Michael Thoss

Multilayer Formulation of the Multiconfiguration Time‐Dependent Hartree Theory

A multilayer (ML) formulation of the multiconfiguration time-dependent Hartree (MCTDH) theory is presented. In this new approach, the single-particle (SP) functions in the original MCTDH method are further expressed employing a time-dependent multiconfigurational expansion. The Dirac–Frenkel variational principle is then applied to optimally determine the equations of motion. Following this strategy, the SP groups are built in several layers, where each top layer SP can contain many more Cartesian degrees of freedom than in the previous formulation of the MCTDH method. As a result, the ML-MCTDH method has the capability of treating substantially more physical degrees of freedom than the original MCTDH method, and thus significantly enhances the ability of carrying out quantum dynamical simulations for complex molecular systems. The efficiency of the new formulation is demonstrated by converged quantum dynamical simulations for systems with a few hundred to a thousand degrees of freedom.

Self-consistent hybrid approach for complex systems: Application to the spin-boson model with Debye spectral density

The self-consistent hybrid approach [H. Wang, M. Thoss, and W. H. Miller, J. Chem. Phys. 115, 2979 (2001), preceding paper] is applied to the spin-boson problem with Debye spectral density as a model for electron-transfer reactions in a solvent exhibiting Debye dielectric relaxation. The population dynamics of the donor and acceptor states in this system is studied for a broad range of parameters, including the adiabatic (slow bath), nonadiabatic (fast bath), as well as the intermediate regime. Based on illustrative examples we discuss the transition from damped coherent dynamics to purely incoherent decay. Using the numerically exact results of the self-consistent hybrid approach as a benchmark, several approximate theories that have been widely used to describe the dynamics in the spin-boson model are tested: the noninteracting blip approximation, the Bloch–Redfield theory, the Smoluchowski-equation treatment of the reaction coordinate (Zusman equations), and the classical path approach (Ehrenfest model...

Semiclassical description of nonadiabatic quantum dynamics: Application to the S1–S2 conical intersection in pyrazine

A recently proposed semiclassical approach to the description of nonadiabatic quantum dynamics [G. Stock and M. Thoss, Phys. Rev. Lett. 78, 578 (1997), X. Sun and W. H. Miller, J. Chem. Phys. 106, 916 (1997)] is applied to the S1–S2 conical intersection in pyrazine. This semiclassical method is based on a transformation of discrete quantum variables to continuous variables, thereby bypassing the problem of a classical treatment of discrete quantum degrees of freedom such as electronic states. Extending previous work on small systems, we investigate the applicability of the semiclassical method to larger systems with strong vibronic coupling. To this end, we present results for several pyrazine models of increasing dimensionality and complexity. In particular, we discuss the quality and performance of the semiclassical approach when the number of nuclear degrees of freedom is increased. Comparison with quantum-mechanical calculations and experimental results shows that the semiclassical method is able to d...

Generalized forward–backward initial value representation for the calculation of correlation functions in complex systems

The capability of two different, recently proposed semiclassical (SC) forward–backward (FB) initial value representations (IVR) to describe quantum interference and coherence effects is investigated. It is shown that depending on the way the observable under consideration is represented by unitary operators one can obtain rather different results. Although the FB-IVR based on an integral representation as a rule is capable of describing quantum interference, a closer analysis reveals that it depends on the observable under consideration if all interference that can be described semiclassically is actually included in the calculation. To overcome this problem a new, generalized FB-IVR method (GFB-IVR) is proposed, which combines the capability of the SC-IVR to describe quantum interference effects independent of the observable and the better convergence properties of the FB-IVR. The performance of this new approach is studied in some detail. In particular, it is shown that the GFB-IVR can describe both the...

Systematic convergence in the dynamical hybrid approach for complex systems: A numerically exact methodology

An efficient method, the self-consistent hybrid method, is proposed for accurately simulating time-dependent quantum dynamics in complex systems. The method is based on an iterative convergence procedure for a dynamical hybrid approach. In this approach, the overall system is first partitioned into a “core” and a “reservoir” (an initial guess). The former is treated via an accurate quantum mechanical method, namely, the time-dependent multiconfiguration self-consistent field or multiconfiguration time-dependent Hartree approach, and the latter is treated via a more approximate method, e.g., classical mechanics, semiclassical initial value representations, quantum perturbation theories, etc. Next, the number of “core” degrees of freedom, as well as other variational parameters, is systematically increased to achieve numerical convergence for the overall quantum dynamics. The method is applied to two examples of quantum dissipative dynamics in the condensed phase: the spin-boson problem and the electronic r...

Singlet fission in pentacene dimers

Significance In the present work, we show compelling evidence for the unprecedented intramolecular singlet fission at room temperature and in dilute solutions within a set of three different regioisomeric pentacene dimers. Pump–probe experiments, which were complemented by theoretical calculations using high-level ab initio multireference perturbation theory methods, corroborate triplet quantum yields as high as 156 ± 5%. To this end, electronic couplings between the two pentacenes in the dimers, by virtue of through-bond or through-space interactions, are decisive in tuning the rates of singlet fission. Singlet fission (SF) has the potential to supersede the traditional solar energy conversion scheme by means of boosting the photon-to-current conversion efficiencies beyond the 30% Shockley–Queisser limit. Here, we show unambiguous and compelling evidence for unprecedented intramolecular SF within regioisomeric pentacene dimers in room-temperature solutions, with observed triplet quantum yields reaching as high as 156 ± 5%. Whereas previous studies have shown that the collision of a photoexcited chromophore with a ground-state chromophore can give rise to SF, here we demonstrate that the proximity and sufficient coupling through bond or space in pentacene dimers is enough to induce intramolecular SF where two triplets are generated on one molecule.

Modeling of ultrafast electron-transfer processes: Validity of multilevel Redfield theory

The capability of multilevel Redfield theory to describe ultrafast photoinduced electron-transfer reactions is investigated. Adopting a standard model of photoinduced electron transfer in a condensed-phase environment, we consider electron-transfer reactions in the normal and inverted regimes, as well as for different values of the electron-transfer parameters, such as reorganization energy, electronic coupling, and temperature. Based on the comparison with numerically exact reference results, obtained using the self-consistent hybrid method, we discuss in some detail the advantages and shortcomings of two different versions of Redfield theory, which employ the time-dependent and stationary Redfield tensor, respectively. The results of the study demonstrate that multilevel Redfield theory, if applied in the appropriate parameter regime, is well suited to describe the ultrafast coherent dynamics of photoinduced electron-transfer reactions.

Semiclassical description of quantum coherence effects and their quenching: A forward-backward initial value representation study

The forward–backward (FB) version of the semiclassical (SC) initial value representation (IVR) is used to study quantum coherence effects in the time-dependent probability distribution of an anharmonic vibrational coordinate and its quenching when coupled to a thermal bath. It is shown that the FB-IVR accurately reproduces the detailed quantum coherent structure in the weak coupling regime, and also describes how this coherence is quenched with an increase of the system–bath coupling and/or the bath temperature. Comparisons are made with other approximations and the physical implications are discussed.

Forward–backward initial value representation for the calculation of thermal rate constants for reactions in complex molecular systems

The semiclassical (SC) initial value representation (IVR) provides a potentially practical way for including quantum effects into classical molecular dynamics simulations. The forward–backward (FB) version of the IVR provides an especially attractive way for calculating time correlation functions, in particular the reactive flux correlation function which determines chemical reaction rates. This paper presents a further analysis and development of the FB-IVR approach. Applications show that it is feasible and accurate for a reaction coordinate coupled to up to 40 degrees of freedom.

Experimental Evidence for Quantum Interference and Vibrationally Induced Decoherence in Single-Molecule Junctions

We analyze quantum interference and decoherence effects in single-molecule junctions both experimentally and theoretically by means of the mechanically controlled break junction technique and density-functional theory. We consider the case where interference is provided by overlapping quasidegenerate states. Decoherence mechanisms arising from electronic-vibrational coupling strongly affect the electrical current flowing through a single-molecule contact and can be controlled by temperature variation. Our findings underline the universal relevance of vibrations for understanding charge transport through molecular junctions.