A&S Graduate Studies

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[PAST EVENT] Siyuan Chen: Physics Dissertation Defense  

April 13, 2023
10am - 1pm
Location
Zoom
Access & Features
  • Open to the public
Siyuan

Siyuan Chen, Final Oral Examination for the Ph.D. Degree, Title: "Ab initio Computations of Structural Properties in Solids by Auxiliary Field Quantum Monte Carlo"
https://simonsfoundation.zoom.us/j/93981365915 Passcode: available upon request. Please email Ellie at [[evwilk]].

Abstract: Determining the accurate structure of a material is a critical step in understanding its physics. Standard electronic structure methods have not achieved systematic accuracy, especially for materials with strong electron correlation effects. Many-body methods can potentially deliver higher accuracy, but they all face significant algorithmic obstacles for structural optimization in solids. I will present a direct, ab initio computation of forces and stresses with auxiliary field quantum Monte Carlo (AFQMC). AFQMC is a many-body computational method that has shown excellent performance in total energy and charge density. Our method for computing forces and stresses requires minimal approximations and can be used to predict the potential energy surface at a much higher efficiency than an energy-only approach. In addition, we propose a fast and robust structural optimization algorithm with statistically noisy forces. Applying this algorithm to our forces and stresses, we demonstrate efficient, accurate, and full degrees-of-freedom optimizations in solids. Further, with our latest improvement on correlated sampling, we show the AFQMC forces can be used to obtain accurate phonon spectra of solids. Finally, we will also demonstrate the possibility of computing Berry phases, polarizations, and the Chern number. Our method paves the way for a wide range of applications, including but not limited to prediction of structures, thermodynamics, ferroelectricity, and topological properties of quantum materials.

Bio: Siyuan Chen was born in Ma’anshan, Anhui, China on August 29, 1998. Since childhood, he was deeply interested and had shown his strong ability in science and math, by winning multiple national science-related competition prizes. At the age of 13, he was admitted into the Class for the Gifted Young, at the University of Science and Technology of China. Despite this class being a multi-disciplinary program, he gradually began to show more interest in physics. He started with experimental quantum information, but soon found the field of computational condensed matter physics matched his interest and knowledge better. After getting his Bachelor of Science degree in 2016, he entered William & Mary and joined the theoretical condensed matter group to work with Prof. Shiwei Zhang. After a summer internship in 2019, he moved to New York City and is currently working at Flatiron Institute. His research focuses on development and application of the auxiliary-field quantum Monte Carlo to approach accurate, universal, and efficient computation of physical properties of materials. After graduation, he will continue as a post-doctoral researcher at University of Chicago.