Nonadiabatic excited-state molecular dynamics: treatment of electronic Decoherence

Within the fewest switches surface hopping (FSSH) formulation, a swarm of independent trajectories is propagated and the equations of motion for the quantum coefficients are evolved coherently along each independent nuclear trajectory. That is, the phase factors, or quantum amplitudes, are retained. At a region of strong coupling, a trajectory can branch into multiple wavepackets. Directly following a hop, the two wavepackets remain in a region of nonadiabatic coupling and continue exchanging population. After these wavepackets have sufficiently separated in phase space, they should begin to evolve independently from one another, the process known as decoherence. Decoherence is not accounted for in the standard surface hopping algorithm and leads to internal inconsistency. FSSH is designed to ensure that at any time, the fraction of classical trajectories evolving on each quantum state is equal to the average quantum probability for that state. However, in many systems this internal consistency requirement is violated. Treating decoherence is an inherent problem that can be addressed by implementing some form of decoherence correction to the standard FSSH algorithm. In this study, we have implemented two forms of the instantaneous decoherence procedure where coefficients are reinitialized following hops. We also test the energy-based decoherence correction (EDC) scheme proposed by Granucci et al. and a related version where the form of the decoherence time is taken from Truhlar's Coherent Switching with Decay of Mixing method. The sensitivity of the EDC results to changes in parameters is also evaluated. The application of these computationally inexpensive ad hoc methods is demonstrated in the simulation of nonradiative relaxation in two conjugated oligomer systems, specifically poly-phenylene vinylene and poly-phenylene ethynylene. We find that methods that have been used successfully for treating small systems do not necessarily translate to large polyatomic systems and their success depends on the particular system under study.Fil: Nelson, Tammie. Los Alamos National Laboratory; Estados UnidosFil: Fernández Alberti, Sebastián. Universidad Nacional de Quilmes; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Roitberg, Adrián. University of Florida; Estados UnidosFil: Tretiak, Sergei. Los Alamos National Laboratory; Estados Unido

Identification of unavoided crossings in nonadiabatic photoexcited dynamics involving multiple electronic states in polyatomic conjugated molecules

Radiationless transitions between electronic excited states in polyatomic molecules take place through unavoided crossings of the potential energy surfaces with substantial non-adiabatic coupling between the respective adiabatic states. While the extent in time of these couplings are large enough, these transitions can be reasonably well simulated through quantum transitions using trajectory surface hopping-like methods. In addition, complex molecular systems may have multiple trivial unavoided crossings between noninteracting states. In these cases, the non-adiabatic couplings are described as sharp peaks strongly localized in time. Therefore, their modeling is commonly subjected to the identification of regions close to the particular instantaneous nuclear configurations for which the energy surfaces actually cross each other. Here, we present a novel procedure to identify and treat these regions of unavoided crossings between non-interacting states using the so-called Min-Cost algorithm. The method differentiates between unavoided crossings between interacting states (simulated by quantum hops), and trivial unavoided crossings between non-interacting states (detected by tracking the states in time with Min-Cost procedure). We discuss its implementation within our recently developed non-adiabatic excited state molecular dynamics framework. Fragments of two- and four-ring linear polyphenylene ethynylene chromophore units at various separations have been used as a representative molecular system to test the algorithm. Our results enable us to distinguish and analyze the main features of these different types of radiationless transitions the molecular system undertakes during internal conversion.Fil: Fernández Alberti, Sebastián. Universidad Nacional de Quilmes; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Roitberg, Adrián. University of Florida; Estados UnidosFil: Nelson, Tammie. Los Alamos National Laboratory; Estados UnidosFil: Tretiak, Sergei. Los Alamos National Laboratory; Estados Unido

Nonadiabatic excited-state molecular dynamics: Numerical tests of convergence and parameters

Nonadiabatic molecular dynamics simulations, involving multiple Born-Oppenheimer potential energy surfaces, often require a large number of independent trajectories in order to achieve the desired convergence of the results, and simulation relies on different parameters that should be tested and compared. In addition to influencing the speed of the simulation, the chosen parameters combined with the frequently reduced number of trajectories can sometimes lead to unanticipated changes in the accuracy of the simulated dynamics. We have previously developed a nonadiabatic excited state molecular dynamics methodology employing Tullys fewest switches surface hopping algorithm. In this study, we seek to investigate the impact of the number of trajectories and the various parameters on the simulation of the photoinduced dynamics of distyrylbenzene (a small oligomer of polyphenylene vinylene) within our developed framework. Various user-defined parameters are analyzed: classical and quantum integration time steps, the value of the friction coefficient for Langevin dynamics, and the initial seed used for stochastic thermostat and hopping algorithms. Common approximations such as reduced number of nonadiabatic coupling terms and the classical path approximation are also investigated. Our analysis shows that, at least for the considered molecular system, a minimum of ∼400 independent trajectories should be calculated in order to achieve statistical averaging necessary for convergence of the calculated relaxation timescales.Fil: Nelson, Tammie. Los Alamos National Laboratory; Estados UnidosFil: Fernández Alberti, Sebastián. Universidad Nacional de Quilmes; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Chernyak, Vladimir. Wayne State University (wayne State University); Estados UnidosFil: Roitberg, Adrián. University of Florida; Estados UnidosFil: Tretiak, Sergei. Los Alamos National High Magnetic Field Laboratory; Estados Unido

Let Digons be Bygones: The Fate of Excitons in Curved π-Systems

We explore the diverse origins of unpolarized absorption and emission of molecular polygons consisting of π-conjugated oligomer chains held in a bent geometry by strain controlled at the vertex units. For this purpose, we make use of atomistic nonadiabatic excited-state molecular dynamics simulations of a bichromophore molecular polygon (digon) with bent chromophore chains. Both structural and photoexcited dynamics were found to affect polarization features. Bending strain induces exciton localization on individual chromophore units of the conjugated chains. The latter display different transition dipole moment orientations, a feature not present in the linear oligomer counterparts. In addition, bending makes exciton localization very sensitive to molecular distortions induced by thermal fluctuations. The excited-state dynamics reveals an ultrafast intramolecular energy redistribution that spreads the exciton equally among spatially separated chromophore fragments within the molecular system. As a result, digons become virtually unpolarized absorbers and emitters, in agreement with recent experimental studies on the single-molecule level.Fil: Ondarse Alvarez, Dianelys.Fil: Nelson, Tammie.Fil: Lupton, John M..Fil: Tretiak, Sergei.Fil: Fernandez-Alberti, Sebastian

Analysis of State-Specific Vibrations Coupled to the Unidirectional Energy Transfer in Conjugated Dendrimers

The nonadiabatic excited-state molecular dynamics (NA-ESMD) method and excited-state instantaneous normal modes (ES-INMs) analyses have been applied to describe the state-specific vibrations that participate in the unidirectional energy transfer between the coupled chromophores in a branched dendrimeric molecule. Our molecule is composed of two-, three-, and four-ring linear poly(phenyleneethynylene) (PPE) units linked through meta-substitutions. After an initial laser excitation, an ultrafast sequential S3 → S 2 → S1 electronic energy transfer from the shortest to longest segment takes place. During each Sn → Sn-1 (n = 3, 2) transition, ES-INM(Sn) and ES-INM(Sn-1) analyses have been performed on Sn and Sn-1 states, respectively. Our results reveal a unique vibrational mode localized on the Sn state that significantly matches with the corresponding nonadiabatic coupling vector dn,(n-1). This mode also corresponds to the highest frequency ES-INM(Sn) and it is seen mainly during the electronic transitions. Furthermore, its absence as a unique ES-INM(S n-1) reveals that state-specific vibrations play the main role in the efficiency of the unidirectional Sn → Sn-1 electronic and vibrational energy funneling in light-harvesting dendrimers.Fil: Soler, Miguel A.. Universidad Nacional de Quilmes; ArgentinaFil: Roitberg, Adrián E.. University of Florida; Estados UnidosFil: Nelson, Tammie. Los Alamos National High Magnetic Field Laboratory; Estados UnidosFil: Tretiak, Sergei. Los Alamos National High Magnetic Field Laboratory; Estados UnidosFil: Fernández Alberti, Sebastián. Universidad Nacional de Quilmes; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Coherent exciton-vibrational dynamics and energy transfer in conjugated organics

Coherence, signifying concurrent electron-vibrational dynamics in complex natural and man-made systems, is currently a subject of intense study. Understanding this phenomenon is important when designing carrier transport in optoelectronic materials. Here, excited state dynamics simulations reveal a ubiquitous pattern in the evolution of photoexcitations for a broad range of molecular systems. Symmetries of the wavefunctions define a specific form of the non-adiabatic coupling that drives quantum transitions between excited states, leading to a collective asymmetric vibrational excitation coupled to the electronic system. This promotes periodic oscillatory evolution of the wavefunctions, preserving specific phase and amplitude relations across the ensemble of trajectories. The simple model proposed here explains the appearance of coherent exciton-vibrational dynamics due to non-adiabatic transitions, which is universal across multiple molecular systems. The observed relationships between electronic wavefunctions and the resulting functionalities allows us to understand, and potentially manipulate, excited state dynamics and energy transfer in molecular materials.Fil: Nelson, Tammie R.. Los Alamos National Laboratory; Estados UnidosFil: Ondarse Alvarez, Dianelys. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Quilmes; ArgentinaFil: Oldani, Andres Nicolas. Universidad Nacional de Quilmes; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Rodríguez Hernández, Beatriz. Universidad Nacional de Quilmes; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Alfonso Hernandez, Laura. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Quilmes; ArgentinaFil: Galindo, Johan F.. Universidad Nacional de Colombia; ColombiaFil: Kleiman, Valeria D.. University of Florida; Estados UnidosFil: Fernández Alberti, Sebastián. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Quilmes; ArgentinaFil: Roitberg, Adrián. University of Florida; Estados UnidosFil: Tretiak, Sergei. Los Alamos National Laboratory; Estados Unido

Sexually transmitted infections in association with area-level prostitution and drug-related arrests

Objectives: Examine the mediators and moderators of area-level prostitution arrests and sexually transmitted infections (STI) using population level data. Methods Using justice and public health STI/HIV data in Marion County (Indianapolis), Indiana, over an 18-year period, we assessed the overall association of area-level prostitution and drug-related arrests and STI /HIV, and mediators and moderators of the relationship. Point-level arrests were geocoded and aggregated by census block group. Results: Results indicate a positive relationship between numbers of prostitution arrests and area-level STI rates. There was a dose-response relationship between prostitution arrests and STI rates when accounting for drug-related arrests. The highest quintile block groups had significantly higher rates of reported chlamydia (IRR: 3.29, 95% CI: 2.82, 3.84), gonorrhea (IRR: 4.73, 95% CI: 3.90, 5.57), syphilis (IRR: 4.28, 95% CI: 3:47, 5.29), and HIV (IRR: 2.76, 95% CI: 2.24, 3.39) compared with the lowest quintile. When including drug arrests, the second (IRR: 1.19, 95% CI: 1.03, 1.38) and the third (IRR: 1.20, 95% CI: 1.02, 1.41) highest quintile block groups had lower IRR for reported rates of chlamydia, indicating that drug arrests mediated the prostitution arrest effect. Conclusions: These findings inform public health agencies and community-based organizations that conduct outreach in these areas to expand their efforts to include harm reduction and HIV/STI testing for both sex workers and individuals experiencing substance use disorder. Another implication of these data is the importance of greater collaboration in public health and policing efforts to address overlapping epidemics that engage both health and legal intervention

Nonadiabatic excited state molecular dynamics: perspectives for a robust future

Thesis (Ph. D.)--University of Rochester. Dept. of Chemistry, 2013.The simulation of nonadiabatic molecular dynamics is an indispensable tool for understanding complex ultrafast photoinduced processes such as charge and energy transfer, and nonradiative relaxation. We have developed a computationally efficient nonadiabatic excited state molecular dynamics (NA-ESMD) framework incorporating quantum transitions among multiple adiabatic excited state potential energy surfaces (PESs) using the fewest-switches surface hopping (FSSH) algorithm. The NA-ESMD methodology allows for simulation of nonadiabatic dynamics in large molecular systems with hundreds of atoms on ~10 ps time scales where multiple coupled excited states are involved. From these calculations, we can learn about energy relaxation (vibrational and electronic) and transfer rates, details of the nonadiabatic couplings and their relationship with molecular motion, and spectroscopic signatures. The results of NA-ESMD simulations can vary drastically depending on the chosen parameters for propagation and adoption of various approximation schemes. Furthermore, neglecting to treat trivial unavoided crossings between excited state PESs can cause quantum transitions to be missed due to the finite valued propagation time step and the strongly localized nonadiabatic couplings. This failure can result in unphysical long-range energy transfer, even between two non-interacting molecules separated by large distances. Here we analyze the considerations involved in selecting parameters, the validity of commonly adopted approximations, and their effects on the simulated dynamics. In order to avoid spurious artifacts arising from state crossings, we have developed an algorithm to detect trivial unavoided crossings between excited state PESs. As we will demonstrate in this work, implementation of this novel method creates an improved NA-ESMD framework that can now be used to simulate systems involving numerous crossings between multiple excited state PESs. The final ingredient for a robust NA-ESMD approach is the treatment of long-lived quantum coherences. In this work, we will evaluate the performance of two computationally low-cost methods for incorporating quantum decoherence into nonadiabatic dynamics. Ultimately, the application of methods developed and tested for small systems involving only a few excited states to large polyatomic systems is not straightforward. The automatic extrapolation to multi-state simulations can be misleading and their validity must be investigated

Conformational disorder in energy transfer: beyond Förster theory

Energy transfer in donor–acceptor chromophore pairs, where the absorption of each species is well separated while donor emission and acceptor absorption overlap, can be understood through a Förster resonance energy transfer model. The picture is more complex for organic conjugated polymers, where the total absorption spectrum can be described as a sum of the individual contributions from each subunit (chromophore), whose absorption is not well separated. Although excitations in these systems tend to be well localized, traditional donors and acceptors cannot be defined and energy transfer can occur through various pathways where each subunit (chromophore) is capable of playing either role. In addition, fast torsional motions between individual monomers can break conjugation and lead to reordering of excited state energy levels. Fast torsional fluctuations occur on the same timescale as electronic transitions leading to multiple trivial unavoided crossings between excited states during dynamics. We use the non-adiabatic excited state molecular dynamics (NA-ESMD) approach to simulate energy transfer between two poly-phenylene vinylene (PPV) oligomers composed of 3-rings and 4-rings, respectively, separated by varying distances. The change in the spatial localization of the transient electronic transition density, initially localized on the donors, is used to determine the transfer rate. Our analysis shows that evolution of the intramolecular transition density can be decomposed into contributions from multiple transfer pathways. Here we present a detailed analysis of ensemble dynamics as well as a few representative trajectories which demonstrate the intertwined role of electronic and conformational processes. Our study reveals the complex nature of energy transfer in organic conjugated polymer systems and emphasizes the caution that must be taken in performing such an analysis when a single simple unidirectional pathway is unlikely.Fil: Nelson, Tammie. Los Alamos National Laboratory; Estados UnidosFil: Fernández Alberti, Sebastián. Universidad Nacional de Quilmes; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Roitberg, Adrián. University of Florida; Estados UnidosFil: Tretiak, Sergei. Los Alamos National Laboratory; Estados Unido

Electronic Delocalization, Vibrational Dynamics, and Energy Transfer in Organic Chromophores

The efficiency of materials developed for solar energy and technological applications depends on the interplay between molecular architecture and light-induced electronic energy redistribution. The spatial localization of electronic excitations is very sensitive to molecular distortions. Vibrational nuclear motions can couple to electronic dynamics driving changes in localization. The electronic energy transfer among multiple chromophores arises from several distinct mechanisms that can give rise to experimentally measured signals. Atomistic simulations of coupled electron-vibrational dynamics can help uncover the nuclear motions directing energy flow. Through careful analysis of excited state wave function evolution and a useful fragmenting of multichromophore systems, through-bond transport and exciton hopping (through-space) mechanisms can be distinguished. Such insights are crucial in the interpretation of fluorescence anisotropy measurements and can aid materials design. This Perspective highlights the interconnected vibrational and electronic motions at the foundation of nonadiabatic dynamics where nuclear motions, including torsional rotations and bond vibrations, drive electronic transitions.Fil: Nelson, Tammie. Los Alamos National High Magnetic Field Laboratory; Estados UnidosFil: Fernández Alberti, Sebastián. Universidad Nacional de Quilmes; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Roitberg, Adrián. University of Florida; Estados UnidosFil: Tretiak, Sergei. Los Alamos National High Magnetic Field Laboratory; Estados Unido