Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Q26: Focus Session: Non-Adiabatic Dynamics: New Insights from Experiment and Theory IV |
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Sponsoring Units: DCP Chair: Scott Reid, Marquette University Room: 204A |
Wednesday, March 4, 2015 2:30PM - 3:06PM |
Q26.00001: Multi-reference vs. single-reference quantum chemical methods in surface hopping dynamics Invited Speaker: Hans Lischka The reliability of quantum chemical methods plays a critical role in performing reliable nonadiabatic dynamics simulations. Unfortunately, the methods for computing excited states including larger regions of the energy surfaces are still computationally expensive or need support from higher level methods. In this talk the capabilities of multireference (MR) versus single reference (SR) methods will be discussed. In terms of SR approaches we focus our attention on the second-order algebraic diagrammatic construction method (ADC(2)). In addition to the direct calculation of nonadiabatic coupling vectors also the method of computing wavefunction overlaps between consecutive time steps is used. Several interesting examples are discussed such as the charge transfer between $\pi $ systems and the photodecativation of adenine. In the latter example an extensive comparison of the results concerning deactivation pathways and decay times is given for different methods including multireference configuration interaction, ADC(2) and time-dependent density functional theory (TDDFT) using various functionals. The surface hopping dynamics simulations are performed on the basis of the public domain program system NEWTON-X [M. Barbatti, M. Ruckenbauer, F. Plasser, J. Pittner, G. Granucci, M. Persico, and H. Lischka, WIREs:CMS 2014, 4, 26-33]. [Preview Abstract] |
Wednesday, March 4, 2015 3:06PM - 3:18PM |
Q26.00002: Unravelling Two State Reactivity: New Insights Combining Experimental, \textit{Ab Initio}, and Statistical Modelling Techniques Shaun Ard, Joshua Melko, Oscar Martinez, Vladimir Ushakov, Anyang Li, Ryan Johnson, Nicholas Shuman, Hua Guo, Jurgen Troe, Albert Viggiano Non-Adiabatic dynamics have long played a role in understanding numerous ion molecule reactions. As calculation techniques have improved, even spin-allowed reactions have been found to be significantly impacted by low lying excited spin states, so-called Two-State reactivity. This talk will focus on recent studies of several canonical examples, FeO$^{+}$ $+$ H$_{2}$FeO$^{+} +$ CH$_{4}$, and Fe$^{+} + $ N$_{2}$O. Experimentally, the kinetics of these reactions are studied from 100 to 700K. Combined with computations of the reaction surface, statistical modelling employing the Statistical Adiabatic Channel Model (SACM) of this near thermal energy range gives unique insight into kinetic details of these systems. Implications of this combined approach, specifically towards better quantifying Two-State reactivity in ion-molecule reactions, will be discussed. [Preview Abstract] |
Wednesday, March 4, 2015 3:18PM - 3:30PM |
Q26.00003: New insights into the nonadiabatic dynamics of proton-coupled electron transfer reactions from the mixed quantum-classical Liouville approach Gabriel Hanna, Farnaz Shakib The nonadiabatic dynamics of model proton-coupled electron transfer (PCET) reactions is studied for the first time using a surface-hopping solution of the mixed quantum-classical Liouville (MQCL) equation. In contrast to Fewest Switches Surface-Hopping (FSSH), which is commonly used in the simulation of PCET, the MQCL approach provides a rigorous treatment of decoherence in dynamics simulations of MQC systems. The studied model consists of a proton and an electron in a donor-acceptor complex (i.e. quantum subsystem) and a collective solvent coordinate (i.e. classical environment). Using this model, both concerted and sequential PCETs are studied under different proton/electron-solvent coupling conditions, and insights into the dynamical principles underlying these reactions are gained. Notably, an analysis of the trajectories reveals that the solvent coordinate spends a large fraction of its time on the mean of two coherently coupled potential energy surfaces (PESs), as opposed to on single PESs as in the FSSH approach. The results of this study not only demonstrate the applicability of the MQCL approach for PCET simulations, but also emphasize the importance of incorporating decoherence effects through mean surface evolution into calculations of PCET rate constants. [Preview Abstract] |
Wednesday, March 4, 2015 3:30PM - 3:42PM |
Q26.00004: Variational state specific solvent models for excited states from time dependent self-consistent field methods Josiah Bjorgaard, Kirill Velizhanin, Sergei Tretiak The effect of a dielectric environment on a molecule can be profound, causing changes in nuclear configuration and electronic structure. Quantum chemical simulation of a solute-solvent system can be prohibitively expensive due to the large number of degrees of freedom attributed to the solvent. To remedy this, the solvent can be treated as a dielectric cavity. Mutual polarization of the solute and solvent must be considered for accurate treatment of an optically excited state (ES) with a state-specific solvent model (SSM). In vacuum, time dependent self-consistent field (TD-SCF) methods (e,g, TD-HF, TD-DFT) give variational excitation energies. With the well known Z-vector equation, a variational ES energy is used to explore the ES potential energy surface (PES) with analytical gradients. Modification of the standard TD-SCF eigensystem to accommodate a SSM creates a nonlinear TD-SCF equation with non-variational excitation energies. This prevents analytical gradients from being formulated so that the ES PES cannot be explored. Here, we show how a variational formulation of existing SSMs can be derived from a Lagrangian formalism and give numerical results for the variability of calculated quantities. Model dynamics using SSMs are showcased. [Preview Abstract] |
Wednesday, March 4, 2015 3:42PM - 4:18PM |
Q26.00005: Electronic Structure and Potential Fitting Methods Suitable For Multistate Reactive Surfaces Invited Speaker: Richard Dawes Part of this talk describes the development of a PES generator (software code) which uses parallel processing on High-Performance Computing (HPC) clusters to construct PESs automatically. Thousands of \textit{ab initio} data are computed at geometries chosen by the algorithm and fit to a functional form. The electronic structure of molecules is difficult to describe continuously across global reactive PESs since it changes qualitatively as bonds are formed and broken along reaction coordinates. I will discuss a high-level \textit{ab initio} method (GDW-SA-CASSCF/MRCI) designed to allow the electronic wavefunction to smoothly evolve across the PES and provide an accurate and balanced description of the various regions. These methods are combined to study a number of small gas-phased molecules from the areas of atmospheric, combustion and interstellar chemistry including a large variational calculation of all the bound vibrational states of ozone and the photodissociation dynamics of the simplest Criegee intermediate (CH$_2$OO). [Preview Abstract] |
Wednesday, March 4, 2015 4:18PM - 4:30PM |
Q26.00006: Phase-space formulation of Nonadiabatic Quantum Processes Andres Estrada-Guerra, Leonardo Pachon Based on Schwinger's exact mapping of the discrete quantum variables onto continuous degrees of freedom and the phase-space path-integral representation of quantum dynamics by Marinov, a phase-space approach is developed here to analyze the quantum features of non-adiabatic processes. Being a phase-space formulation, the associated semiclassical description is by construction an initial value representation and allows for clearly analyzing classical and quantum contributions to the dynamics. The formulated theory is applied to the context of light-harvesting systems in photosynthetic complexes to understand the extent to which quantum effects are determinant in this nonadiabatic process. [Preview Abstract] |
Wednesday, March 4, 2015 4:30PM - 4:42PM |
Q26.00007: Coherent Dynamics Following Strong Field Ionization of Polyatomic Molecules Arkaprabha Konar, Yinan Shu, Vadim Lozovoy, James Jackson, Benjamin Levine, Marcos Dantus Molecules, as opposed to atoms, present confounding possibilities of nuclear and electronic motion upon strong field ionization. The dynamics and fragmentation patterns in response to the laser field are structure sensitive; therefore, a molecule cannot simply be treated as a ``bag of atoms'' during field induced ionization. We consider here to what extent molecules retain their molecular identity and properties under strong laser fields. Using time-of-flight mass spectrometry in conjunction with pump-probe techniques we study the dynamical behavior of these molecules, monitoring ion yield modulation caused by intramolecular motions post ionization. The delay scans show that among positional isomers the variations in relative energies, amounting to only a few hundred meVs, influence the dynamical behavior of the molecules despite their having experienced such high fields (V/{\AA}). Ab initio calculations were performed to predict dynamics along with single and multiphoton resonances in the neutral and ionic states. We propose that single electron ionization occurs within an optical cycle with the electron carrying away essentially all of the energy, leaving behind little internal energy in the cation. Evidence for this observation comes from coherent vibrational motion governed by the potential energy surface of the ground state of the cation. Subsequent fragmentation of the cation takes place as a result of further photon absorption modulated by one- and two-photon resonances, which provide sufficient energy to overcome the dissociation energy. [Preview Abstract] |
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