The most computationally intensive computer program ever developed has been put to work simulating the quantum behaviour of atoms.

The simulations help scientists at the US government's Department of Energy determine the reliability of the country’s ageing stockpile of nuclear weapons without actually detonating the weapons.

The program, called Qbox, runs on the world's most powerful supercomputer, Blue Gene/L, built by IBM and installed at Lawrence Livermore National Laboratory in California, US. It performs very complex quantum calculations to simulate the behaviour of thousands of atoms in three dimensions.

Researchers at Lawrence Livermore and IBM fine-tuned Qbox to make the most of Blue Gene/L's number-crunching abilities. They rewrote key parts of the program to let the supercomputer's processors tackle the relevant equations more efficiently.

Critical routines

"The programmers spent a lot of time finding out where the critical routines were and how to distribute them across Blue Gene," says Jim Sexton, a member of the research team from IBM's Watson Research Center in New York, US. "It's a factor-of-four improvement, so it's basically like having a computer that's four times bigger."

Gene/L consists of 131,072 individual processors wired together using specialised high-speed networking gear. It is capable of a peak performance of 360 trillion calculations per second (teraflops), although the software that normallyruns on the machine functions well below this peak, often consuming only about 5% of the machine’s total processing power.

The enhanced version of Qbox, however, reaches a sustained performance of 207.3 teraflops, a record for any software. It simulates interactions between 1000 molybdenum atoms under high pressure using equations that take the quantum behaviour of electrons into account.

Other software can be used to simulate interactions between billions of atoms, but only using classical molecular dynamics. Performing simulations involving quantum-mechanical behaviour is far more complex and, until now, such quantum simulations have only involved about 50 atoms at a time.

Ensuring safety

The molybdenum simulations will help physicists determine the reliability and stability of warheads in the US stockpile, some of which are now beyond their original storage-time limit.

The project is coordinated by the National Nuclear Security Administration (NNSA), part of the Department of Energy, through its Advanced Simulation and Computing programme.

But Dimitri Kusnezov, head of computing at NNSA, thinks the project could also have important scientific spin-offs, for example, helping materials scientists understand the fundamental properties of materials.

"The combination of this code and this computer has implications for the broad research community well beyond NNSA’s mission of stockpile science," he says.