Do simulations represent the real world on the atomic scale?

Computer simulations hold great promise in accelerating the molecular engineering of green energy technologies, such as new systems for storing electrical energy and the use of solar energy, as well as the capture of carbon dioxide from the environment. However, the predictive power of these simulations depends on the way of confirming that it describes the real world.

Such confirmation is no simple task. Many assumptions go into setting up these simulations. As a result, the simulations must be carefully monitored using an appropriate ‘validation protocol’ that includes experimental measurements.

“We have focused on a solid / liquid interface, because interfaces are ubiquitous in materials, and the between oxides and water is the key 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 US Department of Energy (DOE) Argonne National Laboratory, University of Chicago and the University of California, Davis, have developed a groundbreaking 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 experimental Argonne.

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

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

The validation procedure they designed uses XR reflectivity measurements (XR) as the experimental pillar of the protocol. The team compared XR measurements for an alumina / water interface, performed on bar line 33-ID-D at Argonne’s Advanced Photon Source (APS), with the results obtained by high performance computer simulations at the Argonne Leadership Computing Facility ( ALCF). Both the APS and ALCF are DOE Office of Science User Facilities.

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

“This makes XR an ideal investigation to obtain experimental results that are directly comparable to simulations,” added Katherine Harmon, a graduate student at Northwestern University, a visiting Argonne student and the first author of the article. . The team performed the simulations at the ALCF using the Qbox code, which was designed to study finite temperature properties of materials and molecules using simulations based on quantum mechanics.

“We were able to test different approaches to the theory,” said Francois Gygi of the University of California, Davis, part of the team and lead developer of the Qbox code. The team compared measured XR intensities with those calculated from various simulated structures. They also investigated how X-rays scattering of the electrons in different parts of the sample would interfere to produce the experimentally observed signal.

The effort of the team was more challenging than expected. “Admittedly, it was a bit of a trial and error at the beginning when we tried to understand the right geometry and the right theory that would give us accurate results,” said Maria Chan, a co-author of the study. scientist at Argonne’s Center for Nanoscale Materials, a DOE Office of Science User Facility. “Our back and forth between theory and experiment has paid off, and we have been able to set up a robust validation protocol that can now be used for other interfaces as well.”

“The validation protocol has helped quantify the strengths and weaknesses of the simulations, and provides a way to build more accurate models of fixed / liquid interfaces in the future,” said Kendra Letchworth-Weaver. An assistant professor at James Madison University, developed his software to predict XR signals from simulations during a postdoctoral fellowship in Argonne.

The simulations also shed new insight into the XR measurements themselves. In particular, they showed that the data are not only sensitive to the atomic positions, but also to the electron distribution around each atom in subtle and complex ways. These insights will be beneficial for future experiments on oxide / liquid interfaces.

The interdisciplinary team is part of the Midwest Integrated Center for Computational Materials, headquartered in Argonne, a center for computer materials science supported by DOE. The work is presented in an article entitled “Validation of first-principle calculations of molecular dynamics of oxide / water interfaces with X-ray reflection data”, which appeared in the November 2020 issue of Physical review material.