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Multiexciton generation, by which more than a single electron–hole pair is generated on optical excitation, is a promising paradigm for pushing the efficiency of solar cells beyond the Shockley–Queisser limit of 31%. Utilizing this paradigm, however, requires the onset energy of multiexciton generation to be close to twice the band gap energy and the efficiency to increase rapidly above this onset. This challenge remains unattainable even using confined nanocrystals, nanorods or nanowires. Here, we show how both goals can be achieved in a nanorod heterostructure with type-II band offsets. Using pseudopotential atomistic calculation on a model type-II semiconductor heterostructure we predict the optimal conditions for controlling multiexciton generation efficiencies at twice the band gap energy. For a finite band offset, this requires a sharp interface along with a reduction of the exciton cooling and may enable a route for breaking the Shockley–Queisser limit., Multiple exciton generation could help limit thermalization losses in solar cells, but the efficiency of the process is still limited. Here, the authors show by atomistic calculations that type-II interfaces in nanostructures along with a change in exciton cooling rate favour multiple exciton generation.

International audience; We propose a new type of nanodevice, capable of both path-selectivity and anisotropic lasing, that is based on loss-compensation and amplification by a localized plasmon polariton. The nanodevice is a Y-shaped plasmonic nanostructure embedded in an anisotropic host medium with gain. The anisotropy leads to path selectivity, an effect which is more pronounced once gain is included. Such a device is potentially realizable via bottom-up techniques. The path-selectivity may be coupled with activation of a rotation of the anisotropic host medium for inducing a light-guiding switching functionality.

We develop a stochastic resolution of identity representation to the second-order Matsubara Green's function (sRI-GF2) theory. Using a stochastic resolution of the Coulomb integrals, the second order Born self-energy in GF2 is decoupled and reduced to matrix products/contractions, which reduces the computational cost from O(N5) to O(N3) (with N being the number of atomic orbitals). The current approach can be viewed as an extension to our previous work on stochastic resolution of identity second order Moller-Plesset perturbation theory [T. Y. Takeshita et al., J. Chem. Theory Comput. 13, 4605 (2017)] and offers an alternative to previous stochastic GF2 formulations [D. Neuhauser et al., J. Chem. Theory Comput. 13, 5396 (2017)]. We show that sRI-GF2 recovers the deterministic GF2 results for small systems, is computationally faster than deterministic GF2 for N > 80, and is a practical approach to describe weak correlations in systems with 103 electrons and more.

We propose a general theme, labeled mechanical electrodynamics, where the relative three-dimensional (3-D) orientation of particles with nontrivial geometries is tracked based on the details of the...

A fast method is developed for calculating the random phase approximation (RPA) correlation energy for density functional theory. The correlation energy is given by a trace over a projected RPA response matrix, and the trace is taken by a stochastic approach using random perturbation vectors. For a fixed statistical error in the total energy per electron, the method scales, at most, quadratically with the system size; however, in practice, due to self-averaging, it requires less statistical sampling as the system grows, and the performance is close to linear scaling. We demonstrate the method by calculating the RPA correlation energy for cadmium selenide and silicon nanocrystals with over 1500 electrons. We find that the RPA correlation energies per electron are largely independent of the nanocrystal size. In addition, we show that a correlated sampling technique enables calculation of the energy difference between two slightly distorted configurations with scaling and a statistical error similar to that of the total energy per electron.

We describe the mechanism of graphene fluorination by XeF2, calculated here by a periodic plane-wave DFT. We find that the fluorination of graphene proceeds by simultaneous bonding of two F atoms from XeF2 via transition states that interact a bit asymmetrically with the graphene surface. The fluorination of graphene occurs at the (1, 4) positions of the constituent phenyl ring, eventually covering 25% of the graphene C atoms. Bonding to other positions involves large reaction barriers. We also elucidate the origin of experimental observations (Robinson, J. T.; et al. Nano Lett.2010, 10, 3001) of 100% saturation with F when both sides are allowed to react with XeF2. We find that fluorination at one side facilitates the bonding of F at the (1, 2) position on the other side of graphene by significantly lowering the activation barrier of fluorination, indicating that successive (1, 2) additions eventually lead to 100% coverage of graphene by F. We also discuss the effects of a defect on the graphene surface,...

A method is introduced for simple calculation of charge transfer between very large solvated organic dimers (fullerenes here) from isolated dimer calculations. The individual monomers in noncentrosymmetric dimers experience different chemical environments, so that the dimers do not necessarily represent bulk-like molecules. Therefore, we apply a delocalizing bias directly to the Fock matrix of the dimer system, and verify that this is almost as accurate as self-consistent solvation. As large molecules like fullerenes have a plethora of excited states, the initially excited state orbitals are thermally populated, so that the rate is obtained as a thermal average over Marcus thermal transfers.

We demonstrate a far-field single molecule super-resolution method that maps plasmonic near-fields. The method is largely invariant to fluorescence quenching (arising from probe proximity to a metal), has reduced point-spread-function distortion compared to fluorescent dyes (arising from strong coupling to nanoscopic metallic features), and has a large dynamic range (of 2 orders of magnitude) allowing mapping of plasmonic field-enhancements regions. The method takes advantage of the sensitivity of quantum dot (QD) stochastic blinking to plasmonic near-fields. The modulation of the blinking characteristics thus provides an indirect measure of the local field strength. Since QD blinking can be monitored in the far-field, the method can measure localized plasmonic near-fields at high throughput using a simple far-field optical setup. Using this method, propagation lengths and penetration depths were mapped-out for silver nanowires of different diameters and for different dielectric environments, with a spatial accuracy of ∼15 nm. We initially use sparse sampling to ensure single molecule localization for accurate characterization of the plasmonic near-field with plans to increase density of emitters in further studies. The measured propagation lengths and penetration depths values agree well with Maxwell finite-difference time-domain calculations and with published literature values. This method offers advantages such as low cost, high throughput, and superresolved mapping of localized plasmonic fields at high sensitivity and fidelity.

We present a simple method, time-dependent split (TD-Split) for A → B electron transfer by a TD evaluation of the lowest excitation energy from the ground state of the combined (AB) − system. As an example, we study transfer between substituted fullerenes, primarily PCBM. Electron transfer in such fullerene systems is important as it is often the bottleneck in organic solar cells. The TD-Split method is described in detail, including numerical linearization which reduces the number of required iterations, and comparison to other possible approaches. We also compare to other molecules such as C 60 Me 5 H, and find similar trends as experiment.

The low-lying excited states (La and Lb) of polyacenes from naphthalene to heptacene (N = 2-7) are studied using various time-dependent computational approaches. We perform high-level excited-state calculations using equation of motion coupled cluster with singles and doubles (EOMCCSD) and completely renormalized equation of motion coupled cluster with singles, doubles, and perturbative triples (CR-EOMCCSD(T)) and use these results to evaluate the performance of various range-separated exchange-correlation functionals within linear-response (LR) and real-time (RT) time-dependent density functional theories (TDDFT). As has been reported recently, we find that the range-separated family of functionals addresses the well-documented TDDFT failures in describing these low-lying singlet excited states to a large extent and are as about as accurate as results from EOMCCSD on average. Real-time TDDFT visualization shows that the excited state charged densities are consistent with the predictions of the perimeter free electron orbital (PFEO) model. This corresponds to particle-on-a-ring confinement, which leads to the well-known red-shift of the excitations with acene length. We also use time-dependent semiempirical methods like TD-PM3 and TD-ZINDO, which are capable of handling very large systems. Once reparametrized to match the CR-EOMCCSD(T) results, TD-ZINDO becomes roughly as accurate as range-separated TDDFT, which opens the door to modeling systems such as large molecular assemblies.

Huckel simulations of large finite graphene nanomeshes with lithographically induced holes show sizable band gaps in the conduction while the optical absorption has generally the same semi-metal character as pure graphene. There is a strong dependence of the band gap on the angle between the graphene axis and the periodic hole axis. Simple modification of on-site energies shows that substituents on the edges of the holes could also have a significant effect. These simulations show that graphene nanomeshes, which have been recently fabricated, are potentially useful tunable materials for electronic applications.

We simulate the spin-flip current and transmission function through rings containing elements with a spin–orbit interaction. In a previous study (J. Chem. Phys. 123 (2005) 204714) we predicted that such a system can show spin-birefringence, i.e., a spin current polarized parallel to the molecular axis can flip its direction due to a phase lag due to the spin–orbit interaction. Here we demonstrate the effect in a semi-empirical extended Huckel theory (EHT) molecular simulation. The ring systems studied are naphthalene–bitellurium, gold–porphyrin, and cyclometallated chlorogold, connected to polyacetylene.

We present density functional theory (DFT) and time-dependent DFT (TD-DFT) study of the structures and electronic spectra of small CdSe nanocluster-adenine complexes Cd(n)Se(n)-adenine (n = 3, 6, 10, 13). We examine the changes in the geometries and excitation spectra of the nanoclusters induced by DNA base-binding. By comparing the results calculated for the bare (Cd(n)Se(n)), hydrogen-passivated (Cd(n)Se(n)H(2n)), as well as the corresponding adenine (Ade)-bound clusters (Cd(n)Se(n)-Ade, Cd(n)Se(n)H(2n)-Ade, Cd(n)Se(n)H(2n-2)-Ade), we find that binding with Ade slightly blue-shifts (up to 0.18 eV) the electronic excitations of bare nanoclusters but strongly red-shifts (1.2 eV) those of hydrogen-passivated nanoclusters. Natural bond orbital analysis shows that the LUMO of Cd(n)Se(n)H(2n)-Ade is a pi* orbital located on the purine ring.

We develop an alternative formulation in the energy-domain to calculate the second order Moller−Plesset (MP2) perturbation energies. The approach is based on repeatedly choosing four random energies using a nonseparable guiding function, filtering four random orbitals at these energies, and averaging the resulting Coulomb matrix elements to obtain a statistical estimate of the MP2 correlation energy. In contrast to our time-domain formulation, the present approach is useful for both quantum chemistry and real-space/plane wave basis sets. The scaling of the MP2 calculation is roughly linear with system size, providing a useful tool to study dispersion energies in large systems. This is demonstrated on a structure of 64 fullerenes within the SZ basis as well as on silicon nanocrystals using real-space grids.

Charge carrier localization in extended atomic systems has been described previously as being driven by disorder, point defects or distortions of the ionic lattice. Here we show for the first time by means of first-principles computations that charge carriers can spontaneously localize due to a purely electronic effect in otherwise perfectly ordered structures. Optimally-tuned range-separated density functional theory and many-body perturbation calculations within the GW approximation reveal that in trans-polyacetylene and polythiophene the hole density localizes on a length scale of several nanometers. This is due to exchange-induced translational symmetry breaking of the charge density. Ionization potentials, optical absorption peaks, excitonic binding energies and the optimally-tuned range parameter itself all become independent of polymer length as it exceeds the critical localization scale. Moreover, lattice disorder and the formation of a polaron result from the charge localization in contrast to the traditional view that lattice distortions precede charge localization. Our results can explain experimental findings that polarons in conjugated polymers form instantaneously after exposure to ultrafast light pulses., Comment: 5 pages, 4 figures

A time-dependent formulation for electron-hole excitations in extended finite systems, based on the Bethe-Salpeter equation (BSE), is developed using a stochastic wave function approach. The time-dependent formulation builds on the connection between time-dependent Hartree-Fock (TDHF) theory and configuration-interaction with single substitution (CIS) method. This results in a time dependent Schr\"odinger-like equation for the quasiparticle orbital dynamics based on an effective Hamiltonian containing direct Hartree and screened exchange terms, where screening is described within the Random Phase Approximation (RPA). To solve for the optical absorption spectrum, we develop a stochastic formulation in which the quasiparticle orbitals are replaced by stochastic orbitals to evaluate the direct and exchange terms in the Hamiltonian as well as the RPA screening. This leads to an overall quadratic scaling, a significant improvement over the equivalent symplectic eigenvalue representation of the BSE. Application of the time-dependent stochastic BSE (TDsBSE) approach to silicon and CdSe nanocrystals up to size of ~3000 electrons is presented and discussed., Comment: 12 pages, 5 figures

We examine the effects of self-repulsion on the predictions of charge distribution in biased molecular junctions by the local density functional theory methods. This is done using a functional with explicit long-range exchange term effects [R. Baer, D. Neuhauser, Phys. Rev. Lett. 94 (2005) 043002]. We discuss in detail the new density functional, pointing out some of the remaining difficulties in the theory. We find that in weakly coupled junctions (the typical molecular electronics case) local-density functionals fail to describe correctly the charge distribution in the intermediate bias regime.

We reformulate 1D boson-fermion duality in path-integral terms. The result is a 1D counterpart of the boson-fermion duality in the 2D Chern-Simons gauge theory. The theory is consistent and enables, using standard resummation techniques, to obtain the long-wavelength asymptotics of the collective mode in 1D boson systems at the Tonks-Girardeau regime. The collective mode has the dispersion of Bogoliubov phonons: ω(q) = q(U(q)/m)1/2, where is the bosons density and U(q) is a Fourier component of the two-body potential.

We demonstrate an unexpectedly strong surface-plasmonic absorption at the interface of silver and high-index dielectrics based on electron and photon spectroscopy. The measured bandwidth and intensity of absorption deviate significantly from the classical theory. Our density-functional calculation well predicts the occurrence of this phenomenon. It reveals that due to the low metal-to-dielectric work function at such interfaces, conduction electrons can display a drastic quantum spillover, causing the interfacial electron-hole pair production to dominate the decay of surface plasmons. This finding can be of fundamental importance in understanding and designing quantum nano-plasmonic devices that utilize noble metals and high-index dielectrics., Comment: 5 pages, 5 figures

A stochastic approach to time-dependent density functional theory (TDDFT) is developed for computing the absorption cross section and the random phase approximation (RPA) correlation energy. The core idea of the approach involves time-propagation of a small set of stochastic orbitals which are first projected on the occupied space and then propagated in time according to the time-dependent Kohn-Sham equations. The evolving electron density is exactly represented when the number of random orbitals is infinite, but even a small number (? 16) of such orbitals is enough to obtain meaningful results for absorption spectrum and the RPA correlation energy per electron. We implement the approach for silicon nanocrystals (NCs) using real-space grids and find that the overall scaling of the algorithm is sublinear with computational time and memory., 7 pages, 4 figures

We present a linear-response approach for time-dependent density-functional theories using time-adiabatic functionals. The resulting theory can be performed both in the time and in the frequency domain. The derivation considers an impulsive perturbation after which the Kohn-Sham orbitals develop in time autonomously. The equation describing the evolution is not strictly linear in the wave function representation. Only after going into a symplectic real-spinor representation does the linearity make itself explicit. For performing the numerical integration of the resulting equations, yielding the linear response in time, we develop a modified Chebyshev expansion approach. The frequency domain is easily accessible as well by changing the coefficients of the Chebyshev polynomial, yielding the expansion of a formal symplectic Green's operator.

We use Huckel theory to examine interference effects on conductance of a wire when a `lollypop' side-chain is bonded to it, acting as a resonance cavity. A clear signature of interference is found at these ballistic conducting systems, stronger in small systems. Gating effects are enhanced by the presence of the loop, where the electronic wavefunctions can experience large changes in phase. Using an `interference index', I=mod(S,2)+mod(L,4), where S,L are stick and loop lengths, respectively, we conclude that interference is constructive (destructive) and conductance high (low) when I=0,4(I=2).

The construction of devices based on molecular components depends upon the development of molecular wires with adaptable current–voltage characteristics. Here, we report that quantum interference effects could lead to substantial differences in conductance in molecular wires which include some simple polycyclic aromatic hydrocarbons (PAHs). For molecular wires containing a single benzene, anthracene or tetracene molecule a large peak appears in the electron transmission probability spectrum at an energy just above the lowest unoccupied orbital (LUMO). For a molecular wire containing a single naphthalene molecule, however, this same peak essentially vanishes. Furthermore, the peak can be re-established by altering the attachment points of the molecular leads to the naphthalene molecule. A breakdown of the individual terms contributing the relevant peak confirms that these results are in fact due to quantum interference effects.

The small-bias conductance of the C6 molecule, stretched between two metallic leads, is studied using time-dependent density functional theory within the adiabatic local density approximation. The leads are modeled by jellium slabs, the electronic density and the current density are described on a grid, whereas the core electrons and the highly oscillating valence orbitals are approximated using standard norm-conserving pseudopotentials. The jellium leads are supplemented by a complex absorbing potential that serves to absorb charge reaching the edge of the electrodes and hence mimic irreversible flow into the macroscopic metal. The system is rapidly exposed to a ramp potential directed along the C6 axis, which gives rise to the onset of charge and current oscillations. As time progresses, a fast redistribution of the molecular charge is observed, which translates into a direct current response. Accompanying the dc signal, alternating current fluctuations of charge and currents within the molecule and the metallic leads are observed. These form the complex impedance of the molecule and are especially strong at the plasmon frequency of the leads and the lowest excitation peak of C6. We study the molecular conductance in two limits: the strong coupling limit, where the edge atoms of the chain are submerged in the jellium and the weak coupling case, where the carbon atoms and the leads do not overlap spatially.

Nanoshells have been previously shown to have tunable absorption frequencies that are dependent on the ratio of their inner and outer radii. Inspired by this, we ask: can a nanoshell increase the absorption of a small core system embedded within it? A theoretical model is constructed to answer this question. A core, composed of a “jellium” ball of the density of gold is embedded within a jellium nanoshell of nanometric diameter. The shell plasmon frequency is tuned to the core absorption line. A calculation based the time-dependent density functional theory was performed showing a 10 fold increase in core excitation yield.

We develop a formalism for efficient iterative solutions of scattering problems involving the Maxwell equations. The methods, borrowed from recent advancements in chemical reaction dynamics, represent the scattering wavefunctions on two grids; one used for the initial wave and is one-dimensional; the other is a small three-dimensional grid padded with absorbing-potentials on which the scattered function is represented. The formalism is automatically suitable for scattering studies of transmission, reflection and scattering components of a wave. The simulations can be done with time-dependent wavepackets or direct iterative solution for the Green's function, but the results are rigorous time-independent (frequency-dependent) scattering amplitudes. Model time-dependent simulations involving up to a 100×100×100 grid for the inner wavefunction were numerically done on a simple PC.

A mathematical model is presented that describes several signaling events that occur in cells from patients with chronic myeloid leukemia, i.e. autophosphorylation of the Bcr-Abl oncoprotein and subsequent signaling through the Crkl pathway. Dynamical effects of the drug STI-571 (Gleevec) on these events are examined, and a minimal concentration for drug effectiveness is predicted by simulation. Most importantly, the model suggests that, for cells in blast crisis, cellular drug clearance mechanisms such as drug efflux pumps dramatically reduce the effectiveness of Gleevec. This is a new prediction regarding the efficacy of Gleevec. In addition, it is speculated that these resistance mechanisms might be present from the onset of disease.

Intermolecular kinetic-energy operators are derived (in the rigid monomer approximation) for solute–solventn clusters of the type B–An, where B is a molecule and A is either an atom or a molecule. The operators are obtained for a body-fixed frame embedded in the B moiety and parallel to the principal axes of that species. They are expressed in terms of intermolecular coordinates that represent the projection along the body-fixed axes of position vectors pointing from the center of mass of B to the centers of mass of the A species. The results are particularly useful for calculations on clusters in which A–B interactions dominate over A–A interactions in the intermolecular potential energy surface and/or there is minimal interaction between subsets of the A moieties. This utility is demonstrated in variational calculations of intermolecular states in the (1|1) isomer of anthracene–He2.

A general formalism is developed for the theory of electronic current through a molecule attached to macroscopic electrodes in a given bias. The current is calculated in a non-perturbative many-body formalism. We obtain an intuitively appealing formula. We identify two distinct limits: the Kubo linear response formula valid in the small bias regime and the Landauer formula valid for non-interacting electrons. Finally, we develop a new correlation function formula, which is more amenable to numerical computations and discuss the application of absorbing potentials in the electrodes as a possible means of calculation.

A semiclassical cellular method is proposed. Signals generated by semiclassical techniques generally deteriorate over time as trajectories become chaotic. One approach to remedy this problem has been to have each trajectory weighted by an entire cell of nearby trajectories (Filinov transform). But even in this approach the exponential part of the propagator typically becomes large and positive over time. Here the cellularization (Filinov) parameter is subject to constraints which make it time dependent and trajectory dependent. It also depends on dimensionality, so it ends up as a matrix. Physically, the Filinov transform is done differently in different directions associated with the stability matrix for the phase—essentially a more confined integration in directions where the matrix diverges and a wider integration in other directions. This squelches the contribution from any part of a trajectory that becomes excessively chaotic. A trajectory-dependent cellurized frozen Gaussian is applied here within the Herman–Kluk semiclassical approach. It is tested by looking at a single-particle three-dimensional problem, He attached to a rigid immovable naphtalene, where it is shown to be more accurate than the original HK approach, without the divergence of the correlation function common in the usual cellular dynamics (HK) formulation, and is able to separate a low-lying excited state from the ground state.

The dynamics of a chain of vibrational bonds which develop a classical solitary compression wave is simulated. A converged fully correlated quantum mechanical calculation is compared with a time dependent mean field approach (TDSCF) and with a classical simulation. The dynamics were all generated from the same Hamiltonian. The TDSCF and classical calculations show a fully developed solitary wave with the expected dependence of group velocity on amplitude. The full quantum calculations show a solitary-like wave which propagates for a while but then degrades. The robustness of the compression wave depends on the initial preparation. Evidence of partial recurrence of the wave has also been observed.

Interference dips in electronic absorption spectra caused by coupling between excited states are calculated and interpreted using three methods: an analytical expression, an integratable expression, and wave packet propagation. All of the analyses give similar results. The dips (distantly related to Fano’s antiresonance) are caused by spin-orbit coupling between the states involved in spin-forbidden transitions with narrow bandwidths and the state involved in a spin-allowed transition with a large bandwidth caused by progressions in normal vibrational modes that are displaced between the excited and ground electronic states. The analytical expression involves the assumptions that only one vibrational eigenstate of the “forbidden” electronic state is involved and that the broad background is represented by a Lorentzian-type function. The integratable expression replaces the Lorentzian function by one that represents the band shape caused by a progression of unresolved vibronic peaks. The wave packet propagation method is exact for the model. Analytical expressions for multiple interfering states and integratable expressions for multiple electronic states and multiple vibrational modes are derived. The spectra of Ni(H2O)6 2+ and octahedral CrO6 9- units in ZrO2‚33%Y2O3 doped with chromium(III) are analyzed. Physical meanings and mathematical origins of the interference dips are interpreted.

A method is developed for computing the direct current and alternating current conductance of molecular wires at small bias. The basic ingredients are: linear response theory, time-dependent density functional theory, imaginary potentials, and a jellium model for the metallic leads. The theory is capable of incorporating the effect of realistic charge distributions in the system and electron- electron correlation. We demonstrate the method on the jellium-C3-jellium system. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem 91: 524 -532, 2003

We point out and simulate the possible utility of anti-coherence in molecular electronics. In ballistic transfer through a molecule with a large loop that fulfils a certain phase condition on the loop structure, the transfer would be anti-coherent. By applying one or two control voltages to the molecule, that modify the relative phase through the two parts of the loop, the transfer could be controlled, just like in FET or in XOR gates. The simulations use the absorbing-potential based flux–flux formulae with a Huckel–Hamiltonian in a Landauer formulation, and are numerically equivalent to a weighted time-dependent correlation function.

We use Redfield's relaxation theory in the Davies formulation to study primary charge separation reactions in bacterial photosynthesis. The specific model studied is the standard one (spin-boson), with three states for the system, harmonic oscillators for the bath, and linear ohmic system-bath coupling. The Redfield-Davies formulation, which is equivalent to the secular approximation, is Markovian, of second order in system-bath coupling, and is written for the system's density matrix. The approximation does not suffer from any negative probabilities (which appear in the original Redfield approach) and can therefore be used for long-time processes (20 ps or more here). Our results are in line with previous studies, especially to high bath frequencies. They confirm the usefulness of the Redfield-Davies secular approach as a convenient and simple tool for studying system-bath processes. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem 87: 254-263, 2002

The efficiency of bulk heterojunction (BHJ) organic photovoltaics is sensitive to the morphology of the fullerene network that transports electrons through the device. This sensitivity makes it difficult to distinguish the contrasting roles of local electron mobility (how easily electrons can transfer between neighboring fullerene molecules) and macroscopic electron mobility (how well-connected is the fullerene network on device length scales) in solar cell performance. In this work, a combination of density functional theory (DFT) calculations, flash-photolysis time-resolved microwave conductivity (TRMC) experiments, and space-charge-limit current (SCLC) mobility estimates are used to examine the roles of local and macroscopic electron mobility in conjugated polymer/fullerene BHJ photovoltaics. The local mobility of different pentaaryl fullerene derivatives (so-called 'shuttlecock' molecules) is similar, so that differences in solar cell efficiency and SCLC mobilities result directly from the different propensities of these molecules to self-assemble on macroscopic length scales. These experiments and calculations also demonstrate that the local mobility of phenyl-C60 butyl methyl ester (PCBM) is an order of magnitude higher than that of other fullerene derivatives, explaining why PCBM has been the acceptor of choice for conjugated polymer BHJ devices even though it does not form an optimal macroscopic network. The DFT calculations indicate that PCBM's superior local mobility comes from the near-spherical nature of its molecular orbitals, which allow strong electronic coupling between adjacent molecules. In combination, DFT and TRMC techniques provide a tool for screening new fullerene derivatives for good local mobility when designing new molecules that can improve on the macroscopic electron mobility offered by PCBM. The roles of local and macroscopic electron mobility in conjugated polymer/fullerene BHJ photovoltaics are examined via a combination of density functional theory, flash-photolysis time-resolved microwave conductivity, and space-charge-limit current (SCLC) mobility estimates. The local mobility of different fullerene derivatives ('shuttlecock' molecules) is similar, so differences in solar cell efficiency and SCLC mobilities result directly from the different propensities of these molecules to self-assemble on macroscopic length scales. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We develop a formalism to calculate the quasiparticle energy within the GW many-body perturbation correction to the density functional theory. The occupied and virtual orbitals of the Kohn-Sham Hamiltonian are replaced by stochastic orbitals used to evaluate the Green function G, the polarization potential W, and, thereby, the GW self-energy. The stochastic GW (sGW) formalism relies on novel theoretical concepts such as stochastic time-dependent Hartree propagation, stochastic matrix compression, and spatial or temporal stochastic decoupling techniques. Beyond the theoretical interest, the formalism enables linear scaling GW calculations breaking the theoretical scaling limit for GW as well as circumventing the need for energy cutoff approximations. We illustrate the method for silicon nanocrystals of varying sizes with N_{e}3000 electrons.

A solution of the combined Maxwell-Schrodinger equations for the propagation of an ultraviolet (UV) pulse in a sample of model H2 C -like molecules (with only two electronic states), shows a significant increase of the emission at 5-10 harmonics for a thick-density (line integrated density of 50 a.u. 2 ) vs. a thin-density case. The increase is more than two orders of magnitude larger than the expected factor (the ratio of the squared densities) and is due to a "sharpening" effect whereby Rabi oscillations induce a spatially rapidly varying electric field that has very sharp peaks. Equally important, the molecular degree of freedom is shown to exert a strong influence on the emitted radiation so that multiharmonic emission can be controlled by preparing molecules in specific initial wavepackets. c 2001 John Wiley & Sons, Inc. Int J Quantum Chem 81: 260-267, 2001

The spectra of metal-containing molecules and crystals often contain “interference dips,” i.e., sharp decreases in absorbance at the energy of forbidden or weakly allowed intraconfigurational transitions [1–14]. The loss of absorbance is naively unexpected; the spectrum should consist of the superposition of the absorbances resulting from the transitions to the two different excited electronic states. The spectroscopic dips are routinely interpreted using the theory developed by Fano that was derived for atomic absorption in the presence of a slowly varying background continuum [15]. In molecular and crystal spectroscopy, however, the “background” contains an underlying dispersion part that varies strongly with energy [16–18]. It results from a dipole allowed electronic transition with a bandwidth determined by progressions in normal vibrational modes that are displaced between the excited and ground electronic states. A new treatment of the loss of absorbance that has physical relevance to molecules or to metal ions with strong phonon coupling to host lattices is needed. In this Letter we derive a simple analytical expression for the intensity decrease and quantitatively interpret the phenomenon. The dips are a form of destructive quantum interference. The important molecular properties that govern the interference are the relative energies of the states, the vibrational frequencies in the electronic states, and the coupling properties. Our analytical derivation uses the simplifying assumption that only one vibrational eigenstate of the “forbidden” electronic state is involved. This assumption is accurate for intraconfigurational transitions in metal compounds, where the spin multiplicity changes but the orbital configuration does not [19]. When this type of transition is well separated from more intense transitions, it is observed as a single line (i.e., little or no vibrational progression.) Our analytical expression for the interference dips is compared to the calculation using the full Hamiltonian. The total electron-nuclear Hamiltonian for the coupled excited states is described in terms of the usual diabatic Hamiltonians [20]: H p 2

Shifted contour auxiliary field Monte Carlo is implemented for molecular electronic structure using a plane-waves basis and norm conserving pseudopotentials. The merits of the method are studied by computing atomization energies of H2, BeH2, and Be2. By comparing with high correlation methods, DFT-based norm conserving pseudopotentials are evaluated for performance in fully correlated molecular computations. Pseudopotentials based on generalized gradient approximation lead to consistently better atomization energies than those based on the local density approximation, and we find there is room for designing pseudopotentials better suited for full valence correlation.

We present an approach toward the dynamically exact calculation of intermolecular states in molecule-large molecule complexes. The approach employs an intermolecular Hamiltonian specifically formulated with the case of molecule-large molecule complexes in mind. In addition, it makes use of filter diagonalization techniques to diagonalize that Hamiltonian. The approach is applied to the calculation of J=0 intermolecular states below about 110 cm−1 in the benzene–H2O complex. The results of the calculation are interpreted in terms of five internal rotation states, a doubly degenerate bending mode and a singly degenerate stretching mode, the latter two modes involving the relative translation of the monomer moieties in the complex. The internal rotation states are discussed in the context of the two-dimensional, free internal rotation/water in-plane torsion model of Pribble et al. [J. Chem. Phys. 103, 531 (1995)]. It is shown that that model is largely successful in identifying the important features of the ...

Recently we presented a method for modeling a quantum system to a bath that explicitly correlates the system with the individual bath modes. We do this through representation of the bath by locally propagating Gaussians (LPG), which change in position and momentum but remain Gaussian in form. The explicit correlation of the system to the bath modes enters through the simultaneous use of a different Gaussian for each state (or grid point) of the system. In this work, we look at two possibilities for the LPG method. In the frozen LPG, the width of the Gaussians is kept constant. In the flexible LPG, we relax this condition and allow for the width to be both time dependent and complex. We present a comparative study of these two methods and compare them with both time-dependent self-consistent field calculations (TDSCF) and an exact quantum calculation. The two LPG methods, in comparison with TDSCF, more accurately describe the exact dynamics. The difference is especially noticeable in the case of weak coupling, where the averaging done in TDSCF is an oversimplification of the system.

The shifted-contour auxiliary-field Monte Carlo (SCAFMC) approach has been recently developed by Rom, Charutz and Neuhauser as an extension of the auxiliary-field Monte Carlo (AFMC) method. AFMC replaces the difficult fully interacting electrons problem by an ensemble of simpler problems where the electrons interact with a fluctuating electric field but not with each other. SCAFMC is based on shifting the auxiliary-field contour of integration to pass through the (imaginary) stationary point, leading to numerical stability at long propagation times. The new approach converges to the full CI energy in electronic structure calculations (both ground and low-lying excited states). Here we expand the application of SCAFMC from atomic to molecular problems. First, we calculate ground-state energies of a highly correlated transition-metal system (Cr2) with a moderate (12 orbitals) active space size, and demonstrate that SCAFMC is able to extract the energies accurately. In addition, we use SCAFMC to calculate a ...

We apply two hybrid methods for solving scattering problems affected by resonances, to a four-dimensional reactive surface scattering system. In each method the solution of the problem is divided into two parts: a wave packet propagation, and a resonance calculation; results of the resonance calculation are used to extrapolate the long-time behavior of the system. In the first hybrid method, the propagation is by the multistep Chebyshev method, with calculation of resonances performed by the Lanczos method. In the second, the propagation is done using an implementation of the absorbing boundary condition (ABC) evolution operator, and the resonance calculation by filter diagonalization (FDG). Each method produces accurate scattering results in much less computation time than standard long-time wave packet propagation. The Chebyshev–Lanczos approach proves most capable for the calculation of resonances, but is computationally expensive. The ABC–FDG method is much cheaper to implement, but could not be made to extract accurate data for certain broad, overlapping resonances. This was overcome by propagating longer (still much shorter than for long-time propagation) to allow the elusive resonances time to decay.

Several years ago we introduced a new approach for signal extraction, Filter-Diagonalization, which is capable of handling signals containing millions of points or more. The method was recently extended to handle multidimensional signals, as appear in NMR, by us and separately by Mandelshtam and Taylor. We further extend the original formulation by showing that the method is capable of handling degenerate eigenvalues and eigenstates through the use of a simultaneous diagonalization algorithm. We apply the method to simple two-dimensional NMR signals. Comparisons to a standard FFT treatment show that FDG recovers the salient spectral features using a quarter of the signal required by FFT.

A simple method that incorporates coupling between bath and system and between individual bath modes is proposed. In the new locally propagating gaussian (LPG) approximation, the bath is modeled by Gaussians at each system site that propagate in time. Each site couples to others by the overlap of their Gaussians. In a test case with weak but non-negligible coupling, LPG is nearly as rapid as TDSCF but more closely follows exact calculations. LPG can be extended to larger simulations. Strongly coupled system coordinates will be treated with a grid-type calculation and weakly coupled bath modes will be handled by LPG.

A procedure to control harmonic generation is proposed using initial state control. We show that an initial molecular state can be prepared before shining a strong laser pulse to maximize the output harmonic generation. We demonstrate the method by maximizing emission for the sixth harmonic on a given initial IR pulse in a model H 2 + system using five initial vibrational states.

Filter-diagonalization [M. R. Wall and D. Neuhauser, J. Chem. Phys. 102, 8011 (1995)] is a new method for extracting frequencies and damping constants from a short-time segment of any time-dependent signal, whether of quantum origin or not. The method is efficient and able to handle signals with, e.g., millions of (possibly overlapping) frequencies, since it concentrates on specific spectral ranges. The method was shown to be a powerful tool for extracting eigenstates and normal-modes, and for reducing propagation times, in several recent works by us, by Mandelshtam and Taylor (who recently introduced the box filter) and by other groups. Here we extend the method in several directions: first, we show how it can be used with a filter of any form. Next, we show how the methodology may be extended to treat multi-dimensional signals, of the type that appears, e.g., in 2-D nuclear magnetic resonance (NMR). Finally, we exemplify the performance of the various filters for two types of signals where the time-redu...