Author
Qi Ou
Other affiliations: University of Pennsylvania, Princeton University
Bio: Qi Ou is an academic researcher from Tsinghua University. The author has contributed to research in topics: Density functional theory & Excited state. The author has an hindex of 11, co-authored 26 publications receiving 413 citations. Previous affiliations of Qi Ou include University of Pennsylvania & Princeton University.
Papers
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TL;DR: It is shown that if the difference between two excitation energies equals another excitation energy, the couplings from response theory exhibit an unphysical divergence, which is a result of the adiabatic or static kernel approximation of time-dependent density functional theory leading to an incorrect analytical structure of the quadratic response function.
Abstract: We present a complete derivation of derivative couplings between excited states in the framework of adiabatic time-dependent density functional response theory. Explicit working equations are given and the resulting derivative couplings are compared with derivative couplings from a pseudo-wavefunction ansatz. For degenerate excited states, i.e., close to a conical intersection (CI), the two approaches are identical apart from an antisymmetric overlap term. However, if the difference between two excitation energies equals another excitation energy, the couplings from response theory exhibit an unphysical divergence. This spurious behavior is a result of the adiabatic or static kernel approximation of time-dependent density functional theory leading to an incorrect analytical structure of the quadratic response function. Numerical examples for couplings close to a CI and for well-separated electronic states are given.
80 citations
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TL;DR: Organic light-emitting transistors based on LD-1 are for the first time demonstrated with obvious electroluminescent emission and gate tunable features, which opens the door for a new class of organic semiconductor laser molecules, and is critical for deep-blue optical and laser applications.
Abstract: Here, we design and synthesize an organic laser molecule, 2,7-diphenyl-9H-fluorene (LD-1), which has state-of-the-art integrated optoelectronic properties with a high mobility of 0.25 cm2 V-1 s-1, a high photoluminescence quantum yield of 60.3%, and superior deep-blue laser characteristics (low threshold of Pth = 71 μJ cm-2 and Pth = 53 μJ cm-2 and high quality factor (Q) of ∼3100 and ∼2700 at emission peaks of 390 and 410 nm, respectively). Organic light-emitting transistors based on LD-1 are for the first time demonstrated with obvious electroluminescent emission and gate tunable features. This work opens the door for a new class of organic semiconductor laser molecules and is critical for deep-blue optical and laser applications.
73 citations
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TL;DR: Recent developments in electronic structure theory as directly applicable for modeling photoexcited systems are reviewed, including advances in localized diabatization, which is one approach for generating adiabatic-to-diabatic (ATD) transformations.
Abstract: ConspectusElectronically photoexcited dynamics are complicated because there are so many different relaxation pathways: fluorescence, phosphorescence, radiationless decay, electon transfer, etc. In practice, to model photoexcited systems is a very difficult enterprise, requiring accurate and very efficient tools in both electronic structure theory and nonadiabatic chemical dynamics. Moreover, these theoretical tools are not traditional tools. On the one hand, the electronic structure tools involve couplings between electonic states (rather than typical single state energies and gradients). On the other hand, the dynamics tools involve propagating nuclei on multiple potential energy surfaces (rather than the usual ground state dynamics).In this Account, we review recent developments in electronic structure theory as directly applicable for modeling photoexcited systems. In particular, we focus on how one may evaluate the couplings between two different electronic states. These couplings come in two flavors...
69 citations
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TL;DR: In this paper, the photophysics of benzaldehyde are analyzed through the lens of TD-DFT adiabatic excited states and Boys or Edmiston-Ruedenberg localized diabatic states.
Abstract: The photophysics of benzaldehyde are analyzed through the lens of TD-DFT adiabatic excited states and Boys or Edmiston–Ruedenberg localized diabatic states. We predict rate constants for two processes in excited benzaldehyde: (i) the intersystem crossing from S1 → T2 and (ii) the phosphorescence from T1 → S0. We also study (iii) the conical intersection between T2 and T1 that is putatively responsible for an ultrafast internal conversion process, T2 → T1. In agreement with Ohmori et al. (J. Phys. Chem. 1988, 92 (5), 1086–1093), our results suggest that the S1 → T2 intersystem crossing in benzaldehyde is rapid not only because of a large spin–orbit matrix element (i.e., El-Sayed’s rule) but also because of a fortuitously small energy barrier. Furthermore, when studying the T2 → T1 internal conversion, we find that both Boys and Edmiston–Ruedenberg localization give remarkably stable and accurate diabatic states which will be useful for ongoing studies of dynamics near conical intersections. To our knowledg...
66 citations
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TL;DR: Working within the Tamm-Dancoff approximation, the derivative couplings between time-dependent density-functional theory excited states are calculated by assuming that the Kohn-Sham superposition of singly excited determinants represents a true electronic wavefunction.
Abstract: Working within the Tamm-Dancoff approximation, we calculate the derivative couplings between time-dependent density-functional theory excited states by assuming that the Kohn-Sham superposition of singly excited determinants represents a true electronic wavefunction. All Pulay terms are included in our derivative coupling expression. The reasonability of our approach can be established by noting that, for closely separated electronic states in the infinite basis limit, our final expression agrees exactly with the Chernyak-Mukamel expression (with transition densities from response theory). Finally, we also validate our approach empirically by analyzing the behavior of the derivative couplings around the T1/T2 conical intersection of benzaldehyde.
46 citations
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TL;DR: The theory and fundamental principles of the spin-vibronic mechanism for ISC are presented, followed by empirical rules to estimate the rate of ISC within this regime.
Abstract: Intersystem crossing (ISC), formally forbidden within nonrelativistic quantum theory, is the mechanism by which a molecule changes its spin state. It plays an important role in the excited state decay dynamics of many molecular systems and not just those containing heavy elements. In the simplest case, ISC is driven by direct spin–orbit coupling between two states of different multiplicities. This coupling is usually assumed to remain unchanged by vibrational motion. It is also often presumed that spin-allowed radiationless transitions, i.e. internal conversion, and the nonadiabatic coupling that drives them, can be considered separately from ISC and spin–orbit coupling owing to the vastly different time scales upon which these processes are assumed to occur. However, these assumptions are too restrictive. Indeed, the strong mixing brought about by the simultaneous presence of nonadiabatic and spin–orbit coupling means that often the spin, electronic, and vibrational dynamics cannot be described independe...
505 citations
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TL;DR: This review focuses on the NA-MQC dynamics methods and programs developed in the last 10 years, and stresses the relations between approaches and their domains of application.
Abstract: Nonadiabatic mixed quantum–classical (NA-MQC) dynamics methods form a class of computational theoretical approaches in quantum chemistry tailored to investigate the time evolution of nonadiabatic phenomena in molecules and supramolecular assemblies. NA-MQC is characterized by a partition of the molecular system into two subsystems: one to be treated quantum mechanically (usually but not restricted to electrons) and another to be dealt with classically (nuclei). The two subsystems are connected through nonadiabatic couplings terms to enforce self-consistency. A local approximation underlies the classical subsystem, implying that direct dynamics can be simulated, without needing precomputed potential energy surfaces. The NA-MQC split allows reducing computational costs, enabling the treatment of realistic molecular systems in diverse fields. Starting from the three most well-established methods—mean-field Ehrenfest, trajectory surface hopping, and multiple spawning—this review focuses on the NA-MQC dynamics...
396 citations
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Evgeny Epifanovsky, Andrew T.B. Gilbert1, Andrew T.B. Gilbert2, Xintian Feng3 +235 more•Institutions (54)
TL;DR: The Q-Chem quantum chemistry program package as discussed by the authors provides a suite of tools for modeling core-level spectroscopy, methods for describing metastable resonances, and methods for computing vibronic spectra, the nuclear-electronic orbital method, and several different energy decomposition analysis techniques.
Abstract: This article summarizes technical advances contained in the fifth major release of the Q-Chem quantum chemistry program package, covering developments since 2015. A comprehensive library of exchange-correlation functionals, along with a suite of correlated many-body methods, continues to be a hallmark of the Q-Chem software. The many-body methods include novel variants of both coupled-cluster and configuration-interaction approaches along with methods based on the algebraic diagrammatic construction and variational reduced density-matrix methods. Methods highlighted in Q-Chem 5 include a suite of tools for modeling core-level spectroscopy, methods for describing metastable resonances, methods for computing vibronic spectra, the nuclear-electronic orbital method, and several different energy decomposition analysis techniques. High-performance capabilities including multithreaded parallelism and support for calculations on graphics processing units are described. Q-Chem boasts a community of well over 100 active academic developers, and the continuing evolution of the software is supported by an "open teamware" model and an increasingly modular design.
360 citations
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TL;DR: Representative applications to PCET in solution, proteins, electrochemistry, and photoinduced processes are presented, highlighting the interplay between theoretical and experimental studies.
Abstract: Proton-coupled electron transfer (PCET) is ubiquitous throughout chemistry and biology. This Perspective discusses recent advances and current challenges in the field of PCET, with an emphasis on the role of theory and computation. The fundamental theoretical concepts are summarized, and expressions for rate constants and kinetic isotope effects are provided. Computational methods for calculating reduction potentials and pKa’s for molecular electrocatalysts, as well as insights into linear correlations and non-innocent ligands, are also described. In addition, computational methods for simulating the nonadiabatic dynamics of photoexcited PCET are discussed. Representative applications to PCET in solution, proteins, electrochemistry, and photoinduced processes are presented, highlighting the interplay between theoretical and experimental studies. The current challenges and suggested future directions are outlined for each type of application, concluding with an overall view to the future.
318 citations
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TL;DR: A current, up-to-date review of the surface hopping methodology for solving nonadiabatic problems, 25 years after Tully published the fewest switches surface hopping algorithm is presented.
Abstract: We present a current, up-to-date review of the surface hopping methodology for solving nonadiabatic problems, 25 years after Tully published the fewest switches surface hopping algorithm. After reviewing the original motivation for and failures of the algorithm, we give a detailed examination of modern advances, focusing on both theoretical and practical issues. We highlight how one can partially derive surface hopping from the Schrodinger equation in the adiabatic basis, how one can change basis within the surface hopping algorithm, and how one should understand and apply the notions of decoherence and wavepacket bifurcation. The question of time reversibility and detailed balance is also examined at length. Recent applications to photoexcited conjugated polymers are discussed briefly.
313 citations