20 years ago, two Trieste-based scientists provided the scientific underpinning for computer simulation, setting the stage for the modelling revolution that followed.

Models of Behaviour

Roberto Car and Michele Parrinello

In the modern BC (before the computer) world of science, all of science, but particularly physics, could be neatly divided into two categories: theoretical and experimental.
Over the past two decades, however, computer simulation and modelling--a hybrid of theory and experiment or, more precisely, an intricate blend of theory and experiment in which it is difficult to separate one from the other--has emerged as one of the primary ingredients of modern scientific inquiry. Indeed some researchers now contend that computer simulation and modelling represents a third way of doing science.
One of the most noteworthy advancements in this field took place in Trieste during the 1980s when two Italian physicists, Roberto Car and Michele Parrinello, conducted research at the International School for Advanced Studies (SISSA), next door to ICTP, and at ICTP, on the ab-initio molecular dynamics method, an analytical method that has since assumed the names of its discoverers: the Car-Parrinello method.
The article that they coauthored, "Unified Approach for Molecular Dynamics and Density-Functional Theory," published in Physical Review Letters (22-25 November 1985), remains the fifth most cited article of the publication--with a total of 3,000 citations and counting. For their work, Car and Parrinello shared the 1990 Europhysics Prize of the European Physical Society.
Born in Trieste and educated at Politecnico di Milano, Italy, Roberto Car is currently professor in the Chemistry Department at Princeton University, New Jersey, USA. Michele Parrinello, who was born in Messina, Sicily, is now professor of computational science at the Laboratory of Physical Chemistry of the Eidgenossische Technische Hochschule Zürich (ETH, Swiss Federal Institute of Technology) in Lugano, Switzerland.
The Car-Parrinello method helped to launch a new era of understanding in the esoteric field of electronic structure calculations for solids, liquids and molecules. While the concept may be difficult to understand, it has had an enormous impact in the creation of molecular simulations that have had useful applications in all the basic sciences.
Car and Parrinello, then both in their early 40s, first met in Trieste in 1984 through their affiliation with SISSA and ICTP, which have always promoted inter-institutional interaction between their staff and visiting scientists.
"We hit it off immediately and soon talked about the possibility of working together," says Parrinello. "Roberto was deeply involved in studies on how to improve the electronic structure of crystals, while I had a strong background in statistical mechanics and molecular biology. We thought it could be interesting to combine our expertise."
"At the time, the challenges posed by silicon, particularly liquid silicon, were among the hottest topics in the field of condensed matter physics. Researchers knew, from experimentation, that silicon changes from a semiconductor to a metallic phase when you melt it. But no one could satisfactorily explain how this transition took place. It was an unanswered challenge that we thought we could make a contribution to, through our varied and complementary skills."
The twosome, often working at night when computer capacity was most available, conducted their research and wrote their paper over several months in the winter of 1984. The article, after a series of minor revisions, was published in Physical Review Letters several months later.
"It was an extraordinary time," Parrinello recalls. "I think that, regardless of your profession, it is always important to be in the right place at the right time--and that certainly was the case for Roberto and me in Trieste in the mid 1980s. We later went our separate ways, but we both look back on those years with fondness, especially when meeting on occasions
in Trieste."
In recent years, Car has used his expertise to pursue such research topics as modelling electronic current in nanostructures. "But I still benefit from the collaboration with Michele. The way we approached the problem--through what was then the new field of computer simulation--has given me both a practical tool and lofty inspiration for my subsequent research."
The Car-Parrinello model is now a standard analytical concept not only in condensed matter physics, but in other disciplines as well. Parrinello, for example, is currently interested in the behaviour of water, whose study involves biophysics, biochemistry and, yes, sophisticated computer simulation.
"Even today," Parrinello says, "many of my colleagues insist that computing simulation fails to represent the real world and, as a result, real problems cannot be addressed by such a methodology."
"I agree that computing is no substitute for thinking. But computer simulation has proven useful in so many studies that the verdict should no longer be in dispute: There is overwhelming evidence in favour of using such an approach for both broadening and deepening our scientific understanding of an increasing number of problems. Simulation is here to stay."

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