Bradley F. Habenicht

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.

Ab initio molecular dynamics simulations investigating proton transfer in perfluorosulfonic acid functionalized carbon nanotubes.

Proton dissociation and transfer were examined with ab initio molecular dynamics (AIMD) simulations of carbon nanotubes (CNT) functionalized with perfluorosulfonic acid (-CF(2)SO(3)H) groups with 1-3 H(2)O/SO(3)H. The CNT systems were constructed both with and without fluorine atoms covalently bound to the walls to elucidate the effects of the presence of a strongly hydrophobic environment, the fluorine, on proton dissociation, hydration, and stabilization. The simulations revealed that the dissociated proton was preferentially stabilized as a hydrated hydronium cation (i.e., Eigen like) in the fluorinated CNTs but as a Zundel (H(5)O(2)(+)) cation in the nonfluorinated CNTs. This feature is attributed to the fluorine atoms forming hydrogen bonds with the water molecules coordinated to the central hydronium ion.

The effects of the hydrophobic environment on proton mobility in perfluorosulfonic acid systems: An ab initio molecular dynamics study

Model systems of perfluorosulfonic acid (PFSA) polymers exhibiting regular shaped and idealized channel morphologies were constructed by functionalizing single walled carbon nanotubes (CNTs) with –CF2SO3H groups and adding from 1 to 3H2O/SO3H to investigate structural and chemical factors affecting proton dissociation and transport. No a priori assumptions about either the dissociation or hydration of the protons were assumed and extensive ab initio molecular dynamics (AIMD) simulations were performed and subject to analysis. The importance of the hydrophobic environment was assessed by comparing the hydration of the protons both with and without fluorine atoms attached to the CNT walls. The AIMD trajectories showed that dissociation of the acidic proton increased with increasing density of sulfonic acid groups; however, greater densities also brought about trapping of the dissociated proton. The fluorine atoms accepted hydrogen bonds from the water molecules, stabilized hydrogen bonding, and enhanced proton dissociation. The CNT systems without fluorination of the walls exhibited a propensity for the formation of Zundel cations (H5O2+), while the fluorinated systems favoured hydrated structures involving hydronium ions or hydrated H3O+ species depending on the amount of water in the system.

Ab Initio Study of Phonon-Induced Dephasing of Electronic Excitations in Narrow Graphene Nanoribbons

Vibrational dephasing of the lowest energy electronic excitations in the perfect (16,16) graphene nanoribbon (GNR) and those with the C2-bond insertion and rotation defects is studied with ab initio molecular dynamics. Compared to single-walled carbon nanotubes (SWCNTs) of similar size, GNRs shows very different properties. The dephasing in the ideal GNR occurs twice faster than that in the SWCNTs. It is induced primarily by the 1300 cm (-1) disorder mode seen in bulk graphite rather than by the 1600 cm (-1) C-C stretching mode as in SWCNTs. In contrast to SWCNTs, defects exhibit weaker electron-phonon coupling compared to the ideal system. Therefore, defects should present much less of a practical problem in GNRs compared to SWCNTs. The predicted optical line widths can be tested experimentally.

Two-electron Rabi oscillations in real-time time-dependent density-functional theory

We investigate the Rabi oscillations of electrons excited by an applied electric field in several simple molecular systems using time-dependent configuration interaction (TDCI) and real-time time-dependent density-functional theory (RT-TDDFT) dynamics. While the TDCI simulations exhibit the expected single-electron Rabi oscillations at a single resonant electric field frequency, Rabi oscillations in the RT-TDDFT simulations are a two-electron process. The existence of two-electron Rabi oscillations is determined both by full population inversion between field-free molecular orbitals and the behavior of the instantaneous dipole moment during the simulations. Furthermore, the Rabi oscillations in RT-TDDFT are subject to an intensity threshold of the electric field, below which Rabi oscillations do not occur and above which the two-electron Rabi oscillations occur at a broad range of frequencies. It is also shown that at field intensities near the threshold intensity, the field frequency predicted to induce Rabi oscillations by linear response TDDFT only produces detuned Rabi oscillations. Instead, the field frequency that yields the full two-electron population inversion and Rabi oscillation behavior is shown to be the average of single-electron transition frequencies from the ground S0 state and the doubly-excited S2 state. The behavior of the two-electron Rabi oscillations is rationalized via two possible models. The first model is a multi-photon process that results from the electric field interacting with the three level system such that three level Rabi oscillations may occur. The second model suggests that the mean-field nature of RT-TDDFT induces paired electron propagation.

Adsorption and Diffusion of 4d and 5d Transition Metal Adatoms on Graphene/Ru(0001) and the Implications for Cluster Nucleation

To explore the possibility of using the graphene moiré superstructure formed on Ru(0001) (g/Ru(0001)) as a template to self-assemble super-lattices of metal nanoparticles as model catalysts, it is desirable to know the minimum-energy adsorption sites, adsorption energies, and diffusion properties of small metal species on this surface. Toward that end, density functional theory calculations have been carried out to investigate the adsorption and diffusion of 18 4d (Y–Ag) and 5d (La–Au) transition metal adatoms on g/Ru(0001), using small surface models representing different regions of the g/Ru(0001) surface. For each adatom, adsorption is the strongest in the fcc region and the weakest in the mound region of the moiré. Diffusion within the fcc region is facile for most adatoms, but an additional barrier is imposed by the corrugation of the graphene moiré for traversing between neighboring fcc regions. Overall, the earlier 4d and 5d metal adatoms have stronger adsorption energies and higher diffusion barriers on g/Ru(0001) than the later ones. The results are then interpreted to provide a better understanding of the conditions necessary to achieve dense super-lattices of monodisperse metal clusters on g/Ru(0001).

Peak-Shifting in Real-Time Time-Dependent Density Functional Theory

In recent years, the development and application of real-time time-dependent density functional theory (RT-TDDFT) has gained momentum as a computationally efficient method for modeling electron dynamics and properties that require going beyond a linear response of the electron density. However, the RT-TDDFT method within the adiabatic approximation can unphysically shift absorption peaks throughout the electron dynamics. Here, we investigate the origin of these time-dependent resonances observed in RT-TDDFT spectra. Using both exact exchange and hybrid exchange-correlation approximate functionals, adiabatic RT-TDDFT gives time-dependent absorption spectra in which the peaks shift in energy as populations of the excited states fluctuate, while exact wave function methods yield peaks that are constant in energy but vary in intensity. The magnitude of the RT-TDDFT peak shift depends on the frequency and intensity of the applied field, in line with previous studies, but it oscillates as a function of time-dependent molecular orbital populations, consistent with a time-dependent superposition electron density. For the first time, we provide a rationale for the direction and magnitude of the time-dependent peak shifts based on the molecular electronic structure. For three small molecules, H2, HeH(+), and LiH, we give contrasting examples of peak-shifting to both higher and lower energies. The shifting is explained as coupled one-electron transitions to a higher and a lower lying state. Whether the peak shifts to higher or lower energies depends on the relative energetics of these one-electron transitions.

Adsorption and diffusion of the Rh and Au adatom on graphene moiré/Ru(0001).

Detailed density functional theory calculations have been performed to investigate the adsorption and diffusion of the Rh and Au adatom on the graphene moiré superstructure on Ru(0001). The adsorption energies of each adatom in all of the non-equivalent C-top and C6 ring center sites on the graphene moiré have been calculated. The resulting potential energy surfaces encompass the entire graphene moiré unit cell and shows that the adsorption of both Rh1 and Au1 is most stable in the fcc region on the graphene moiré. The minimum-energy diffusion path between adjacent moiré cells is identified to run mostly directly between the fcc and hcp regions for Au1, but deviates toward the mound region for Rh1. The global diffusion barrier is estimated to be 0.53 eV for Rh1 and 0.71 eV for Au1, corresponding to a hopping rate between adjacent moiré cells of ~10(3) s(-1) and ~1 s(-1) at 298 K, respectively. The consequences of different hopping rates to cluster nucleation have been explored by performing Monte Carlo-based statistical analysis, which suggests that diffusing species other than adatoms need to be taken into account to develop an accurate description of cluster nucleation and growth on this surface.

Ab initio simulations of the effects of nanoscale confinement on proton transfer in hydrophobic environments.

Carbon nanotubes (CNTs) were functionalized with -CF(2)SO(3)H groups and hydrated with 1-3 water molecules per sulfonic acid group to investigate proton dissociation and transport in confined, hydrophobic environments. The distance between sulfonate groups was systematically varied from 6 to 8 Å, and three different CNTs were used to determine the effects of nanoscale confinement. The inner walls of the CNT were either functionalized with fluorine atoms to provide a localized negative charge or left bare to provide a more delocalized charge distribution. The use of ab initio molecular dynamics permitted the study of sulfonate solvation, proton dissociation, and the formation of a hydrogen bonding network without a priori assumptions. It was shown that decreasing the distance between sulfonate groups increased proton dissociation, as well as the interactions between water molecules. As the sulfonate distance increased, connectivity among the water molecules decreased as they formed more isolated clusters around the sulfonate groups. The sulfonate distance and geometry were the most dominant factors in proton dissociation; however, the hydrophobic environment and nanoscale confinement became more important as the distance between sulfonate groups increased.

Nanotube devices: Watching electrons in real time.

Terahertz measurements allow the electronic properties of carbon nanotube transistors to be explored at high frequencies, which should hasten the development of new devices based on these materials.