Eric Neuscamman, one of chemistry’s newest assistant professors, succinctly summarizes his research with a simple question, “How do we predict how electrons glue things together?”
Neuscamman is an electronic structure theorist, a researcher in a branch of theoretical chemistry where questions about even simple molecules can, if not asked carefully, explode into computational nightmares. His job is to gain insight into fundamental topics while avoiding those nightmares.
He is the oldest of three children of a Chevron petroleum geologist and a college-educated stay-at-home mother. He was born in 1984 in Denver, CO, but spent his early years in Livermore, CA. When he was five years old the family moved to Beaconsfield, England, a town to the northwest of London, almost halfway to Oxford. “In England,” says Neuscamman, “I was enrolled in an international elementary school. It had great teachers, and I remember it as a damp but positive experience.”
His father was transferred back to Chevron headquarters, then in San Ramon, CA, so the family returned to Livermore, where Neuscamman graduated from high school in 2002 and where he lives today with his wife, Stephanie, and their two children.
For college, Neuscamman attended UCLA, where he spent his first two years studying chemical engineering. “But I grew dissatisfied,” he says, “because I wanted to understand more fundamentally the physics of how electrons create molecules. I grew more interested in quantum mechanics and switched to physical chemistry.” As a third-year student he studied NMR with Yung-Ya Lin, who had earned his Ph.D. at Berkeley in 1998 with Alex Pines.
Neuscamman had enough credits to graduate in 2006 with B.S. degrees in both physical chemistry and chemical engineering, with a math minor. “Although I didn’t pursue it,” he says, “I’m thankful for my engineering background. Numerical optimization techniques have come in handy.”
Next Neuscamman was off to Cornell University in upstate New York, an experience he described as, among other things, “a working education in buying proper winter clothing.” There he studied in the group of Garnet Chan, whose research explored quantum many-particle systems by using numerical simulations.
Classic mechanics has its n-body problem, which Issac Newton and others struggled to solve in the late 1600s. Small planets and other objects travel around large stars in neatly defined elliptical orbits. However, when several bodies of similar size orbit around each other, their gravitational fields interact, making the calculations of their orbits much more complicated.
Likewise, quantum mechanics has its many-body problem, which arises in atomic scale systems of interacting particles. Instead of planets occupying distinct positions in smooth orbits, electrons may be arranged in space in many different ways at once. The quantum mechanical challenge is precisely that many arrangements can exist simultaneously, and that the number of probable arrangements grows extremely rapidly with the number of electrons.
For Neuscamman, that’s a problem. Keeping track of quantum interactions as more particles are added to a system makes simple models scale factorially, which means even the largest supercomputers can become hopelessly inadequate. Neuscamman has addressed this problem by borrowing an insight from the art of sculpting, where, as an old adage states, the sculptor simply starts with a large block of stone and chips away the unnecessary parts.
Says Neuscamman, “A straightforward approach to quantum chemistry fails due to the quantum many-body problem, in which the size of the Hilbert space grows factorially with the system size. In traditional quantum chemistry, the most common approach has been to start with a small subsection of Hilbert space and then to sparingly add more flexibility only as necessary to achieve accurate results.
“I’ve shown it may be more effective to pursue a subtractive strategy, in which an initially crude approximation that covers more of Hilbert space than necessary is cleaned up by deleting unnecessary pieces. Crucially, this paring down need not require inspecting the details of the system’s factorial complexity and can thus be achieved at a polynomially scaling cost.
“Chemistry is ultimately about understanding and controlling collections of electrons,” he adds, “and theoretical chemists make predictions that are useful for this purpose. If you’re trying to make fuels from sunlight, for example, which of three expensive possible experiments do you choose? Theoretical guidance helps make more reliable predictions about which experiment will yield the most insight.
“Advances in electron structure theory are needed in order to improve our predictive power about chemical catalysis, molecular light harvesting and other critical applications of chemistry to the big problems society is facing.”