The Story of Berni Alder and the Birth of Molecular Dynamics
In the mid-20th century, as computers were in their infancy, a visionary physicist named Berni Julian Alder pioneered a revolutionary approach to understanding nature. He transformed the scientific method itself, establishing computer simulation as a third pillar of discovery alongside traditional theory and experimentation 1 . Through seemingly simple simulations of bouncing balls, Alder and his colleagues invented molecular dynamics, a technique that would forever change how we study everything from the flow of water to the design of life-saving drugs 2 4 .
Alder's career, spanning more than 65 years until his death in 2020 at age 95, transformed statistical mechanics, many-body physics, and chemistry 1 . His work earned him the National Medal of Science in 2009, yet his greatest legacy lies in the powerful simulation tools used by scientists worldwide to explore nature at the atomic scale 4 7 .
Awarded to Berni Alder in 2009 for his pioneering contributions
A technique now used worldwide for atomic-scale exploration
Berni Alder's scientific journey began amid tremendous adversity. Born to Jewish parents in Duisburg, Germany, in 1925, his family fled to Zurich, Switzerland, when the Nazis came to power in 1933 1 4 . In 1941, they made a daring escape to the United States via a sealed train from neutral Switzerland to Spain, then Portugal, where they boarded a ship to the U.S. 4 7 .
Born in Duisburg, Germany to Jewish parents
Family fled to Zurich, Switzerland when Nazis came to power
Escaped to the United States via Spain and Portugal
Served as a radar technician in the U.S. Navy
Joined Lawrence Livermore National Laboratory
After serving as a radar technician in the U.S. Navy during World War II, Alder pursued his education at the University of California, Berkeley, where he earned undergraduate and master's degrees in chemistry 2 . He continued his studies at the California Institute of Technology under physical chemist John Gamble Kirkwood, where he first began exploring the use of mechanical computers to understand molecular behavior 2 5 .
In 1955, Alder joined the Lawrence Livermore National Laboratory, where he found the perfect environment to pursue his computational ideas 2 . The laboratory, well-funded during the Cold War, had embraced advanced computing from its founding days, providing Alder with access to increasingly powerful electronic computers 1 2 .
One of the most fundamental questions Alder tackled involved the nature of phase transitions—how and why materials change from solid to liquid to gas. The prevailing scientific wisdom held that solids formed because attractive interactions between molecules minimized their energy in a regular crystal lattice 2 . Alder questioned this assumption, wondering whether a system of hard spheres—imagined as perfectly elastic billiard balls with no attractive forces—could undergo a phase transition purely through compression 3 7 .
This question was highly controversial at the time. At scientific meetings, applied mathematicians would regularly take votes, with about half believing such a phase transition was possible and half not 7 . Traditional mathematics couldn't resolve the debate—the problem required a new approach.
In 1957, Alder and Wainwright made a remarkable discovery: as they compressed the system of hard spheres, it underwent a transition from liquid to solid 1 2 . This was revolutionary because it demonstrated that some systems crystallize at high density not to minimize their energy, but to maximize their entropy—a measure of disorder 2 .
The regular arrangement of spheres in a crystal actually allows more space for movement than in a liquid arrangement for hard spheres 2 . This overturned the textbook explanation that solids form primarily due to attractive interactions between molecules.
| Year | Discovery | Scientific Impact |
|---|---|---|
| 1957 | Liquid-solid phase transition in hard spheres | Showed crystallization can be entropy-driven, not energy-driven |
| 1962 | Phase transition in two-dimensional elastic disks | Contradicted established theories, inspired new physics |
| 1970 | Algebraic decay of velocity autocorrelation function | Revealed slow relaxation in fluids, reforming linear response theory |
| 1980 | Definitive calculation of interacting electron gas | Enabled success of density functional methods |
Alder followed this discovery with a 1962 simulation of two-dimensional elastic disks that again contradicted established theory 1 . Powerful arguments going back to Landau and Peierls suggested that true long-range ordered states couldn't exist in 2D systems with continuous symmetry. Yet Alder and Wainwright found clear evidence of a first-order transition 1 . This contradiction helped motivate the groundbreaking 1973 theory of Kosterlitz and Thouless regarding phase transitions in 2D systems, which later earned the Nobel Prize in Physics in 2016 1 .
The molecular dynamics method pioneered by Alder relies on several key components, each playing a critical role in enabling realistic simulations of atomic and molecular behavior.
| Component | Function | Role in Simulation |
|---|---|---|
| Hard Sphere Potential | Represents atoms as perfectly rigid, colliding spheres | Simplifies calculations while capturing essential physics of repulsive interactions |
| Boundary Conditions | Defines how particles behave at simulation boundaries | Prevents edge effects; allows small systems to represent bulk material |
| Time Integration Algorithms | Calculates particle positions and velocities over time | Evolves the system forward through discrete time steps |
| Initial Configuration | Starting positions and velocities of all particles | Provides baseline from which simulation progresses |
| Collision Detection | Identifies when and where particles collide | Ensures accurate tracking of particle interactions |
Alder, together with collaborator Thomas Wainwright and programmer Mary Ann Mansigh, developed what would become known as molecular dynamics simulation 4 5 . Their approach was elegantly simple in concept yet revolutionary in execution:
Alder specifically chose hard spheres because their dynamics could be exactly determined, "silencing criticism that the results were the product of inaccurate computer arithmetic" 2 . The team employed many standard features of molecular dynamics still used today: boundary conditions in space and time, error estimation, and identification of phase transition signals 5 .
Having established the fundamentals with hard spheres, Alder continually expanded molecular dynamics into new domains. In the 1970s, he began investigating quantum many-body systems 1 . This work culminated in a definitive 1980 calculation of the interacting electron gas, which crucially enabled the success of the density functional method now ubiquitous in computational physics, chemistry, and materials science 1 5 .
Helped found the University of California, Davis Department of Applied Science in 1963
Served as editor of the Journal of Computational Physics, nurturing the growing field
The Berni J. Alder CECAM Prize was established in his honor
Reflecting on the unimaginable growth of computing power—increasing by a factor of 10¹² from Livermore's first electronic machines—Alder noted that what they also didn't foresee was "how these methods would permeate all of physics, chemistry, biology, materials science" 7 .
Berni Alder's work transformed our approach to scientific discovery. As he once noted, molecular dynamics revealed that "hydrodynamics applies on a very small scale"—meaning that for a few hundred atoms over less than a billionth of a second, scientists could quantitatively reproduce the precise movement of water in an ocean wave 3 .
His message to young scientists captured his lifelong passion: "When you really make a discovery, when you for the first time know something which nobody else knows... there's no higher reward in this world and no higher high that you can get than working in science and achieving a goal that's been waiting there for a long time" 3 .
Through a seemingly simple system of colliding spheres, Berni Alder unlocked a new window into nature's secrets, creating tools that continue to reveal the hidden workings of our world at the atomic scale. His legacy lives on every time a researcher uses molecular dynamics to understand disease, design new materials, or explore fundamental physics.