Car Parrinello

In the 1980s, the dynamics of the molecules was calculated empirically, even though it was theoretically possible to calculate in an approximate manner the actual quantum mechanical molecule conditions “ab initio”, i.e. without using any concrete measurements, by applying the so-called Born-Oppenheimer approximation (BO approximation). The BO approximation method, however, was time-consuming and needed a computer capacity that was not available at the time: in the 1980s the computing power of a “supercomputer” was a few gigaflops; nowadays, a conventional computer’s processor can manage up to 25 gigaflops (25 billion computer operations per second).

Actual image with “ab initio”

However, the only correct way to model the actual forces exerted on an atom is “ab initio”. This means that to illustrate how chemical compounds break up and new ones form, the structure of the electrons, their energy state, has to be calculated for every position of the ion with a quantum mechanical equation. Calculating the electron structure was Roberto Car’s specialist field.

“Sometimes, ignorance is bliss“, jokes Parrinello, who has been a professor at ETH Zurich since 2001. Car and Parrinello saw beyond the opinions of the experts, who dismissed their idea of calculating both the molecular dynamics and the electron structure in one go as impossible. That meant combining the BO approximation with so-called quantum mechanical Density Functional Theory (DFT), which determines the structure of the electrons. “We knew a bit about each other’s field, but nowhere near enough. Otherwise we wouldn’t have thought it possible to combine both in one approach”, recalls Parrinello.

Tricks of the trade

For Parrinello, the starting point was to describe the element silicon. Under certain temperature and pressure conditions, silicon displays the characteristics of both metals and non-metals, and is an important semi-conductor. As these changes brought on by chemical reactions could not be determined with the Born-Oppenheimer approximation and the computer capacities that were available at the time, the two scientists resorted to a trick: they expanded the so-called Lagrange function for describing a physical system so that not every position of the ion and the forces exerted had to be calculated one step at a time. Using the Car-Parrinello method, the forces have to be determined once at the beginning of the simulation, while later the ion propagates itself like a wave with its electrons. The electron follows the ion quasi-adiabatically, without an exchange with its environment.

And so the first quantum mechanical calculation of the molecular state – “ab initio” – was successful. The Car-Parrinello method has been developed and honed for different applications over the years and has produced a lot of important information in various research disciplines. For instance, the former ETH-Zurich professor Artem Oganov used the method during his time at ETH Zurich to show that the bottom 150 kilometres of the Earth’s mantle did not – as had always been assumed – consist of the mineral perovskite, but rather a modified, layered variety that Oganov christened post-perovskite. The properties of the new mineral were able to explain conclusively the seismic discontinuity of this field for the first time.

Over the years, the two scientists have won numerous prestigious awards for their development, including the Dirac Medal in 2009. According to the commemorative publication to mark Parrinello’s 60th birthday back in 2005, the article describing the method published 25 years ago in “Physical Review Letters” was one of the most frequently cited interdisciplinary papers of all time.

Method with a long-term effect

With the leaps and bounds that have been made in computer technology over the last decade, the method is no longer as essential as it was 20 years ago; however, it is still effective in research today. Alessandro Curioni, the head of the Computational Science Group at the IBM Research Lab in Rüschlikon, who uses the method and had a hand in developing it further together with Parrinello and other researchers, stresses the importance of the Car-Parrinello method for molecular dynamics. An important side effect was also to see how Car and Parrinello’s little “trick” solved a large problem, shaping the scientists’ mode of thinking and opening up new horizons for them. “In the long run, the many ideas the Car-Parrinello method has generated for new quantum mechanical approaches to solving a problem will probably be even more important than the method itself”, says Curioni.

(Article with courtesy of ETH Life, ETH Zurich)