Statistical Thermodynamics & Molecular Simulations (STMS) Seminar Series
These seminar series are aimed at providing a virtual platform for sharing scientific research in the area of statistical mechanics, molecular simulations, and computational materials science. In recent months, the coronavirus pandemic has stopped all large in-person scientific gatherings, including conferences and department seminars, and it is not clear that the situation will improve any time soon. STMS is aimed at filling this gap, and provide a venue for dissemination of research findings and exchange of ideas in the age of COVID. This model is being currently used by several other scientific communities, and can potentially continue even beyond the pandemic if successful.
Each seminar will be a 60-minute event and will comprise of a long-form (30-minute) talk by a principal investigator or a senior research scientists from academia or industry and a short-form (15-minute) presentation by a graduate student or a postdoc. The remainder of the event will be dedicated to Q&A (10 minutes for the PI, 5 minutes for the student/postdoc). Long-form speakers will be chosen by the STMS Organizing Committee, while we encourage suggestions from the community at large. Student and postdoctoral speakers, however, need to be nominated by their advisors. Seminars will take place on Fridays, from 11 AM-12 PM. During 2021, we expect to hold two seminar per month, at the last two Fridays of each month.This event's talks:
Calculation of thermoelastic properties of crystals using harmonically mapped averaging
Prof. David Kofke (State University of New York at Buffalo)
Abstract: Mechanical properties of materials are key to their function, and often molecular models are parameterized by comparison to experimental data for these properties. Statistical thermodynamics provides a connection between mechanical properties and free energy derivatives with respect to some external strain. For example, the stress tensors and elastic constants are expressed as first and second derivatives of the free energy, respectively, with respect to deformation. For crystalline systems, the harmonic model provides a (simple) first-order approximation to these properties, which has increasingly accurate results as the temperature decreases, but fails to describe the system at high temperatures due to anharmonic effects.
Evaluation of properties while including anharmonic effects typically requires molecular simulation, which reports the “full” properties, with harmonic and anharmonic contributions taken together. This is unfortunate, given that the harmonic treatment captures much of the behavior already via noise-free analytical calculations. This advantageous starting point is discarded when evaluating the property using conventional averaging.
Harmonically mapped averaging (HMA) is a framework to reformulate the ensemble averages of properties. It yields expressions that start with the analytic, approximate harmonic behavior, and adds the anharmonic contribution as an ensemble average, providing in essence a direct measurement of the anharmonic contributions to the properties. Notably, HMA does not alter the molecular simulation sampling, so several properties may be measured at once in a single simulation.
We consider application of HMA to the thermoelastic properties of crystalline systems. Unlike previous applications, we examine methods based on different formulations of the harmonic model, including in particular the more approximate Einstein model employed previously for other properties, as well as a formulation based on a full normal-mode harmonic treatment. We consider the performance and relative benefits of each approach, and discuss performance improvements that in some cases are not (so far) as impressive as seen previously for other properties, e.g., we observe only halving of uncertainties, rather than a reduction of ten-fold or more.
Speaker Bio: David Kofke received his B.S. in chemical engineering in 1983 from Carnegie Mellon University, and his Ph.D. in 1988 from the University of Pennsylvania, where he worked under the supervision of Eduardo Glandt. Since 1989 he has been on the chemical engineering faculty of the University at Buffalo (SUNY), where he served as department chair for the years 2006-2012; he is now a SUNY Distinguished Professor and holds the Walter E. Schmid Chair. Prof. Kofke’s research in methods and applications of molecular simulation has resulted in nearly 170 refereed publications to date. Prof. Kofke’s research aims to develop and improve methods to compute material properties accurately and reliably from molecular models. Present interests proceed in two directions: (1) calculating structure and properties of crystalline materials, and (2) calculating and applying cluster integrals to compute properties of supercritical fluids. He is also active in development of object-oriented molecular simulation software for research and education. Among other awards, Prof. Kofke is the recipient of the triennial John M. Prausnitz Award for applied chemical thermodynamics in 2004, the 2007 Jacob F. Schoellkopf Medal, and the 2012 Himmelblau Award from the CAST division of AIChE. Prof. Kofke is a member since 1999 of the Board of Trustees of CACHE, where he served as President in 2010-2012. In 2014 he was elected Fellow of the AIChE and the AAAS. Since 2016, he has been an Associate Editor of Journal of Chemical & Engineering Data.
Discovery and design of hyperuniform atomic-scale low-dimensional materials
Dr. Duyu Chen (University of California, Santa Barbara)
Abstract: Disordered hyperuniformity is a recently discovered novel state of many-body systems that possesses vanishing normalized infinite-wavelength density fluctuations and a hidden long-range order similar to a perfect crystal, and yet is statistically isotropic with no Bragg peaks like aliquid or glass. In this talk I will present a series of research in which my collaborators and I extend the concept of disordered hyperuniformity to atomic-scale low-dimensional materials. In particular, we discover a hyperuniformity-preserving topological transformation in low-dimensional networks that involves continuous introduction of Stone-Wales (SW) defects, and the resulting DHU network structures capture the salient features of amorphous low-dimensional materials such as graphene and silica. Our network models reveal unique electronic transport mechanisms and mechanical behaviors associated with different classes of disorder in low-dimensional materials. In particular, we find that when adding disorder in a hyperuniform manner, silica and pyrite systems exhibit a transition from insulating/semiconducting to metallic behavior, which is in contrast to the conventional wisdom of landmark "Anderson localization'' that disorder generally diminishes electronic transport. Our studies will open up many novel potential applications in optoelectronics, thermoelectrics, and quantum devices.
Speaker Bio: Dr. Duyu Chen is currently a postdoctoral research scientist in the Materials Research Laboratory at University of California, Santa Barbara advised by Prof. Glenn H. Fredrickson. Prior to this, he earned my Ph.D. from the Department of Chemistry at Princeton University working with Prof. Salvatore Torquato, and was mainly trained as a soft-matter theorist. He received his B.S. from University of Science and Technology of China through the Physical Science Honors Bachelor Program with a major in chemical physics and also holds a M.S. in Business Technologies from Tepper School of Business at Carnegie Mellon University.