Volume 135, Issue 5, 07 August 2011

Graphene is a single sheet of carbon atoms that constitutes the basic building block of macroscopic graphite crystals. Held together by a backbone of overlapping sp^{2} hybrids, graphene's 2p orbitals form π state bands that delocalize over an entire 2dimensional macroscopic carbon sheet leading to a number of unusual characteristics that include large electrical and thermal conductivities. Recent discoveries have provided simple methods (e.g., mechanical cleavage of graphite) for preparing laboratory scale samples that can be used to investigate the fundamental physical and chemical characteristics of graphene. In addition, a number of techniques have emerged that show promise for producing largescale samples with the ultimate goal of developing devices that take advantage of graphene's unusual properties. As large samples become available, the possibility grows for applications of this material in solar cell technology (as flexible, transparent electrodes), in composite material development, and in electronic devices.
 PERSPECTIVES


Perspective: The dawning of the age of graphene
View Description Hide DescriptionGraphene is a single sheet of carbon atoms that constitutes the basic building block of macroscopic graphite crystals. Held together by a backbone of overlapping sp^{2} hybrids, graphene's 2p orbitals form π state bands that delocalize over an entire 2dimensional macroscopic carbon sheet leading to a number of unusual characteristics that include large electrical and thermal conductivities. Recent discoveries have provided simple methods (e.g., mechanical cleavage of graphite) for preparing laboratory scale samples that can be used to investigate the fundamental physical and chemical characteristics of graphene. In addition, a number of techniques have emerged that show promise for producing largescale samples with the ultimate goal of developing devices that take advantage of graphene's unusual properties. As large samples become available, the possibility grows for applications of this material in solar cell technology (as flexible, transparent electrodes), in composite material development, and in electronic devices.
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 COMMUNICATIONS


Communication: Effective temperature and glassy dynamics of active matter
View Description Hide DescriptionA systematic expansion of the manybody master equation for active matter, in which motors power configurational changes as in the cytoskeleton, is shown to yield a description of the steady state and responses in terms of an effective temperature. The effective temperature depends on the susceptibility of the motors and a Peclet number which measures their strength relative to thermal Brownian diffusion. The analytic prediction is shown to agree with previous numerical simulations and experiments. The mapping also establishes a description of aging in active matter that is also kinetically jammed.
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 ARTICLES

 Theoretical Methods and Algorithms

A new internally contracted multireference configuration interaction method
View Description Hide DescriptionWe present a new internally contracted multireference configuration interaction (MRCI) method which, at the same time, efficiently handles large active orbital spaces, long configuration expansions, and many closedshell orbitals in the reference function. This is achieved by treating the closedshell orbitals explicitly, so that all required coupling coefficients and density matrices only depend on active orbital labels. As a result, closedshell orbitals are handled as efficiently as in a closedshell singlereference program, and this opens up the possibility to perform highaccuracy MRCI calculations for much larger molecules than before. The enormously complex equations are derived using a new domainspecific computer algebra system and semiautomatically implemented using a newly developed integrated tensor framework. The accuracy and efficiency of the MRCI method is demonstrated with applications to dioxygencopper complexes with different ligands, some of which involve more than 30 atoms, and to spinstate splittings of ferrocene.

Soret motion in nonionic binary molecular mixtures
View Description Hide DescriptionWe study the Soret coefficient of binary molecular mixtures with dispersion forces. Relying on standard transport theory for liquids, we derive explicit expressions for the thermophoretic mobility and the Soret coefficient. Their sign depends on composition, the size ratio of the two species, and the ratio of Hamaker constants. Our results account for several features observed in experiment, such as a linear variation with the composition; they confirm the general rule that small molecules migrate to the warm, and large ones to the cold.

Quantum optimal control for the full ensemble of randomly oriented molecules having different fieldfree Hamiltonians
View Description Hide DescriptionWe have presented the optimal control theory formulation to calculate optimal fields that can control the full ensemble of randomly oriented molecules having different fieldfree Hamiltonians. The theory is applied to the fiftyfifty mixture of randomly oriented ^{133}CsI and ^{135}CsI isotopomers and an optimal field is sought to achieve isotopeselective vibrational excitations with high efficiency. Rotational motion is frozen and two total times (T’s) of electric field duration, 460 000 and 920 000 a.u. (11.1 and 22.2 ps), are chosen in the present calculation. As a result, the final yields for T = 460 000 and 920 000 a.u. are calculated to be 0.706 and 0.815, respectively. The relatively high final yield obtained for T = 920 000 a.u. strongly suggests that a single laser pulse can control the full ensemble of randomly oriented nonidentical molecules. The result is quite encouraging in terms of the application to isotopeseparation processes.

Inherent structures for soft longrange interactions in twodimensional manyparticle systems
View Description Hide DescriptionWe generate inherent structures, local potentialenergy minima, of the “kspace overlap potential” in twodimensional manyparticle systems using a cooling and quenching simulation technique. The ground states associated with the kspace overlap potential are stealthy (i.e., completely suppress single scattering of radiation for a range of wavelengths) and hyperuniform (i.e., infinite wavelength density fluctuations vanish). However, we show via quantitative metrics that the inherent structures exhibit a range of stealthiness and hyperuniformity depending on the fraction of degrees of freedom χ that are constrained. Inherent structures in two dimensions typically contain fiveparticle rings, wavy grain boundaries, and vacancyinterstitial defects. The structural and thermodynamic properties of the inherent structures are relatively insensitive to the temperature from which they are sampled, signifying that the energy landscape is relatively flat along the directions sampled, with wide shallow local minima and devoid of deep wells. Using the nudgedelasticband algorithm, we construct paths from groundstate configurations to inherent structures and identify the transition points between them. In addition, we use point patterns generated from a random sequential addition (RSA) of hard disks, which are nearly stealthy, and examine the particle rearrangements necessary to make the configurations absolutely stealthy. We introduce a configurational proximity metric to show that only small local, but collective, particle rearrangements are needed to drive initial RSA configurations to stealthy disordered ground states. These results lead to a more complete understanding of the unusual behaviors exhibited by the family of “collectivecoordinate” potentials to which the kspace overlap potential belongs.

Timedependent density functional theory excited state nonadiabatic dynamics combined with quantum mechanical/molecular mechanical approach: Photodynamics of indole in water
View Description Hide DescriptionWe present a combination of timedependent density functional theory with the quantum mechanical/molecular mechanical approach which can be applied to study nonadiabatic dynamical processes in molecular systems interacting with the environment. Our method is illustrated on the example of ultrafast excited statedynamics of indole in water. We compare the mechanisms of nonradiative relaxation and the electronic state lifetimes for isolated indole, indole in a sphere of classical water, and indole + 3H_{2}O embedded in a classical water sphere. In the case of isolated indole, the initial excitation to the S_{2} electronic state is followed by an ultrafast internal conversion to the S_{1} state with a time constant of 17 fs. The S_{1} state is long living (30 ps) and deactivates to the ground state along the N–H stretching coordinate. This deactivation mechanism remains unchanged for indole in a classical water sphere. However, the lifetimes of the S_{2} and S_{1} electronic states are extended. The inclusion of three explicit water molecules opens a new relaxation channel which involves the electron transfer to the solvent, leading eventually to the formation of a solvated electron. The relaxation to the ground state takes place on a time scale of 60 fs and contributes to the lowering of the fluorescence quantum yield. Our simulations demonstrate the importance of including explicit water molecules in the theoretical treatment of solvated systems.

Alternative wavefunction ansatz for including explicit electronproton correlation in the nuclearelectronic orbital approach
View Description Hide DescriptionThe nuclearelectronic orbital (NEO) approach treats specified nuclei quantum mechanically on the same level as the electrons with molecular orbital techniques. The explicitly correlated HartreeFock (NEOXCHF) approach was developed to incorporate electronnucleus dynamical correlation directly into the variational optimization of the nuclearelectronic wavefunction. In the original version of this approach, the HartreeFock wavefunction is multiplied by , where is a geminal operator expressed as a sum of Gaussian type geminal functions that depend on the electronproton distance. Herein, a new wavefunctionansatz is proposed to avoid the computation of five and sixparticle integrals and to simplify the computation of the lower dimensional integrals involving the geminal functions. In the new ansatz, denoted NEOXCHF2, the HartreeFock wavefunction is multiplied by rather than . Although the NEOXCHF2 ansatz eliminates the integrals that are quadratic in the geminal functions, it introduces terms in the kinetic energy integrals with no known analytical solution. A truncated expansion scheme is devised to approximate these problematic terms. An alternative hybrid approach, in which the kinetic energy terms are calculated with the original NEOXCHF ansatz and the potential energy terms are calculated with the NEOXCHF2 ansatz, is also implemented. Applications to a series of model systems with up to four electrons provide validation for the NEOXCHF2 approach and the treatments of the kinetic energy terms.

Rigorous coarsegraining for the dynamics of linear systems with applications to relaxation dynamics in proteins
View Description Hide DescriptionReduceddimensionality, coarsegrained models are commonly employed to describe the structure and dynamics of large molecular systems. In those models, the dynamics is often described by Langevin equations of motion with phenomenological parameters. This paper presents a rigorous coarsegraining method for the dynamics of linear systems. In this method, as usual, the conformational space of the original atomistic system is divided into master and slave degrees of freedom. Under the assumption that the characteristic timescales of the masters are slower than those of the slaves, the method results in Langevintype equations of motion governed by an effective potential of mean force. In addition, coarsegraining introduces hydrodynamiclike coupling among the masters as well as nontrivial inertial effects. Application of our method to the longtimescale part of the relaxation spectra of proteins shows that such dynamic coupling is essential for reproducing their relaxation rates and modes.

Ab initio quantum Monte Carlo study of the binding of a positron to alkalimetal hydrides
View Description Hide DescriptionQuantum Monte Carlo methods are used to investigate the binding of a positron to the alkalimetal hydrides, XH (X = Na and K). We obtain positron affinities for the NaH and KH molecules of 1.422(10) eV and 2.051(39) eV, respectively. These are considerably larger than the previous results of 1.035 eV and 1.273 eV obtained from multireference single and doubleexcitation configuration interaction calculations. Together with our previous results for [LiH;e^{+}] [Y. Kita et al., J. Chem. Phys.131, 134310 (2009)], our study confirms the strong correlation between the positron affinity and dipole moment of alkalimetal hydrides.

Fractional spin in reduced densitymatrix functional theory
View Description Hide DescriptionWe study the behavior of different functionals of the onebody reduced density matrix (1RDM) for systems with fractional zcomponent of the total spin. We define these systems as ensembles of integer spin states. It is shown that, similarly to density functional theory, the error in the dissociation of diatomic molecules is directly related to the deviation from constancy of the atomic total energies as functions of the fractional spin. However, several functionals of the 1RDM show a size inconsistency which leads to additional errors. We also investigate the difference between a direct evaluation of the energy of an ensemble of integerspin systems and a direct minimization of the energy of a fractionalspin system.
 Advanced Experimental Techniques

Nuclear magnetic relaxation in water revisited
View Description Hide DescriptionIn this study, we revisited nuclear magnetic relaxation of ^{1}H in water at very low Larmor frequencies that has been studied intensively in earlier years. We make use of the recently developed superconducting quantum interference device based ultralow field NMR technique, which enables much easier access to the longitudinal spinlattice relaxation timeT _{1} and the transversal spinspin relaxation timeT _{2} below several kHz than traditional field cycling methods. Our data reproduce and complement the earlier results, in that they corroborate the finding of an exchange process with a correlation time of about 0.34 ms at room temperature which can be attributed to the migration of hydronium and hydroxyl ions in neutral water via hydrogen bridges. The corresponding relaxation process is driven by the interaction of the protons with ^{17}O and contributes to the T _{1} and the T _{2} relaxation rate by about 0.12 s^{−1}. In addition, we found evidence of a very slow exchange process at about 100 Hz that has hitherto not been reported.
 Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Leastsquares analysis of overlapped boundfree absorption spectra and predissociation data in diatomics: The C(^{1}Π_{ u }) state of I_{2}
View Description Hide DescriptionAbsorption spectra are recorded at low resolution but high quantitative precision for I_{2} vapor at 35 °C and 64 °C. These and literature spectra are analyzed by leastsquares quantum spectral simulation of the overlapped A ← X, B ← X, and C(^{1}Π_{ u }) ← X transitions, with the aid of a pseudocontinuum model for the discrete regions of the A ← X and B ← Xspectra. The analysis yields improved descriptions of the smallR regions of the A and Bstate potentials, which are known precisely at larger R from discrete spectroscopy. The C potential is determined at small R from its C ← Xabsorption, at intermediate R from literature data for B → Cpredissociation, and at large R from its known van der Waals well. The estimates of the electronic transition moment function μ _{ e }(R) for the B–X transition expand upon precise results from a recent determination by a different method. For the C–X and A–X transitions, the Rdependence of the transition moment functions resembles that found previously for these systems in Br_{2}. Of the spectroscopic properties, the C ← Xspectrum is most altered from the previous analysis, being now ∼20% weaker. For B → Cpredissociation, no derived C potential has yielded computed rates in adequate statistical agreement with the analyzed experimental data.

Collision limited reaction rates for arbitrarily shaped particles across the entire diffusive Knudsen number range
View Description Hide DescriptionAerosol particle reactions with vapor molecules and molecular clusters are often collision rate limited, hence determination of particlevapor molecule and particlemolecular cluster collision rates are of fundamental importance. These collisions typically occur in the mass transfer transition regime, wherein the collision kernel (collision rate coefficient) is dependent upon the diffusive Knudsen number, Kn D . While this alone prohibits analytical determination of the collision kernel, aerosol particle vapor molecule collisions are further complicated when particles are nonspherical, as is often the case for particles formed in high temperature processes (combustion). Recently, through a combination of mean first passage time simulations and dimensional analysis, it was shown that the collision kernel for spherical particles and vapor molecules could be expressed as a dimensionless number, H, which is solely a function of Kn D . In this work, it is shown through similar mean first passage times and redefinitions of H and Kn D that the H(Kn D ) relationship found for spherical particles applies for particles of arbitrary shape, including commonly encountered agglomerate particles. Specifically, it is shown that to appropriately define H and Kn D , two geometric descriptors for a particle are necessary: its Smoluchowski radius, which defines the collision kernel in the continuum regime (Kn D →0) and its orientationally averaged projected area, which defines the collision kernel in the free molecular regime (Kn D →∞). With these two parameters, as well as the properties of the colliding vapor molecule (mass and diffusion coefficient), the particlevapor molecule collision kernel in the continuum, transition, and free molecular regimes can be simply calculated using the H(Kn D ) relationship.

Fulldimensional multi configuration time dependent Hartree calculations of the ground and vibrationally excited states of He_{2,3}Br_{2} clusters
View Description Hide DescriptionQuantum dynamics calculations are reported for the tetra, and pentaatomic van der Waals He_{N}Br_{2} complexes using the multiconfiguration timedependent Hartree (MCTDH) method. The computations are carried out in satellite coordinates, and the kinetic energy operator in this set of coordinates is given. A scheme for the representation of the potential energy surface based on the sum of the threebody HeBr_{2} interactions at CSSD(T) level plus the HeHe interaction is employed. The potential surfaces show multiple close lying minima, and a quantum description of such highly floppy multiminima systems is presented. Benchmark, fulldimensional converged results on ground vibrational/zeropoint energies are reported and compared with recent experimental data available for all these complexes, as well as with previous variational quantum calculations for the smaller HeBr_{2} and He_{2}Br_{2} complexes on the same surface. Some lowlying vibrationally excited eigenstates are also computed by block improved relaxation calculations. The binding energies and the corresponding vibrationally averaged structures are determined for different conformers of these complexes. Their relative stability is discussed, and contributes to evaluate the importance of the multipleminima topology of the underlying potential surface.

Revisiting falloff curves of thermal unimolecular reactions
View Description Hide DescriptionMaster equations for thermal unimolecular reactions and the reverse thermal recombination reactions are solved for a series of modelreaction systems and evaluated with respect to broadening factors. It is shown that weak collision center broadening factors can approximately be related to the collision efficiencies β _{ c } through a relation ≈ max {β _{ c } ^{0.14}, 0.64(±0.03)}. In addition, it is investigated to what extent weak collision falloff curves in general can be expressed by the limiting low and high pressure rate coefficients together with central broadening factors F _{ cent } only. It is shown that there cannot be one “best” analytical expression for broadening factors F(x) as a function of the reduced pressure scale x = k _{0}/k _{ ∞ }. Instead, modelled falloff curves of various reaction systems, for given k _{0}, k _{ ∞ }, and F _{ cent }, fall into a band of about 10% width in F(x). A series of analytical expressions for F(x), from simple symmetric to more elaborate asymmetric broadening factors, are compared and shown to reproduce the band of modelled broadening factors with satisfactory accuracy.

Geometrical structure of benzene and naphthalene: Ultrahighresolution laser spectroscopy and ab initio calculation
View Description Hide DescriptionGeometrical structures of the isolated benzene and naphthalene molecules have been accurately determined by using ultrahighresolution laser spectroscopy and ab initio calculation in a complementary manner. The benzene molecule has been identified to be planar and hexagonal (D _{6h }) and the structure has been determined with accuracies of 2 × 10^{−14} m (0.2 mÅ; 1 Å = 1 × 10^{−10} m) for the C–C bond length and 1.0 × 10^{−13} m (1.0 mÅ) for the C–H bond length. The naphthalene molecule has been identified to be symmetric with respect to three coordinate axes (D _{2h }) and the structure has been determined with comparable accuracies. We discuss the effect of vibrational averaging that is a consequence of zeropoint motions on the uncertainty in determining the bond lengths.

Pressure and temperature dependence of dissociative and nondissociative electron attachment to CF_{3}: Experiments and kinetic modeling
View Description Hide DescriptionThe kinetics of electron attachment to CF_{3} as a function of temperature (300–600 K) and pressure (0.75–2.5 Torr) were studied by variable electron and neutral density attachment mass spectrometry exploiting dissociative electron attachment to CF_{3}Br as a radical source. Attachment occurs through competing dissociative (CF_{3} + e^{−} → CF_{2} + F^{−}) and nondissociative channels (CF_{3} + e^{−} → CF_{3} ^{−}). The rate constant of the dissociative channel increases strongly with temperature, while that of the nondissociative channel decreases. The rate constant of the nondissociative channel increases strongly with pressure, while that of the dissociative channel shows little dependence. The total rate constant of electron attachment increases with temperature and with pressure. The system is analyzed by kinetic modeling in terms of statistical theory in order to understand its properties and to extrapolate to conditions beyond those accessible in the experiment.

Experimental and theoretical studies of neutral Mg_{ m }C_{ n }H_{ x } and Be_{ m }C_{ n }H_{ x } clusters
View Description Hide DescriptionNeutral Mg_{ m }C_{ n }H_{ x } and Be_{ m }C_{ n }H_{ x }clusters are investigated both experimentally and theoretically for the first time. Single photon ionization at 193 nm is used to detect neutral cluster distributions through time of flight mass spectrometry. Mg_{ m }C_{ n }H_{ x } and Be_{ m }C_{ n }H_{ x }clusters are generated through laser ablation of Mg or Be foil into CH_{4}/He expansion gas. A number of members of each cluster series are identified through isotopic substitution experiments employing ^{13}CH_{4} and CD_{4} instead of CH_{4} in the expansion gas. An oscillation of the vertical ionization energies (VIEs) of Mg_{ m }C_{ n }H_{ x }clusters is observed in the experiments. The VIEs of Mg_{ m }C_{ n }H_{ x }clusters are observed to vary as a function of the number of H atoms in the clusters.Density functional theory(DFT) and ab initio (MP2) calculations are carried out to explore the structures and ionization energies of Mg_{ m }C_{ n }H_{ x }clusters. Many Be_{ m }C_{ n }H_{ x }clusters are also generated and detected in the experiments. The structures and VIEs of Be_{ m }C_{ n }H_{ x }clusters are also studied by theoretical calculations. Calculational results provide a good and consistent explanation for the experimental observations, and are in general agreement with them for both series of clusters.

Ultrafast dynamics of aniline in the 294234 nm excitation range: The role of the πσ* state
View Description Hide DescriptionThe ultrafast relaxation of jetcooled aniline was followed by timeresolved ionization, after excitation in the 294234 interval. The studied range of energy covers the absorption of the two bright ππ ^{*} excitations, S_{1} and S_{3}, and the almost dark S_{2} (πσ ^{*}) state. The employed probe wavelengths permit to identify different ultrafast time constants related with the coupling of the involved electronic surfaces. A τ _{1} = 165 ± 30 fs lifetime is attributed to dynamics along the S_{2} (πσ ^{*}) repulsive surface. Other relaxation channels as the S_{1}→S_{0} and S_{3}→S_{1} internal conversion are also identified and characterized. The work provides a general view of the photophysics of aniline, particularly regarding the role of the πσ ^{*} state. This state appears as minor dissipation process due to the ineffective coupling with the bright S_{1} and S_{3} states, being the S_{1}→S_{0} internal conversion the main nonradiative process in the full studied energy range. Additionally, the influence of the offresonance adiabatic excitation of higher energy electronic states, particularly S_{3}, is also observed and discussed.