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Quantum coherence plays an important role in exciton dynamics such as singlet fission, which may be determined by molecular physical properties, including energy levels, electronic couplings, and electron–phonon couplings, and by geometric properties, including packing configuration and exciton delocalization. However, the global picture of quantum coherence in high-dimensional multistate systems is still blurred. Here, we perform nonadiabatic molecular dynamics simulation for singlet fission in tetracene clusters and demonstrate that the topology of quantum coherence in terms of the global structure of the coupled multistate system may significantly modulate fission dynamics. In particular, quantum coherence in the spin-specified models could be protected by its topological structure from external perturbations. Our work suggests that the topology of quantum coherence is indispensable in the understanding and control of quantum dynamics, which may find potential implementations to singlet fission and quantum computation. [ABSTRACT FROM AUTHOR]

The delocalization of excitonic states and the interstate quantum coherence are of great importance in understanding fundamental mechanisms in exciton dynamics such as singlet fission. The accurate theoretical description on this key component requires dynamic simulations to be performed at the molecular level in a nonadiabatic framework. Here, we apply the recently developed nonadiabatic active state trajectory method to simulate fission dynamics in tetracene clusters of up to 10 monomers. It is shown that a global view of the topology of quantum coherence in terms of molecular details such as packing configurations, spatial delocalization of states, and the topology of coherent regime plays an important role in modulating fission dynamics, which suggests a new focus for nonadiabatic control of exciton dynamics and provides valuable dynamical information and physical insights for artificial design. [ABSTRACT FROM AUTHOR]

Full semiclassical (SC) initial value representation (IVR) for time correlation functions involves a double phase space average over a set of two phase points, each of which evolves along a classical path. Conventionally, the two initial phase points are sampled independently for all degrees of freedom (DOF) in the Monte Carlo procedure. Here, we present an efficient importance sampling scheme by including the path correlation between the two initial phase points for the bath DOF, which greatly improves the performance of the SC-IVR calculations for large molecular systems. Satisfactory convergence in the study of quantum coherence in vibrational relaxation has been achieved for a benchmark system-bath model with up to 21 DOF. [ABSTRACT FROM AUTHOR]

An efficient time-dependent (TD) Monte Carlo (MC) importance sampling method has recently been developed [G. Tao and W. H. Miller, J. Chem. Phys. 135, 024104 (2011)] for the evaluation of time correlation functions using the semiclassical (SC) initial value representation (IVR) methodology. In this TD-SC-IVR method, the MC sampling uses information from both time-evolved phase points as well as their initial values, and only the 'important' trajectories are sampled frequently. Even though the TD-SC-IVR was shown in some benchmark examples to be much more efficient than the traditional time-independent sampling method (which uses only initial conditions), the calculation of the SC prefactor-which is computationally expensive, especially for large systems-is still required for accepted trajectories. In the present work, we present an approximate implementation of the TD-SC-IVR method that is completely prefactor-free; it gives the time correlation function as a classical-like magnitude function multiplied by a phase function. Application of this approach to flux-flux correlation functions (which yield reaction rate constants) for the benchmark H + H2 system shows very good agreement with exact quantum results. Limitations of the approximate approach are also discussed. [ABSTRACT FROM AUTHOR]

An efficient time-dependent importance sampling method is developed for the Monte Carlo calculation of time correlation functions via the initial value representation (IVR) of semiclassical (SC) theory. A prefactor-free time-dependent sampling function weights the importance of a trajectory based on the magnitude of its contribution to the time correlation function, and global trial moves are used to facilitate the efficient sampling the phase space of initial conditions. The method can be generally applied to sampling rare events efficiently while avoiding being trapped in a local region of the phase space. Results presented in the paper for two system-bath models demonstrate the efficiency of this new importance sampling method for full SC-IVR calculations. [ABSTRACT FROM AUTHOR]

The frozen Gaussian approximation to the quantum propagator may be a viable method for obtaining 'on the fly' quantum dynamical information on systems with many degrees of freedom. However, it has two severe limitations, it rapidly loses normalization and one needs to know the Gaussian averaged potential, hence it is not a purely local theory in the force field. These limitations are in principle remedied by using the Herman-Kluk (HK) form for the semiclassical propagator. The HK propagator approximately conserves unitarity for relatively long times and depends only locally on the bare potential and its second derivatives. However, the HK propagator involves a much more expensive computation due to the need for evaluating the monodromy matrix elements. In this paper, we (a) derive a new formula for the normalization integral based on a prefactor free HK propagator which is amenable to 'on the fly' computations; (b) show that a frozen Gaussian version of the normalization integral is not readily computable 'on the fly'; (c) provide a new insight into how the HK prefactor leads to approximate unitarity; and (d) how one may construct a prefactor free approximation which combines the advantages of the frozen Gaussian and the HK propagators. The theoretical developments are backed by numerical examples on a Morse oscillator and a quartic double well potential. [ABSTRACT FROM AUTHOR]

The combination of optical-Kerr-effect (OKE) spectroscopy and molecular dynamics simulations provided with an ability to delve into the librational dynamics of liquids, revealing, in the process, some surprising commonalities among aromic liquids. Benzene and biphenyl, have remarkably similar OKE spectra despite marked differences in their shapes, sizes and moments of inertia-and even more chemically distinct aromatics tend to have noticeable similarities in their spectra.

Doping can be employed to enhance the electrical conductivity and electrochemical performance of LiFePO4, a promising material for Li-ion batteries. However, the microscopic mechanism of doping is not fully understood. In this study, ab initio density functional theory (DFT) with the generalized gradient approximation (GGA) + U calculations was performed on both bulk and surface Sn-doped LiFePO4. Our results indicate that surface doping is preferred over bulk or subsurface doping because it shows a lower doping energy and surface energy. The doping effect appears to be local, and the effect of the Li vacancy (VLi) distribution was examined. The multivalent Sn doping may facilitate the formation of an Fe2+/Fe3+ complex with the involvement of an effective intermediate Sn3+ component, which complements the existing charge transfer model for LiFePO4. The effective Sn3+ –Fe3+/2VLi complex may exist on the LiFePO4 surfaces, providing possible surface design schemes to control charge transfer. The results suggest that the Sn dopant could modulate band gap and local charge transfer, and improve the electrochemical performance at the last stage of the charging process with no capacity loss. However, an optimized doping concentration may exist for electrochemically inactive doping with an unfavorable doping energy. [ABSTRACT FROM AUTHOR]

Molecular doping of monolayer MoS2 provides a great opportunity to modulate its electronic properties for the potential applications in high performance devices. Density functional theory computations are performed to investigate the charge transfer and electrostatic potential modulation upon the adsorption of pentacene molecule on the surface of MoS2 monolayer (ML). Theoretical calculations indicate that interfacial charge transfer is negligible between pentacene and 2H-MoS2 ML while significant in the pentacene/1T-MoS2 ML complex, which is attributed to the match of energy levels near the Fermi level in the latter case. Moreover, molecular doping of pentacene induces substantial structure changes of the substrate resulting in large adsorption energy, which helps stabilize the 1T-MoS2 ML. Depending on different substrate phases and doping configurations, the interfacial dipole barrier and related work function of MoS2 ML may be modulated in a wide range of the order of about 1 eV. The findings therefore shed light on the possibility of developing the desired organic/inorganic complex for electrical and optoelectronic devices by molecular doping via charge transfer modulation and interface engineering. [ABSTRACT FROM AUTHOR]