The National Institute for Computational Sciences

Water and Quantum Mechanics

Scientists Apply Kraken in Quest for Unprecedented Understanding of H2O

By Jennifer Bailey


Supercomputers have allowed for a wide variety of research that otherwise would be impossible. They have greatly increased the speed of calculation and are frequently applied in solving problems of size or complexity not feasible on a typical computer.

A group of researchers from Temple University used the Kraken supercomputer's prowess to perform very complex liquid studies based on quantum mechanics. In addition, they have investigated rare metals. Charles Swartz, a graduate student at Temple, and Xifan Wu, Ph.D., a researcher and assistant professor at Temple, have developed a variety of ab initio (Latin for "from the beginning") molecular dynamics (AIMD) techniques for the project.

Water at Nanoscale

Wu explains that in AIMD the forces are computed by electronic-structure calculations, which allow for complex simulations without relying on adjustable parameters. AIMD connects Newton's equations of motion and Schrodinger's equation that describes how the state of a physical system changes over time.

Two of their techniques include the hybrid functional AIMD and the GW-based electron excitation theory. The hybrid functional AIMD has a high computational cost, but is achievable on processors such as Kraken, Wu says.

The GW-based electron excitation theory uses the GW approximation, which estimates the self-energy of a many-body system of electrons. Self-energy is the contribution to the particle's energy from interactions between the particle and the system in which it resides. The GW approximation shows the expansion of the self-energy by looking at a single particle Green's function (G) and the screened Coulomb interaction (W).

Green's function is a correlation function showing the impulse response of the rate of change of a type of quantity. The Coulomb interaction is an electrostatic interaction of forces between electrically charged particles.

Using their new techniques, Swartz and Wu studied the hydrogen-bond structure in liquid water and liquid solutions, followed by electron excitation, which will be compared with experiments.

"Liquid water is the origin of life. However, it has not been completely understood yet, at least at the level of quantum mechanics," Wu explains. This level is nanoscale, or 1–100 nanometers. A nanometer is a billionth of a meter.

Autoprotonation of water is where hydroxide and hydronium are produced and recombined, and it is behind a wide range of occurrences in physics, chemistry, and biology. Specifically, Swartz and Wu are looking at the dissolution structures of hydroxide and hydronium, which are important in understanding proton transfer in water.

A significant function of the presence of hydronium and hydroxide is to be able to determine a solution's pH. For example, in pure water, an equal number of hydroxide and hydronium ions exist, so pH is 7 (neutral).

Rare Earth Metals

The research group is also investigating a new family of room-temperature multiferroic rare earth metals (RFeO3, with R representing a variety of rare earth metals). Lutetium (Lu) in the molecule LuFeO3 is of particular interest as it is, Wu says, "a rare case material with coexisting spontaneous electric and magnetic polarizations." "Eventually the next-generation devices will be made based on the switchability by both applied and magnetic fields," he explains. Due to the possible applications, room-temperature multiferroics is currently one of the most intensely studied areas.

Multiferroics are materials that exhibit multiple primary ferroic order parameters simultaneously. The four basic primary ferric order parameters are ferromagnetism, ferroelectricity, ferroelasticity, and ferrotoroidicity, although this last one has been under debate.

Ferromagnetism is the basic mechanism by which certain materials form either permanent magnets or are attracted to magnets. Ferroelectricity is a characteristic of certain materials that have a spontaneous electric polarization that can be reversed by an external electric field. Ferrotoroidicity is a phase transition to spontaneous long-range order of microscopic magnetic toroidal moments. However, ferrotoroidicity is the subject of debate due to the difficulty of distinguishing it from antiferromagnetic order.

Superlattices

In conjunction with multiferroics, the group also is designing other functional superlattices. A superlattice is a structure composed of layers of a variety of materials; it typically has semiconducting properties.

Wu stresses the importance of understanding that "functional superlattices are artificial materials, which means they do not exist in nature and could have more enhanced functional properties than bulk materials," and he adds that a significant benefit of using superlattices is that they "provide a perfect way for materials scientists to design functionality by tuning, or adjusting, the interface." The interface is where electrons are confined between two semiconductors. Changing the interface adjusts conditions for charges to flow through the structure.

Publications

Recently, Swartz and Wu had "Ab initio studies of ionization potentials of hydrated hydroxide and hydronium" published in the journal Physical Review Letters and they have also submitted "Structural origin of spin orientation in hexagonal LuFeO3: A bridge between ferroelectricity and ferromagnetism" for publication.

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Article posting date: 28 May 2014

About JICS and NICS: The Joint Institute for Computational Sciences (JICS) was established by the University of Tennessee and Oak Ridge National Laboratory (ORNL) to advance scientific discovery and state-of-the-art engineering, and to further knowledge of computational modeling and simulation. JICS realizes its vision by taking full advantage of petascale-and-beyond computers housed at ORNL and by educating a new generation of scientists and engineers well versed in the application of computational modeling and simulation for solving the most challenging scientific and engineering problems. JICS runs the National Institute for Computational Sciences (NICS), which had the distinction of deploying and managing the Kraken supercomputer. NICS is a leading academic supercomputing center and a major partner in the National Science Foundation's eXtreme Science and Engineering Discovery Environment, known as XSEDE. In November 2012, JICS sited the Beacon system, which set a record for power efficiency and captured the number one position on the Green500 list of the most energy-efficient computers.