Greater Boston Area Theoretical Chemistry Lecture Series

2015-2016 Speaker Schedule

On systems with and without excess energy in environment: ICD and other interatomic mechanisms

10/14/15 4:00pm

MIT Building 4, Room 163

Lorenz Cederbaum

Universitat Heidelberg




Lorenz Cederbaum

How does a microscopic system like an atom or a small molecule get rid of the excess electronic energy it has acquired, for instance, by absorbing a photon? If this microscopic system is isolated, the issue has been much investigated and the answer to this question is more or less well known. But what happens if our system has neighbors as is usually the case in nature or in the laboratory? In a human society, if our stress is large, we would like to pass it over to our neighbors. Indeed, this is in brief what happens also to the sufficiently excited microscopic system. A new mechanism of energy transfer has been theoretically predicted and verified in several exciting experiments. This mechanism seems to prevail “everywhere” from the extreme quantum system of the He dimer to water and even to quantum dots. The transfer is ultrafast and typically dominates other relaxation pathways.
Can there be interatomic/intermolecular processes in environment when the system itself (again, an atom or small molecule) does not possess excess energy? The answer to this intriguing question is yes. The possible processes are introduced and discussed. Examples and arguments are presented which make clear that the processes in question play a substantial role in nature and laboratory. Work on the interatomic processes discussed can be found in the Bibliography

Statistical Physics of Adaptation

10/21/15 4:00pm

MIT Building 4, Room 163

Jeremy England

MIT




Jeremy England

Many-body systems that are driven far from thermal equilibrium can exhibit a seemingly endless range of different "self-organization" phenomena, whether during long periods of transient relaxation over a hierarchy of timesacles, or in an ergodic steady-state. Indeed, the range of possible behaviors is so diverse that it includes (but is not limited to) everything that living things do! In the face of such phenomenological diversity, it is difficult to articulate any thermodynamic commonality that might be analogous to the tendency to minimize free energy observed in equilibrated systems. Here, we try to exploit recent fundamental progress in our understanding of far-from-equilibrium dynamics to develop predictive thermodynamic principles for a general class of driven self-organized systems. We find there is a language in which Darwinian selection in biological systems may be thought of as a special case of a more general physical tendency for "dissipative adaptation" that arises from the correlation between irreversible changes in shape and the absorption of external work. We close by exploring this hypothesis in different simulation frameworks.

Theoretical spectroscopy using density functional theory: new ideas for long-standing problems

11/17/15 4:00pm

MIT Building 4, Room 163

Leeor Kronik

Weizmann Institute of Science




Leeor Kronik

Accurate prediction of the electronic structure and optical properties is essential for rational design of materials for novel (opto)electronic applications. Quantities of interest include, e.g., the bandgap, band dispersion and band width, optical absorption, exciton binding energies, and more. Preferably, we would like to predict such quantities using density functional theory (DFT), because the relative computational simplicity afforded by DFT allows us to attack realistic problems. Unfortunately, despite many other successes, DFT has traditionally struggled with prediction of the above quantities. Specifically, research has been fraught with very difficult questions as to the extent to which spectroscopic conclusions can be drawn from DFT even in principle, followed by serious concerns as to the reliability of typical DFT approximations in practice. In this lecture, I will start with a tutorial overview on DFT. I will then focus on new formal and practical approaches which offer fresh answers to the above long-standing questions. In particular, I will show that DFT can, in many cases, mimic successfully the quasi-particle picture of many-body theory, allowing for quantitative calculations of both single- and two-particle excitations. I will show how this is achieved for finite systems, present initial generalizations to solids, and discuss limitations and remaining challenges.

Plasmonic Nanostructures: Artificial Molecules

12/02/15 4:00pm

MIT Building 4, Room 163

Peter Nordlander

Rice University




Peter Nordlander

The recent observation that metallic nanoparticles possess plasmon resonances that depend sensitively on the shape of the nanostructure has led us to a fundamentally new understanding of the plasmon resonances supported by metals of various geometries. This picture-“plasmon hybridization”, reveals that the collective electronic resonances in metallic nanostructures are mesoscopic analogs of the wave functions of simple atoms and molecules, interacting in a manner that is analogous to hybridization in molecular orbital theory. The new theoretical insight gained through this approach provides an important conceptual foundation for the development of new plasmonic structures that can serve as chemical and biosensors, substrates for surface enhanced spectroscopies, and subwavelength plasmonic waveguides and active optical devices. In the first part of the lecture I will introduce the field of plasmonics and the theoretical techniques that we use to model plasmonic phenomena. In the second part of the talk, I will review several hot applications including aluminum, graphene, molecular plasmonics, quantum plasmonics, plasmonic Fano resonances, and plasmon induced hot carrier generation for photodetection and photocatalysis.

Targeting Excited States with Quantum Monte Carlo

02/10/16 4:00pm

MIT Building 4, Room 163

Eric Neuscamman

University of California Berkeley




Eric Neuscamman

Essentially all of our current methods for modeling electronically excited states are constrained either by their algorithmic cost or by their connection to the ground state. I will discuss two developments in Monte Carlo methods relevant to this situation. The first is the extension of linear response equation of motion theory to variational Monte Carlo, which can be seen as fitting neatly into the current methodological paradigm alongside other linear response theories and which shares most of their advantages and weaknesses. The second development, the unlocking of a new excited state variational principle via Monte Carlo integration, is not so easily categorized and appears to present an altogether new avenue in excited state modeling in which the entire variational freedom of an ansatz is molded around an individual excited state. Results for both methods will be presented, many of which compare favorably against both equation of motion coupled cluster and multireference benchmark data. Finally, potential future uses and improvements for these new methods will be discussed.

Thermodynamics and Kinetics of Deeply Supercooled Water: a Computational Perspective

02/24/16 4:00pm

MIT Building 4, Room 163

Pablo Debenedetti

Princeton University




Pablo Debenedetti

Water, like any other liquid, can be cooled below the equilibrium freezing temperature and still remain in the liquid state: it is then said to be supercooled. Large quantities of supercooled water exist in clouds and play an important role in ice formation, latent heat release, and in the atmosphere’s overall radiative balance. The physical properties of supercooled water have been a source of continued interest since the early ‘70s, when sharp increases in compressibility and heat capacity upon cooling were first reported. One intriguing hypothesis that has been formulated to explain this behavior is the existence of a metastable phase transition between two different liquids at deeply supercooled conditions. The preponderance of experimental evidence is consistent with this hypothesis, although no definitive proof exists to date. State-of-the-art free energy techniques provide clear evidence of a metastable transition between two distinct liquid phases in a molecular model of water. The fact that a phase transition is metastable implies that the possibility of observing it, whether in the computer or in experiments, depends on system size and on the duration of the observation. Understanding the manner in which force field details influence the existence and observability of liquid-liquid transitions is currently a subject of intense study. Although freezing is a ubiquitous phenomenon, large gaps in understanding persist regarding the detailed microscopic mechanism and the rate of ice formation at atmospherically-relevant conditions. Using state-of-the-art computational methods designed to probe rare events, we are able to study the early stages of ice nucleation at deeply supercooled conditions. We observe a competition between cubic and hexagonal ice polymorphs. Transition states are rich in the kinetically-favored cubic ice, rather than in the thermodynamically stable hexagonal ice. These examples illustrate the power of modern computational techniques rooted in statistical mechanics, as well as the considerable challenges that still lie ahead in the quest for accurate and predictive depictions of complex phenomena.

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03/09/16 4:00pm

MIT, Room TBA

Nandini Ananth

Cornell University




Nandini Ananth

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03/23/16 4:00pm

MIT, Room TBA

Eugene Shakhnovich

Harvard University




Eugene Shakhnovich

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03/30/16 4:00pm

MIT, Room TBA

Ivet Bahar

University of Pittsburgh




Ivet Bahar

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04/06/16 4:00pm

MIT, Room TBA

Ksenia Bravaya

Boston University




Ksenia Bravaya

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04/13/16 4:00pm

MIT, Room TBA

Francesco Paesani

University of California, San Diego




Francesco Paesani

TBA

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04/20/16 4:00pm

MIT, Room TBA

Artur Izmaylov

University of Toronto




Artur Izmaylov

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04/27/16 4:00pm

MIT, Room TBA

Joel Eaves

University of Colorado at Boulder




Joel Eaves

TBA

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05/04/16 4:00pm

MIT, Room TBA

Chris Wolverton

Northwestern University




Chris Wolverton

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Past Schedules