Greater Boston Area Theoretical Chemistry Lecture Series

2017-2018 Speaker Schedule

Reduced Hierarchal Equations of Motion Approach to Multi-dimensional Spectroscopies

09/13/17 4:15pm

MIT Building 4, Room 237

Yoshitaka Tanimura

Kyoto University, Japan




Yoshitaka Tanimura

Quantum coherence and its dephasing or relaxation by coupling to an environment plays an important role in nonadiabatic transitions, photoexcitation and tunneling processes. I first introduce a system-bath model to explain how fluctuation and dissipation arises from the environment, and the concept will be illustrated with some examples from chemistry and biology. Then I derive the quantum hierarchal Fokker-Planck equations (QHFPE) on the basis of the Feynman-Vernon influence functional formalism, which can deal with strong system-bath coupling and non-Markovian noise.
By applying hierarchal formalism to multi-electric states, we can investigate laser excitation and photoisomerization process described by multi-electric state numerically rigorously. As an example, we computed Wigner distributions for excited, ground, and coherent states. We then investigated excited state dynamics involving transitions among these states by analyzing electronic spectra with various values of the heat bath parameters. Our results provide predictions for spectroscopic measurements of photoexcitation and photoisomerization dynamics.
References
Y. Tanimura, Real-Time and Imaginary-Time Quantum Hierarchal Fokker-Planck Equations, J. Chem. Phys. 142, 144110 (2015).
T. Ikeda and Y. Tanimura, Probing photoisomerization processes by means of multi-dimensional electric spectroscopy: The Multi-state quantum hierarchal Fokker-Planck Equation approach, J. Chem. Phys. 146, 014102 (2017)

The Dynamics of Coupled Nuclear-Electronic Motion: At the Intersection of Statistical Mechanics, Electronic Structure Theory, and Solid State Physics

10/25/17 4:15pm

MIT Building 4, Room 237

Joe Subotnik

University of Pennsylvania




Joe Subotnik


In this talk, I will give an overview of the theory of mixed quantum-classical nuclear-electronic motion both in solution and at metal surfaces, focusing on both stochastic and mean-field descriptions. This theory gives a clear and intuitive picture of electronic relaxation in a condensed environment, e.g. how photo-excitation can lead to macroscopic charge transfer and how the second law of thermodynamics (i.e. entropy growth) emerges from microscopically reversible dynamics. I will attempt to give an introductory but also deep account of this rich field of study. Towards the end, I will highlight open questions, especially at metal surfaces, where propagating ab initio calculations remains very daunting.

Developing robust force-fields for complex materials: From electrolytes for energy storage to atmospheric processes to biomimetic polymers

02/07/18 4:15pm

MIT Building 4, Room 163

Revati Kumar

Louisiana State University




Revati Kumar


Recent advances in computational materials science have made ab initio/quantum simulations, at the density functional level, increasingly feasible from a computational standpoint. Unfortunately, the accessible length and time scales are still limited, of the order of a few nanometers in length and tens of picoseconds in time, necessitating the development of empirical force-fields that can capture the structure and dynamics at significantly longer scales. The focus of my research is the development of these types of classical models or force-fields, both atomistic and coarse-grained, that are computationally efficient yet capture the essential physics of the problem. Furthermore, my group also works on developing reactive force-fields that allow for changes in bond topology. We use a bottom-up approach to develop these models based on reference data from higher level simulations. The first part of my talk will outline the development of these types of models. The second part of my talk will focus on some interesting applications that my group is working on. There are three main thrusts: solvation and aggregation of biomimetic polypeptoids, gas-liquid nucleation in the presence of acidic defects (relevant to atmospheric chemistry), and finally the study of novel electrolytes for next generation rechargeable battery technologies like sodium-air batteries.

Machine Learning Approach to Connect Time-Series Data of Single-Molecule Experiments with Molecular Dynamics Simulations on Protein Folding Dynamics

02/14/18 4:15pm

MIT Building 4, Room 163

Yuji Sugita

RIKEN, Japan




Yuji Sugita


How proteins fold into native structures in physiological conditions is one of the most fundamental questions in molecular biology and biophysics. Until now, a huge number of theoretical, computational, and experimental studies have been carried out to answer this question. In the lecture, I first introduce standard computational approaches, such as coarse-grained simulations based on the Go-model and enhanced conformational sampling algorithms like replica-exchange molecular dynamics (REMD) methods. Then, I explain our machine learning approach connecting time-series data of single-molecule experiments with molecular dynamics simulations. This method is free from the force-field bias and can provide the conformational ensembles of proteins that match with the single-molecule experimental data. I also compare the usage of single-molecule time-series data to the ensemble-average data in the machine learning approach.

Exploiting Time and Length Scales in Quantum Simulations of Spectroscopy and Dynamics

02/21/18 4:15pm

MIT Building 4, Room 163

Ryan Steele

University of Utah




Ryan Steele


Part 1 – Vibrational Signatures of Electronic Properties in Energy and Biology: Recent experimental developments in cold ion techniques have revolutionized the ability of vibrational spectroscopy to probe structure and reactivity. With ever-increasing reliance on theory/computation for interpretation of these challenging spectra, accurate and cost-efficient computational treatments of anharmonicity are required. New local-mode approaches for anharmonic simulations, when coupled with an underlying quantum chemistry treatment of the electronic structure, allow for a detailed examination of the manner in which electronic properties couple to vibrational motions. These new techniques have been used to explain anomalous signatures in open-shell ions (oxidized water clusters) and definitively assign the structure of amino acids (protonated tryptophan).
Part 2 – Real-Time Dynamics of Reactive Ions: Quantum chemistry methods can also be used to propagate trajectories for detailed simulations of molecular dynamics (MD) and spectroscopy. The many-orders-of-magnitude computational cost increase of these methods over more traditional classical potentials, however, makes them prohibitive for many of the chemical systems for which they are most needed. The second half of this seminar will discuss the manner in which the inherent timescales of quantum chemistry components can be exploited for accelerated ab initio MD simulations. Using multiple-timestep techniques and other predictive models, the real-time dynamics of water oxidation processes and biological ion motion will be explored.

Quantum chemistry in nanoscale and solid-state materials

02/28/18 4:15pm

MIT Building 4, Room 163

Tim Berkelbach

University of Chicago




Tim Berkelbach


Density-functional theory and its descendants are the workhorse methods of computational materials science, despite a number of now well-known shortcomings. In a different direction, I will provide a broad overview of systematically-improvable wavefunction-based methods for condensed-phase electronic structure calculations. I will review the challenges of electronic structure in bulk materials, touching on aspects of many-body theory, diagrammatic techniques, and open-source software. With motivating examples drawn from recent work on low-dimensional semiconductors, I will describe our ongoing research agenda aimed at using deterministic methods (especially coupled-cluster theory) and stochastic methods (quantum Monte Carlo) for the accurate description of condensed-phase electronic structure, excited-state properties, and spectroscopic observables.

Stark control of electrons

03/14/18 4:15pm

MIT Building 4, Room 163

Ignacio Franco

University of Rochester




Ignacio Franco


A general goal in our quest to control matter and energy is the design of strategies to control electronic properties and electron dynamics using coherent laser sources. In addition to its interest at a fundamental level, lasers permit manipulation on an ultrafast timescale opening the way to control the ability of matter to chemically react, conduct charge, absorb light, or other properties, in a femto to attosecond timescale.

In this talk, I will summarize our efforts to understand electronic decoherence processes in molecules that are deleterious to interference-based scenarios for the laser control. In addition, I will discuss how, through Stark effects, non-resonant light of intermediate intensity (non-perturbative but non-ionizing) can be used to generate “laser-dressed” molecules and materials with non-equilibrium properties that can be very different from those observed by matter near thermodynamics equilibrium. In particular, I will discuss how Stark effects can be employed to turn transparent nanomaterials into broadband absorbers, and to generate currents in nanoscale junctions.

Cluster free energies from simple simulations of small systems, with help from number theory

03/28/18 4:15pm

MIT Building 4, Room 163

James Kindt

Emory University




James Kindt


This lecture will start with a review of the statistical thermodynamic basis for the conventional treatment of chemical equilibrium, and a discussion of how this treatment breaks down for small systems.  This fundamental derivation inspired a “brute-force” strategy based on explicit enumerations of integer partitions, to allow ensemble-averaged equilibrium concentrations to be related to cluster free energies even for small systems.   A more elegant and computationally cheaper approach called “PEACH” (Partition-Enabled Analysis of Cluster Histograms) formulated in collaboration with members of the number theory group at Emory, will then be presented.  Evidence from coarse-grained simulations of an anionic surfactant will be presented to validate the PEACH approach.  A case where the method worked suspiciously well (nanodroplet formation from methyl t-butyl ether) will then be discussed, along with how it led us to a simple strategy to reduce complications from non-ideal effects in aggregation equilibria.  Finally, PEACH analysis of equilibria involving NaCl clustering in three solvent environments will be used to illustrate qualitatively different pathways towards formation of an ordered precipitate from supersaturated solution.

Reduced-scaling many-body electronic structure: old ideas that really work

04/04/18 4:15pm

MIT Building 4, Room 163

Ed Valeev

Virginia Tech




Ed Valeev


I will review recent progress in reduced-scaling electronic structure methodology. Many-body electronic structure methods, like coupled-cluster in chemistry and Green's function methods in solid state physics, are capable of significantly higher accuracy than the density functional theory, but their application is hindered by their significantly higher computational cost. Recent rapid advances in reduced-scaling formulation of the coupled-cluster and other many-body theories, in combination with numerical regularization of Coulomb Hamiltonian singularities, reduces or eliminates the cost gap between the many-body methodology and DFT. I will review the key ideas (most of them quite old) behind the modern reduced-scaling electronic structure methods that allow to expose and utilize the sparsity of the electronic wave function, and discuss some of our recent work in this area.

DFTB: recent developments and remaining challenges

04/25/18 4:15pm

MIT Building 4, Room 163

Qiang Cui

Boston University




Qiang Cui


In this seminar I’ll provide an overview for the status and recent developments of the Den- sity Functional Tight Binding (DFTB) model, especially those motivated by biophysical and solid/liquid interface applications. Following an introduction to DFTB, I’ll discuss the treatment of non-covalent interactions with DFTB3, a third-order variant of the DFTB model. In particular, I’ll present improvement of electronic polarization in DFTB3 and benchmark of the model using both gas phase and condensed phase examples. Next, I’ll present progress and challenges for the development of DFTB3 for (transition) metal ions, which play important roles in many metal- loenzymes. I’ll discuss several applications to illustrate the value and remaining issues of DFTB based simulations. If time permits, I’ll also comment on the use of DFTB3(/MM) as the low-level method to drive sampling in multi-level QM/MM free energy calculations.

Intermolecular Interactions

05/02/18 4:15pm

MIT Building 4, Room 163

Mark Gordon

Iowa State University




Mark Gordon


Intermolecular interactions are ubiquitous in chemistry, biochemistry, and materials. Theoretical approaches to the study of intermolecular interactions that are based on accurate fragmentation methods will be discussed, followed by consideration of interesting applications, including ion solvation, excited state hydrogen transfer and ionic liquids.

Ultra-Fast Relaxation, Decoherence, and Localization of Photoexcited States in Pi-Conjugated Polymers

05/08/18 4:15pm

MIT Building 4, Room 163

William Barford

University of Oxford




William Barford


We investigate the energetic relaxation and spatial localization of photoexcited states in conformationally disordered pi-conjugated polymers, choosing poly(para-phenylene vinylene) as a model system. Assuming vertical excitations, the initial photoexcited eigenstates are obtained via the disordered Frenkel model. The subsequent relaxation and localization of the excited states is determined via the disordered Frenkel-Holstein model coupled to a dissipative environment. In particular, we solve the Lindblad master equation via the time evolving block decimation (TEBD) and quantum jump trajectory methods.
The values of the model parameters physically relevant to polymer systems naturally lead to a separation of time scales, with the ultra-fast dynamics corresponding to energy transfer from the exciton to the internal phonon modes (i.e., the C-C bond oscillations), while the longer time dynamics correspond to damping of these phonon modes by the external dissipation. Associated with these time scales, we investigate the following processes that are indicative of the system relaxing onto the emissive chromophores of the polymer: 1) Exciton-polaron formation occurs on an ultra-fast time scale, with the associated exciton-phonon correlations present within half a vibrational time period of the C-C bond oscillations. 2) Exciton decoherence is driven by the decay in the vibrational overlaps associated with exciton-polaron formation, occurring on the same time scale. 3) Exciton density localization is driven by the external dissipation, arising from ‘wavefunction collapse’ occurring as a result of the system-environment interactions. We also show how fluorescence anisotropy measurements can be used to investigate the exciton decoherence process during the relaxation dynamics.

Chemistry in Strong Magnetic Fields

05/16/18 4:15pm

MIT Building 4, Room 163

Trygve Helgaker

University of Oslo, Norway




Trygve Helgaker


In strong magnetic fields, chemistry changes: electronic states change their character, atoms and molecules change their shape, and their interactions with radiation are affected, often in a dramatic manner. Perhaps most surprisingly, new bonding mechanisms occur, giving rise to molecules that do not exist on Earth but may exist elsewhere such as in the atmospheres of magnetic white dwarfs. The exotic chemistry of atoms and molecules in strong magnetic fields provides a fresh perspective on the familiar chemistry on Earth; at the same time, it provides a stress test for quantum chemistry, whose methods have been developed for Earth-like conditions. Density-functional theory, for example, must be re-examined and adapted for the molecules in strong magnetic fields and such modifications have relevance also for the calculation of magnetic properties such as shielding constants and magnetizabilities.
In the talk, I will give an overview of chemistry in strong magnetic fields and discuss the how the methods of quantum chemistry such as coupled-cluster theory and density-functional theory must be modified and adapted to study molecules and their electronic structure in magnetic fields.

My Longterm Relationship with a Difficult Molecule

05/23/18 4:15pm

MIT Building 4, Room 163

John Stanton

University of Florida




John Stanton


This talk reviews the spectroscopy of the NO3radical, which is both fasci-nating for theoreticians as well as an important player in the environment. In particular, the quest to fully understand the spectroscopy of NO3– which has not yet been achieved – has been a “hobby” of mine for essentially the entirety of my scientific career. I will attempt to present a brief history of the science done on this molecule since 1881 as a contextual frame for my own theoretical efforts to understand its spectra and electronic states. The journey starts during my graduate school days, when I was naive and knew nothing. It continues through current times. While I still don’t know very much, I have learned a lot along the way that I will share with you.

Past Schedules