Calendar

• Seminar by Dr. Amartya Bose, Princeton University, USA on November 30, 2021 at 5.30 pm via zoom platform

  Title :

  Classical Trajectory Methods for Approximate Quantum Dynamics

  Abstract :

  Chemistry is the study of large systems of molecules.  Accurate simulations of quantum dynamics scale exponentially with the number of degrees of freedom, making them impractical.  Thus, approximations have to be made.  In the previous talk, I discussed my work involving the system-solvent decomposition using real-time path integral.  In this talk, I would like to focus on problems that are not amenable to such decompositions such as calculating the infrared and Raman spectra or bulk transport properties like diffusion coefficient.  Often dynamical quantum interference gets washed away in condensed phases under ambient conditions leading to an effective utilization of semiclassical^1-3 and other classical trajectory-based approximations^4-6.  The simplest of these approximations incorporates quantum dispersion effects into a phase space distribution called the Wigner distribution³, defined as a multidimensional Fourier transform.  Computational simulation of this distribution is complicated by the Monte Carlo “sign problem.”  I have developed various methods to overcome these problems.  First, I will introduce a very simple adiabatic switching method^{7, 8} that can approximate the Wigner distribution using classical trajectories.  Then I would describe an exact path integral method⁹ that is systematically convergeable and mitigates the sign problem through a physically motivated series expansion.  Other approaches of extracting quantum information from classical trajectories would also be presented¹⁰.  I will show my current work in comparing various augmented classical trajectory methods in simulating spectra and density of states of bulk water using ab initio deep neural network force fields^11-13.

  Finally, I will discuss my future research plans utilizing many of the frameworks that I have developed to address open research questions in Chemistry, and also talk about avenues of further improving these methods.

  References

  1. Van Vleck, J. H., The Correspondence Principle in the Statistical Interpretation of Quantum Mechanics. Proc. Natl. Acad. Sci. 1928, 14 (2), 178-188.
  2. Herman, M. F.; Kluk, E., A semiclasical justification for the use of non-spreading wavepackets in dynamics calculations. Chem. Phys. 1984, 91 (1), 27-34.
  3. Wigner, E., On the Quantum Correction For Thermodynamic Equilibrium. Phys. Rev. 1932, 40 (5), 749-759.
  4. Craig, I. R.; Manolopoulos, D. E., Quantum statistics and classical mechanics: Real time correlation functions from ring polymer molecular dynamics. J. Chem. Phys. 2004, 121 (8), 3368-3373.
  5. Cao, J.; Voth, G. A., The formulation of quantum statistical mechanics based on the Feynman path centroid density. I. Equilibrium properties. J. Chem. Phys. 1994, 100 (7), 5093-5105.
  6. Cao, J.; Voth, G. A., The formulation of quantum statistical mechanics based on the Feynman path centroid density. II. Dynamical properties. J. Chem. Phys. 1994, 100 (7), 5106-5117.
  7. Bose, A.; Makri, N., Wigner phase space distribution via classical adiabatic switching. J. Chem. Phys. 2015, 143 (11), 114114.
  8. Bose, A.; Makri, N., Wigner Distribution by Adiabatic Switching in Normal Mode or Cartesian Coordinates and Molecular Applications. J. Chem. Theory Comput. 2018, 14 (11), 5446-5458.
  9. Bose, A.; Makri, N., Coherent State-Based Path Integral Methodology for Computing the Wigner Phase Space Distribution. J. Phys. Chem. A 2019, 123 (19), 4284-4294.
  10. Bose, A.; Makri, N., Quasiclassical Correlation Functions from the Wigner Density Using the Stability Matrix. J. Chem. Inf. Model 2019, 59 (5), 2165-2174.
  11. Wang, H.; Zhang, L.;  Han, J.; E, W., DeePMD-kit: A deep learning package for many-body potential energy representation and molecular dynamics. Comput. Phys. Commun. 2018, 228, 178-184.
  12. Zhang, L.; Han, J.;  Wang, H.;  Car, R.; E, W., Deep Potential Molecular Dynamics: A Scalable Model with the Accuracy of Quantum Mechanics. Phys. Rev. Lett. 2018, 120 (14), 143001.
  13. Zhang, L.; Chen, M.;  Wu, X.;  Wang, H.;  E, W.; Car, R., Deep neural network for the dielectric response of insulators. Phys. Rev. B 2020, 102 (4), 041121.
• Seminar by Dr. Amartya Bose, Princeton University, USA on November 29, 2021 at 5.30 pm via zoom platform

  Title :

  Exact Quantum Dynamics of Open Systems

  Abstract :

  An ability to accurately simulate the dynamics of quantum systems is essential to understanding all of Chemistry and much of Physics.  While quantum mechanics is, in principle, linear in time, simulations scale exponentially with degrees of freedom.  This is a huge problem when it comes to simulating large systems involving quantum transport.  Broadly, computational approaches can be classified as ones based on “augmented” classical trajectories and ones that are based on a “system-solvent decomposition.” In this first presentation, I would discuss the work that I have done on path integral methodologies for simulating system-solvent dynamics.  This technique of decomposing the problem into a system, treated at a more accurate level of theory, and a solvent, that is typically traced over, is particularly effective if the quantum effects are limited to a small subspace of the problem.  While various formalisms exist to simulate a system-solvent problem, they are typically plagued by computational bottlenecks.  Methods of speeding up such simulations^{1, 2} would be presented.  Recently, I have developed an exciting simulation framework combining various tensor network approaches with Feynman-Vernon influence functionals^3-5.  The most exciting method in this family combines ideas from density matrix renormalization group (DMRG) with influence functionals to give rise to a very promising framework that even in its most primitive form is able to solve problems involving dissipative molecular aggregates and spin chains with unprecedented efficiency.  I would discuss these developments, illustrating their wide-ranging applicability through simulations of essentially quantum mechanical, non-adiabatic processes like electron⁴ and excitonic energy⁶ transfer.  Recent results exploring energy transfer in photosynthetic complexes along with the effect of local vibrations and spatial inhomogeneities will also be shown.

   

  References

  1. Bose, A.; Makri, N., Non-equilibrium reactive flux: A unified framework for slow and fast reaction kinetics. J. Chem. Phys. 2017, 147 (15), 152723.
  2. Bose, A.; Makri, N., Quantum‐classical path integral evaluation of reaction rates with a near‐equilibrium flux formulation. Int. J. Quantum Chem. 2021, 121 (10).
  3. Bose, A., A Pairwise Connected Tensor Network Representation of Path Integrals. arXiv pre-print server 2021.
  4. Bose, A.; Walters, P. L., A tensor network representation of path integrals: Implementation and analysis. arXiv pre-print server 2021.
  5. Bose, A.; Walters, P. L., A Multisite Decomposition of the Tensor Network Path Integrals. arXiv pre-print server 2021.
  6. Bose, A.; Makri, N., All-Mode Quantum–Classical Path Integral Simulation of Bacteriochlorophyll Dimer Exciton-Vibration Dynamics. J. Phys. Chem. B 2020, 124 (24), 5028-5038.
• Seminar by Dr. Keshab Karmakar, Indian Association for the Cultivation of Science, Kolkata, on November 22, 2021 at 4.00 pm via zoom platform

  Title :

  ZnO Nanorod-Templated Multi-layered Two-Dimensional MoS₂/ MoO₃ Photoanodes for Solar Fuel Generation

  Abstract :

  Photoelectrochemical (PEC) water splitting is one of the sustainable ways to convert abundant solar energy into solar fuel (hydrogen) with zero carbon dioxide emissions. Herein, multidimensional nano-heterostructures based photoelectrodes are employed for PEC water splitting where the multi-layered two-dimensional (2D) structure of MoS₂ and MoO₃ are coupled separately on the well-aligned arrays of one-dimensional (1D) ZnO nanorods template, with the expected effective synergic effects. The advantages of catalytically active sites of the 2D layered structure of transition-metal dichalcogenides/oxides are combined with the distinctive dimensionality-dependent phenomena of 1D structure for photoelectrochemical water splitting. A low onset potential and boosted broadband light absorption is evinced by the ZnO/MoS₂ and ZnO/MoO₃ photoanodes, with significantly high photocurrent densities of 2.04 and 0.67 mA cm⁻² at 1.23 V versus reversible hydrogen electrode, thereby enhancing the photocurrent up to 334% and 43%, respectively, compared to that of pristine ZnO nanorods. Consequently, photoanodes also exhibit enhanced applied bias photon-to-current conversion efficiency, owing to superior spatial photo-induced exciton separation and transportation triggered by favourable interfacial band alignment at 2D−1D heterointerfaces. Moreover, favourable band alignment promotes hole transportation at the heterostructure/electrolyte interface and boost the surface oxygen evolution reaction, leading to enhanced photoelectrochemical performance.

• Seminar by Dr. Kalaivanan Nagarajan, University of Strasbourg, France on December 7, 2021 at 4.00 pm in AG-66

  Title :

  Chemistry under Vibrational Strong Coupling

  Abstract :

  Light-matter strong coupling in general, vibrational strong coupling (VSC) in particular, offers exciting possibilities in molecular/material science. Formation of vibro-polaritonic states leads to a large modification in chemical reactivity and other molecular properties under strong coupling conditions.1-2 This emerging field of research is highly multi-disciplinary, and much of its potential has yet to be explored.

  The thrust of my future research will be focused on understanding the fundamental mechanism(s) of VSC and to explore its potential applications in different fields of chemistry. For instance, so far only about a dozen chemical reactions have been studied under VSC and so much remains to be understood to explain how VSC induces such large changes in reactivity. In another direction, the perturbation of solute-solvent interactions under VSC clearly opens a new pathway in the field of chemical equilibria, supramolecular assembly, and crystal engineering that need to be further investigated.3-4 VSC undoubtedly will become a useful tool in chemistry and material science. It will also contribute to the fundamental understanding of the role of vacuum field in common molecular processes.

  References:

  1. Sau, A.; Nagarajan, K.; Patrahau, B.; Lethuillier-Karl, L.; Vergauwe, R. M. A.; Thomas, A.; Moran, J.; Genet, C.; Ebbesen, T. W., Modifying Woodward–Hoffmann Stereoselectivity Under Vibrational Strong Coupling. Angew. Chem. Int. Ed. 2021, 60, 5712-5717.2. Nagarajan, K.; Thomas, A.; Ebbesen, T. W., Chemistry under Vibrational Strong Coupling. J. Am. Chem. Soc. 2021, 143, 16877-16889.3. Joseph, K.; Kushida, S.; Smarsly, E.; Ihiawakrim, D.; Thomas, A.; Paravicini-Bagliani, G. L.; Nagarajan, K.; Vergauwe, R.; Devaux, E.; Ersen, O.; Bunz, U. H. F.; Ebbesen, T. W., Supramolecular Assembly of Conjugated Polymers under Vibrational Strong Coupling. Angew. Chem. Int. Ed. 2021, 60, 19665-19670.4. Hirai, K.; Ishikawa, H.; Chervy, T.; Hutchison, J. A.A.; Uji-i, H., Selective crystallization via vibrational strong coupling. Chem. Sci. 2021, 12, 11986-11994.

• Seminar by Dr. Kalaivanan Nagarajan, University of Strasbourg, France on December 6, 2021 at 4.00 pm in AG-66

  Title :

  Interaction of Light with Matter: From Real to Virtual Photons

  Abstract :

  Light-matter interactions are perpetual sources of stimulation in experimental and theoretical sciences. Such interactions can involve real or virtual photons and examples of both cases will be presented. The first part of the talk will focus on the photo-physics of organic molecules. After excitation, among the various intramolecular processes, singlet-triplet intersystem crossing is an important channel both fundamentally and technologically. In organic chromophores, the quantum yield of intersystem crossing is often low due its spin forbidden nature. Strategies to enhance intersystem crossing such as heavy atom effect, molecular twist, and singlet fission in different core-twisted aromatic molecules will be presented and discussed.1-2In the second part, an overview of the light-matter strong coupling will be presented where virtual photons (a.k.a. vacuum field) play a key role. In the strong coupling regime, hybrid light-matter states (polaritonic states), are formed by the interaction of a molecular transition with the electromagnetic field. The formation of such hybrid states results in the modification of various physical and chemical properties of the molecules/materials as it has been demonstrated over the past decade. 3-4

  References:

  1. Nagarajan, K.; Mallia, A. R.; Reddy, V. S.; Hariharan, M., Access to Triplet Excited State in Core-Twisted Perylenediimide. J. Phys. Chem. C 2016, 120, 8443-8450.2. Nagarajan, K.; Mallia, A. R.; Muraleedharan, K.; Hariharan, M., Enhanced intersystem crossing in core-twisted aromatics. Chem. Sci. 2017, 8, 1776-1782.3. Ebbesen, T. W., Hybrid Light–Matter States in a Molecular and Material Science Perspective. Acc. Chem. Res. 2016, 49, 2403-2412.4. Nagarajan, K.; George, J.; Thomas, A.; Devaux, E.; Chervy, T.; Azzini, S.; Joseph, K.; Jouaiti, A.; Hosseini, M. W.; Kumar, A.; Genet, C.; Bartolo, N.; Ciuti, C.; Ebbesen, T. W., Conductivity and Photoconductivity of a P-Type Organic Semiconductor under Ultrastrong Coupling. ACS Nano 2020, 14, 10219-10225.