Do the simulations represent the real world on an atomic scale?

Computer simulations hold great promise for accelerating the molecular engineering of green energy technologies, such as new systems for storing electrical energy and using solar energy, as well as capturing carbon dioxide from the environment. However, the predictive power of these simulations depends on having a way to confirm that they do describe the real world.

Such confirmation is not a simple task. Many assumptions go into the configuration of these simulations. Accordingly, simulations should be carefully checked using an appropriate “validation protocol” involving experimental measurements.

“We focused on a solid / liquid interface because interfaces are ubiquitous in materials, and those between oxides and water are essential in many energy applications.” – Giulia Galli, theorist with a joint appointment at Argonne and the University of Chicago

To meet this challenge, a team of scientists from the Argonne National Laboratory of the United States Department of Energy (DOE), the University of Chicago and the University of California at Davis, developed a revolutionary validation protocol for simulations of the atomic structure of the interface between a solid (a metal oxide) and liquid water. The team was led by Giulia Galli, a theorist with a joint appointment at Argonne and the University of Chicago, and Paul Fenter, an experimenter from Argonne.

“We focused on a solid / liquid interface because interfaces are ubiquitous in materials, and those between oxides and water are essential in many energy applications,” said Galli.

“To date, most validation protocols have been designed for bulk materials, ignoring interfaces,” Fenter added. “We felt that the atomic-scale structure of surfaces and interfaces in realistic environments would present a particularly sensitive, and therefore difficult, validation approach.”

The validation procedure they designed uses high-resolution X-ray reflectivity (XR) measurements as the experimental pillar of the protocol. The team compared XR measurements for an aluminum oxide / water interface, conducted at the 33-ID-D beamline at Argonne’s Advanced Photon Source (APS), with the results obtained by running high-performance computer simulations at Argonne Leadership Computing Facility (ALCF). Both APS and ALCF are facilities for users of the DOE Science Office.

“These measurements detect the reflection of very high energy x-ray beams from an oxide / water interface,” said Zhan Zhang, physicist in Argonne’s X-ray Science division. At the beam energies generated at APS, the wavelengths of the X-rays are similar to the interatomic distances. This allows researchers to directly probe the molecular-scale structure of the interface.

“This makes XR an ideal probe for obtaining experimental results directly comparable to simulations,” added Katherine Harmon, Northwestern University graduate student, guest student in Argonne and first author of the article. The team performed the simulations at ALCF using the Qbox code, which is designed to study finite temperature properties of materials and molecules using simulations based on quantum mechanics.

“We were able to test several approximations of the theory,” said François Gygi of the University of California at Davis, team member and lead developer of the Qbox code. The team compared the measured XR intensities with those calculated from several simulated structures. They also studied how x-rays scattered by electrons in different parts of the sample would interfere to produce the signal observed experimentally.

The team’s effort turned out to be more difficult than expected. “Admittedly, it was a bit of trial and error at the beginning when we were trying to figure out the right geometry to adopt and the right theory that would give us accurate results,” said Maria Chan, co-author of the study and scientist at the Argonne Center for Nanometric Materials, a user installation of the DOE Office of Science. “However, our back and forth between theory and experience paid off and we were able to set up a robust validation protocol that can now be deployed for other interfaces as well.”

“The validation protocol helped quantify the strengths and weaknesses of the simulations, providing a pathway towards building more accurate models of solid / liquid interfaces in the future,” said Kendra Letchworth-Weaver. Assistant professor at James Madison University, she developed software to predict XR signals from simulations during a postdoctoral fellowship at Argonne.

The simulations also provide new insight into the XR measurements themselves. In particular, they have shown that the data is sensitive not only to atomic positions, but also to the electronic distribution surrounding each atom in subtle and complex ways. This information will prove beneficial for future experiments on the oxide / liquid interfaces.

The interdisciplinary team is part of the Midwest Integrated Center for Computational Materials, headquartered in Argonne, a materials science computer center supported by the DOE. The work is presented in an article titled “Validation of Molecular Dynamics Calculations of Fundamental Principles of Oxide / Water Interfaces with X-Ray Reflectivity Data”, which appeared in the November 2020 issue of Physical examination equipment.