Going big to study small

Researchers look for structures with the lowest energy because that’s how nature tends to arrange things. To conduct this search, Davenport’s team computationally arranges atoms into different shapes and models their potential energy.

“If two atoms are far apart then the energy is two times the sum of the isolated atoms,” Davenport explains. “If I allow those two atoms to come close together they will form a chemical bond. It is the chemical bond energy that we are really looking for.”

As part of their search, the investigators use quantum mechanics to simulate the arrangement of atoms in different configurations and then attempt to determine which has the lowest energy.

“If I have three atoms I can put them into a triangle. If I have four I can put them into a tetrahedron, or if I have eight I can put them into a cube,” Davenport says. “I can then vary the cube edge to see how the energy varies. If the cube edge is very small, usually the energy will be high, because these atoms are too close together and repel each other. If I take them far apart they also have a high energy because they like to bind together. So someplace in between there is an optimum and we search for those optima.”

Scaling up

Researchers at BNL’s Computational Science Center have modeled nanoparticles for several years and succeeded in calculating the crystalline structures of small clusters of up to 150 atoms of gold and palladium. Their results agree with some empirical evidence.

Up to now, the group has verified its results with cutting-edge optical experiments that are slowly becoming available. “There are molecular beam experiments in which these particles can sometimes be created and can be stuck on surfaces and examined with electron microscopes, but it is difficult,” Davenport adds. The researchers also have looked at hydrogen as though it was stuck on a palladium nanostructure, but do not yet have experimental confirmation of their results. They’re now working to scale up their models to examine the structures of particles consisting of hundreds to thousands of atoms.

The researchers chose to model metals first because of previously existing evidence that they would make good storage media and because of past experience at BNL, where other groups have modeled metals for other potential applications.

Davenport’s team has used a number of DOE-developed computer codes in its work, including NWChem, which was developed at Pacific Northwest National Laboratory; GAMESS, a product of Ames Laboratory; and a home-grown code dubbed LASTO.

In addition to the nanoscience work, other researchers at BNL’s Computational Science Center are working on magnetohydrodynamics, with a particular focus on ITER, a fusion research project undertaken by an international consortium including the United States. The ITER reactor, being built in France, will be used to create and study conditions required to produce electricity through nuclear fusion, rather than fission, the process behind present nuclear power plants. Nuclear fusion, the process that takes place in the Sun, holds the potential for solving the world’s energy problems because it would be a virtually limitless energy source independent from finite fossil fuels.

Brookhaven researchers also are involved in modeling the intense collisions produced in Brookhaven’s Relativistic Heavy Ion Collider (RHIC). Activated in 2000, the RHIC is used to study what the universe may have looked like in the first few moments after its creation. It does so by crashing beams of gold ions head-on in a subatomic collision. Davenport’s department is helping increase the intensity of the beam in the collider by simulating the motion of particles in the accelerator.

Some of this computational science work is done on other computers at the Computational Science Center, including the 500-processor Galaxy, a conventional Beowulf cluster. But it is New York Blue that has become the shining star of a very small universe.