Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session C32: Theory and Experiment for Complex Systems in Gas Phase (A)Focus
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Sponsoring Units: DCP Chair: Cristina Puzzarini Room: BCEC 204A |
Monday, March 4, 2019 2:30PM - 3:06PM |
C32.00001: Theory meets experiment in gas-phase spectroscopy: the VMS bridge Invited Speaker: Vincenzo Barone The impressive advances in hardware developments, the availability of powerful graphical interfaces together with the set-up of effective and user-friendly software are leading to a new generation of virtual tools able to deal effectively with the complex systems and phenomena of current interest in the fields of molecular spectroscopy, chemistry (also astrochemistry) and biology. Moving from the presentation of theoretical results as a collection of numbers corresponding to models (often oversimplified) to the vis-à-vis comparison between “in silico” and “in vitro” spectral outcomes, possibly combined with 3D renderings and natural interfaces, should at last overcome the diffidence of experimentalists towards theoreticians and bridge the gap between experiment and theory. Among the available virtual instruments, we focus here on the multifrequency spectrometer (VMS) our group has been developing in the last few years (see, V. Barone, WIREs Comput Mol Sci 6, 86-110 (2016)), which allows a vis-à-vis comparison of experimental spectra with their simulated counterparts as well as the interpretation of the results in terms of the interplay among different well defined effects. The main building blocks of this tool are, apart from powerful 3D pre- and post-processing tools: (a) first-principle approaches based on composite schemes involving post-Hartree-Fock methods complemented by models based on the density functional theory for the proper description of stationary points and their local environment on the pertinent potential energy surfaces; (b) integrated perturbative/variational treatments for describing nuclear motions beyond the rigid rotor / harmonic oscillator level. In this presentation I will sketch the present status of VMS and the ongoing efforts toward increasing its range of application with special reference to different spectroscopies in the gas-phase. Some case studies will be selected to better illustrate the above concepts. |
Monday, March 4, 2019 3:06PM - 3:42PM |
C32.00002: Broadband Rotational Spectroscopy of Odorants and their Complexes: Experiment and Theory in Concert Invited Speaker: Maria Eugenia Sanz Studies of large molecules and their complexes in gas phase using spectroscopic techniques provide experimental data on their structures and interactions that can be directly compared with predictions from theory. One of the most powerful methods for structural determination is rotational spectroscopy, which can identify different coexisting conformations and determine actual atom positions, thus enabling calculation of bond lengths and angles. The development of broadband rotational spectroscopy, allowing fast collection of broad segments of the spectrum at once, has dramatically extended the range and complexity of the molecular systems that can be tackled and opened new opportunities to benchmark theoretical methods. |
Monday, March 4, 2019 3:42PM - 4:18PM |
C32.00003: Exploring conformational landscapes of chiral molecules in their solvation and self-aggregation clusters Invited Speaker: Yunjie Xu Our research program focuses on applying and developing new spectroscopic tools to bridge the gap of our understanding in terms of structural and dynamical properties of an isolated chiral molecule and of it in liquid phase and in solution. Using broadband rotational spectroscopy aided by high level ab initio calculations, we probe structural diversity and emerging bulk behavior in trimeric and tetrameric aggregates of the transiently chiral 2-fluoroethanol (FE). We show that the FE tetramer is an intriguing system at the interface between gas- and bulk-phase behavior, where the conformational specificity seen in the gas-phase is still experimentally relevant but plays a diminished role relative to the intermolecular topology and cooperative stability. In the second example, we examine conformational landscapes of tetrahydro-2-furoic acid (THFA), a chiral carboxylic acid, and its dimer and its complexes with water molecules in a jet expansion using rotational spectroscopy. We also apply vibrational circular dichroism and Raman optical activity to probe how intermolecular interactions with water and THFA itself influence the aforementioned conformational preference in condense phase directly. The interplay between experiment and theory is essential for all the work described. |
Monday, March 4, 2019 4:18PM - 4:30PM |
C32.00004: Conformational Preferences of Gas Phase Peptides and Synthetic Foldamers: Challenges to Theory and Experiment Timothy Zwier, Karl N Blodgett, Christopher P Harrilal, Joshua L Fischer, John T Lawler, Dewei Sun This talk will describe combined experimental/computational studies of the conformational preferences of isolated, beta-peptide foldamers as neutrals, probing the development of mixed 10/12 helices with size. We will also describe our work on cryo-cooled protonated peptides in the gas phase, including a study of peptides capable of forming salt bridges. In both contexts, the need for accurate and fast exploration of the conformational landscape is crucial to success. The prospects and limits of both theory and experiment will be discussed. |
Monday, March 4, 2019 4:30PM - 4:42PM |
C32.00005: Quantum Dynamics of Intramolecular Double Hydrogen Transfer in Porphycene Yair Litman, Jeremy Oscar Richardson, Takashi Kumagai, Mariana Rossi The making and breaking of H-bonds on highly anharmonic potential energy surfaces involved in proton and hydrogen transfer reactions require a full-dimensional quantum mechanical treatment of not only electrons, but also of nuclei. Here we demonstrate that dealing with this complexity is necessary for achieving predictive simulations that can solve puzzling properties of these reactions by addressing the intramolecular double hydrogen transfer (DHT) in porphycene[1]. Our theoretical treatment combines dispersion corrected hybrid density-functional theory calculations and path-integral ring-polymer methods. Our simulations predict the position and width of the N-H stretching band of porphycene and DHT rates in excellent agreement with experiments, thus confirming our determination of the tunneling pathways and the anharmonic mode couplings that play a role in this reaction. They also confirm the importance of the usually ignored competition between concerted and stepwise DHT pathways for this system. Our general theoretical approach provides a quantitative framework for a deeper physical understanding of hydrogen transfer dynamics in complex systems. [1] Y. Litman, J. O. Richardson, T. Kumagai, M. Rossi. arXiv:1810.05681 (2018) |
Monday, March 4, 2019 4:42PM - 4:54PM |
C32.00006: A Stochastic Non-empirical approach in the problem of the reaction path seeking Iurii Nagornov, Ryosuke Akashi In the atomistic simulation methods, the escaping trajectories, which is rendered rare because of the potential energy barrier, and free-energy landscape along them are usually calculated by applying artificial force and/or empirical collective variables. Our motivation is to develop the method free from such artificial forces and collective variables. We developed a non-empirical scheme to search for the minimum-energy escape paths from the minima of the potential surface to unknown saddle points nearby. This method employs only the local gradient and diagonal part of the Hessian matrix of the potential [R.Akashi, YSN, J. Phys. Soc. Jpn. 87, 063801 (2018)]. |
(Author Not Attending)
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C32.00007: N-Body networks for molecular simulation Brandon Anderson, Risi Kondor, Truong Son Hy We introduce a novel deep learning architecture for learning molecular force fields. Our architecture, called CGnet, “bakes in” the underlying rotational invariance of the problem using the Clebsch-Gordan operator. We apply CGnet to the molecular N-Body problem of learning force-fields from ab-initio molecular dynamics simulations. We find state-of-the-art results when applied to the MD-17 dataset. We also train CGnets on QM9 and find competitive results. Finally, we provide a set of tools, called FastCG, along with an interface with PyTorch and Tensorflow, to allow for fast and efficient calculation of CG products, along with their analytical gradients. |
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