Sean A. Fischer

Decoherence-induced surface hopping

A simple surface hopping method for nonadiabatic molecular dynamics is developed. The method derives from a stochastic modeling of the time-dependent Schrödinger and master equations for open systems and accounts simultaneously for quantum mechanical branching in the otherwise classical (nuclear) degrees of freedom and loss of coherence within the quantum (electronic) subsystem due to coupling to nuclei. Electronic dynamics in the Hilbert space takes the form of a unitary evolution, intermittent with stochastic decoherence events that are manifested as a localization toward (adiabatic) basis states. Classical particles evolve along a single potential energy surface and can switch surfaces only at the decoherence events. Thus, decoherence provides physical justification of surface hopping, obviating the need for ad hoc surface hopping rules. The method is tested with model problems, showing good agreement with the exact quantum mechanical results and providing an improvement over the most popular surface hopping technique. The method is implemented within real-time time-dependent density functional theory formulated in the Kohn-Sham representation and is applied to carbon nanotubes and graphene nanoribbons. The calculated time scales of non-radiative quenching of luminescence in these systems agree with the experimental data and earlier calculations.

Regarding the validity of the time-dependent Kohn–Sham approach for electron-nuclear dynamics via trajectory surface hopping

The implementation of fewest-switches surface-hopping (FSSH) within time-dependent Kohn-Sham (TDKS) theory [Phys. Rev. Lett. 95, 163001 (2005)] has allowed us to study successfully excited state dynamics involving many electronic states in a variety of molecular and nanoscale systems, including chromophore-semiconductor interfaces, semiconductor and metallic quantum dots, carbon nanotubes and graphene nanoribbons, etc. At the same time, a concern has been raised that the KS orbital basis used in the calculation provides only approximate potential energy surfaces [J. Chem. Phys. 125, 014110 (2006)]. While this approximation does exist in our method, we show here that FSSH-TDKS is a viable option for computationally efficient calculations in large systems with straightforward excited state dynamics. We demonstrate that the potential energy surfaces and nonadiabatic transition probabilities obtained within the TDKS and linear response (LR) time-dependent density functional theories (TDDFT) agree semiquantitatively for three different systems, including an organic chromophore ligating a transition metal, a quantum dot, and a small molecule. Further, in the latter case the FSSH-TDKS procedure generates results that are in line with FSSH implemented within LR-TDDFT. The FSSH-TDKS approach is successful for several reasons. First, single-particle KS excitations often give a good representation of LR excitations. In this regard, DFT compares favorably with the Hartree-Fock theory, for which LR excitations are typically combinations of multiple single-particle excitations. Second, the majority of the FSSH-TDKS applications have been performed with large systems involving simple excitations types. Excitation of a single electron in such systems creates a relatively small perturbation to the total electron density summed over all electrons, and it has a small effect on the nuclear dynamics compared, for instance, with thermal nuclear fluctuations. In such cases an additional, classical-path approximation can be made. Third, typical observables measured in time-resolved experiments involve averaging over many initial conditions. Such averaging tends to cancel out random errors that may be encountered in individual simulated trajectories. Finally, if the flow of energy between electronic and nuclear subsystems is insignificant, the ad hoc FSSH procedure is not required, and a straightforward mean-field, Ehrenfest approach is sufficient. Then, the KS representation provides rigorously a convenient and efficient basis for numerically solving the TDDFT equations of motion.

Energy-Specific Linear Response TDHF/TDDFT for Calculating High-Energy Excited States.

An energy-specific TDHF/TDDFT method is introduced in this article for excited state calculations. This approach extends the conventional TDHF/TDDFT implementation to obtain excited states above a predefined energy threshold. The method introduced and developed in this work enables computationally efficient yet rigorous calculations of energy-specific spectra, e.g., X-ray absorption involving extremely high-energy transitions. All transitions are solved in the full molecular orbital space, and orthogonality to the ground state and lower-lying excited states is preserved for each high-energy excited state. Encouraging computational savings are observed in calculating the targeted energy spectrum, while the transition energies, as well as oscillator strengths, remain identical to the results from the standard implementation.

Ab initio nonadiabatic molecular dynamics of wet-electrons on the TiO(2) surface.

The electron transfer (ET) dynamics of wet-electrons on a TiO(2) surface is investigated using state-of-the-art ab initio nonadiabatic (NA) molecular dynamics (MD). The simulations directly mimic the time-resolved experiments [Science 2005, 308, 1154] and reveal the nature of ET in the wet-electron system. Focusing on the partially hydroxylated TiO(2) surface with 1-monolayer water coverage, and including electronic evolution, phonon motions, and electron-phonon coupling, the simulations indicate that the ET is sub-10 fs, in agreement with the experiment. Despite the large role played by low frequency vibrational modes, the ET is fast due to the strong coupling between the TiO(2) surface and water. The average ET for the system has equal contributions from the adiabatic and NA mechanisms, even though a very broad range of individual ET events is seen in the simulated ensemble. Thermal phonon motions induce a large fluctuation of the wet-electron state energy, generate frequent crossings of the donor and acceptor states, and drive the adiabatic mechanism. The rapid phonon-assisted NA tunneling from the wet-electron state to the TiO(2) surface is facilitated by the strong water-TiO(2) electronic interaction. The motions of molecular water have a greater effect on the ET dynamics than the hydroxyl vibrations. The former contribute to both the wet-electron state energy and the water-TiO(2) electronic coupling, while the latter changes only the energy and not the coupling. Delocalized over both water and TiO(2), wet-electrons are supported by a new type of state that is created at the interface due to the strong water-TiO(2) interaction and that cannot exist separately in either material. Similar states are present in a number of other systems with strong interfacial coupling, including certain dye-sensitized semiconductors and metal-liquid interfaces. The ET dynamics involving such interfacial states share many universal features, such as an ultrashort time scale and weak-dependence on temperature, surface defects, and other system details.

Ice-nucleating bacteria control the order and dynamics of interfacial water

Specialized bacteria trigger ice formation by controlling the molecular structure and energy transfer in interfacial water. Ice-nucleating organisms play important roles in the environment. With their ability to induce ice formation at temperatures just below the ice melting point, bacteria such as Pseudomonas syringae attack plants through frost damage using specialized ice-nucleating proteins. Besides the impact on agriculture and microbial ecology, airborne P. syringae can affect atmospheric glaciation processes, with consequences for cloud evolution, precipitation, and climate. Biogenic ice nucleation is also relevant for artificial snow production and for biomimetic materials for controlled interfacial freezing. We use interface-specific sum frequency generation (SFG) spectroscopy to show that hydrogen bonding at the water-bacteria contact imposes structural ordering on the adjacent water network. Experimental SFG data and molecular dynamics simulations demonstrate that ice-active sites within P. syringae feature unique hydrophilic-hydrophobic patterns to enhance ice nucleation. The freezing transition is further facilitated by the highly effective removal of latent heat from the nucleation site, as apparent from time-resolved SFG spectroscopy.

Surface hopping with Ehrenfest excited potential

Given the exponentially scaling cost of full quantum calculations, approximations need to be employed for the simulation of the time evolution of chemical systems. We present a modified version of surface hopping that has the potential to treat larger systems. This is accomplished through an Ehrenfest-like treatment of the excited states, thereby reducing the dynamics to transitions between the ground state and a mean-field excited state. A simplified description of the excited states is achieved, while still allowing for an accurate description of disparate reaction channels. We test our mean-field approximation for the excited states on a series of model problems. Results are compared to the standard surface hopping procedure, with its explicit treatment of all excited states, and the traditional Ehrenfest approach, with its averaging together of all states.

Excited states and optical absorption of small semiconducting clusters: Dopants, defects and charging

Spatially confined semiconductors form an exciting type of material with potential for numerous applications, many of which rely on the materials optical properties and excitation dynamics. For successful realization of the applications a detailed analysis of excitations in these materials is needed, extending beyond the ideal case and including various defects, such as photoionization, doping and surface dangling bonds. High-level ab initio electronic structure calculations on two representative classes of semiconducting clusters have lead to much progress on achieving a thorough understanding of the excitation properties of these materials when they are altered with charging, doping or dangling bonds. The calculations show that the defects introduce new intra-band transitions, blue-shift the optical absorption spectra and push the normal excitonic and multiexcitonic transitions to higher energies. Generally, doping and charging have similar effects on the excited state properties, while introduction of dangling bonds cause less severe changes, since the latter defect can be partially accommodated by reorganization in the local bonding pattern.

The role of surface defects in multi-exciton generation of lead selenide and silicon semiconductor quantum dots

Multi-exciton generation (MEG), the creation of more than one electron-hole pair per photon absorbed, occurs for excitation energies greater than twice the bandgap (E(g)). Imperfections on the surface of quantum dots, in the form of atomic vacancies or incomplete surface passivation, lead to less than ideal efficiencies for MEG in semiconductor quantum dots. The energetic onset for MEG is computed with and without surface defects for nanocrystals, Pb(4)Se(4), Si(7), and Si(7)H(2). Modeling the correlated motion of two electrons across the bandgap requires a theoretical approach that incorporates many-body effects, such as post-Hartree-Fock quantum chemical methods. We use symmetry-adapted cluster with configuration interaction to study the excited states of nanocrystals and to determine the energetic threshold of MEG. Under laboratory conditions, lead selenide nanocrystals produce multi-excitons at excitation energies of 3 E(g), which is attributed to the large dielectric constant, small Coulomb interaction, and surface defects. In the absence of surface defects the MEG threshold is computed to be 2.6 E(g). For lead selenide nanocrystals with non-bonding selenium valence electrons, Pb(3)Se(4), the MEG threshold increases to 2.9 E(g). Experimental evidence of MEG in passivated silicon quantum dots places the onset of MEG at 2.4 E(g). Our calculations show that the lowest multi-exciton state has an excitation energy of 2.5 E(g), and surface passivation enhances the optical activity of MEG. However, incomplete surface passivation resulting in a neutral radical on the surface drives the MEG threshold to 4.4 E(g). Investigating the mechanism of MEG at the atomistic level provides explanations for experimental discrepancies and suggests ideal materials for photovoltaic conversion.

Trp-Cage Folding on Organic Surfaces.

Trp-cage is an artificial miniprotein that is small, stable, and fast folding due to concerted hydrophobic shielding of a Trp residue by polyproline helices. Simulations have extensively characterized Trp-cage; however, the interactions of Trp-cage with organic surfaces (e.g., membranes) and their effect on protein conformation are largely unknown. To better understand these interactions we utilized a combination of replica-exchange molecular dynamics (REMD) and metadynamics (MetaD), to investigate Trp-cage folding on self-assembled monolayers (SAMs). We found that, with REMD and MetaD, Trp-cage strongly binds to neutral CH3 surfaces (-25kT) and moderately adsorbs to anionic COOH interfaces (-7.6kT), with hydrophobic interactions driving CH3 adhesion and electrostatic attractions driving COOH adhesion. Similar to solid-state surfaces, SAMs facilitate a number of intermediate Trp-cage conformations between folded and unfolded states. Regarding Trp-cages aromatic groups in neutral CH3 systems, Tyr becomes oriented parallel to the surface in order to maximize hydrophobic interactions while Trp remains caged perpendicular to the surface; however, Trp can reorient itself parallel to the interface as the miniprotein more closely binds to the surface. In contrast, Tyr and Trp are both repelled from COOH surfaces, though the Trp-cage still adheres to the anionic interface via Lys and its N-terminated Asn residue.

Conditions for Directional Charge Transfer in CdSe Quantum Dots Functionalized by Ru(II) Polypyridine Complexes

Thermodynamic conditions governing the charge transfer direction in CdSe quantum dots (QD) functionalized by either Ru(II)-trisbipyridine or black dye are studied using density functional theory (DFT) and time-dependent DFT (TDDFT). Compared to the energy offsets of the isolated QD and the dye, QD-dye interactions strongly stabilize dye orbitals with respect to the QD states, while the surface chemistry of the QD has a minor effect on the energy offsets. In all considered QD/dye composites, the dyes always introduce unoccupied states close to the edge of the conduction band and control the electron transfer. Negatively charged ligands and less polar solvents significantly destabilize the dyes occupied orbitals shifting them toward the very edge of the valence band, thus, providing favorite conditions for the hole transfer. Overall, variations in the dyes ligands and solvent polarity can progressively adjust the electronic structure of QD/dye composites to modify conditions for the directed charge transfer.