Volume 130, Issue 24, 28 June 2009

The presence of a chiral surface can alter the characteristics of nearby solvent molecules such that, on average, these molecules become chiral. The extent of this induced chirality and its dependence on the surface and solvent characteristics are explored in this article. Three surfaces employed in chiral chromatography are examined: The WhelkO1 interface, a phenylglycinederived chiral stationary phase (CSP), and a leucinederived CSP. All three interfaces are “brush type” in that the chiral molecules are attached to the underlying substrate via an achiral tether. The solvents consist of ethanol, a binary hexane/ethanol solvent, 2propanol, and a binary hexane/2propanol solvent.Molecular dynamics simulations of the solvated chiral interfaces form the basis of the analysis. The chirality induced in the solvent is assessed based on a chirality index originally proposed by Osipov et al. [Mol. Phys.84, 1193 (1995)]. Solventchirality will depend on the solvent position relative to the surface. For this reason, a positiondependent chirality index is analyzed in detail.
 COMMUNICATIONS


Local explicitly correlated coupledcluster methods: Efficient removal of the basis set incompleteness and domain errors
View Description Hide DescriptionWe propose an explicitly correlated local LCCSDF12 method in which the basis set incompleteness error as well as the error caused by truncating the virtual orbital space to pairspecific local domains are strongly reduced. This is made possible by including explicitly correlated terms that are orthogonalized only to the pairspecific configuration space. Thus, the contributions of excitations outside the domains are implicitly accounted for by the explicitly correlated terms. It is demonstrated for a set of 54 reactions that the reaction energies computed with the new LCCSDF12 method and triplezeta basis sets deviate by at most 2.5 kJ/mol from conventional CCSD complete basis set results (RMS: 0.6 kJ/mol). The local approximations should make it possible to achieve linear scaling of the computational cost with molecular size.
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 ARTICLES

 Theoretical Methods and Algorithms

Quantum mechanics with the basis set guided by Ehrenfest trajectories: Theory and application to spinboson model
View Description Hide DescriptionIn this article a method of numerical solution of the Schrödinger equation is proposed. The approach corrects the Ehrenfest approximation by using several trajectories/configurations with their amplitudes coupled within and across configurations, thus making the method formally exact. Accurate results are obtained for the spinboson model with up to 2000 bath modes treated on fully quantum level without approximations.

Relativistic, QED, and nuclear mass effects in the magnetic shielding of
View Description Hide DescriptionThe magnetic shielding of is studied. The complete relativistic corrections of order , leading QED corrections of order , and finite nuclear masseffects of order are calculated with high numerical precision. The resulting theoretical predictions for are the most accurate to date among all elements and support the use of as a NMR standard.

Variable Lieb–Oxford bound satisfaction in a generalized gradient exchangecorrelation functional
View Description Hide DescriptionWe propose a different way to satisfy both gradient expansion limiting behavior and the Lieb–Oxford bound in a generalized gradient approximation exchange functional by extension of the Perdew–Burke–Ernzerhof (PBE) form. Motivation includes early and recent exploration of modified values for the gradient expansion coefficient in the PBE exchangecorrelation functional (cf. the PBEsol functional) and earlier experience with a numerical cutoff for large in a version of the deMon molecular code. For either the original PBE or the PBEsol choice of the gradient coefficient, we find improved performance from using an dependent (spatially varying) satisfaction of the Lieb–Oxford bound which quenches to uniform electron gas behavior at large . The mean absolute deviations (MADs) in atomization energies for a widely used test set of 20 small molecules are reduced by about 22% relative to PBE and PBEsol. For these small molecules, the bond length MADs are essentially unchanged.

A new class of highly efficient exact stochastic simulation algorithms for chemical reaction networks
View Description Hide DescriptionWe introduce an alternative formulation of the exact stochastic simulation algorithm (SSA) for sampling trajectories of the chemical master equation for a wellstirred system of coupled chemical reactions. Our formulation is based on factoredout, partial reaction propensities. This novel exact SSA, called the partialpropensity direct method (PDM), is highly efficient and has a computational cost that scales at most linearly with the number of chemical species, irrespective of the degree of coupling of the reaction network. In addition, we propose a sorting variant, SPDM, which is especially efficient for multiscale reaction networks.

Relationships between the thirdorder reactivity indicators in chemical densityfunctional theory
View Description Hide DescriptionRelationships between thirdorder reactivity indicators in the closed system [, ], open system [, ], and density pictures are derived. Our method of derivation unifies and extends known results. Among the relationships is a link between the thirdorder response of the energy to changes in the density and the quadratic response of the density to changes in external potential. This provides a link between hyperpolarizability and the system’s sensitivity to changes in electron density. The dual descriptor is a unifying feature of many of the formulas we derive.

From power law intermittence to macroscopic coherent regime
View Description Hide DescriptionWe address the problem of establishing which is the proper form of quantum master equation generating a survival probability identical to that corresponding to the nonergodic sequence of “light on” and “light off” fluorescence fluctuations in blinking quantum dots. We adopt a theoretical perspective based on the assumption that the abrupt transitions from the light on to light off state are the results of many collisions between system and environment, properly described by the Lindblad equation, and that between two consecutive collisions the system dynamics are frozen. This generates a quantum master equation belonging to the recently proposed class of generalized Lindblad equations, with a time convoluted structure, involving in the specific case of this paper both the unitary and the nonunitary contribution of the Lindblad equation. This is the property that under the lowfrequency condition makes the new class of generalized Lindblad equation generates the required survival probability. We make the conjecture that this equation corresponds to the cooperative dynamics of many units that, in isolation, are described by the ordinary Lindblad equation. When the time scale of the unitary term of the Lindblad equation is shorter than the dephasing time, the cooperation generates a surprisingly extended macroscopic coherence.

Decoherence and dissipation in a molecular system coupled to an environment: An application of semiclassical hybrid dynamics
View Description Hide DescriptionApplying the recently developed semiclassical hybrid dynamics [Grossmann, J. Chem. Phys.125, 014111 (2006)], we study the decay of interference patterns in the reduced density as well as of the purity in a Morse oscillator test system due to the interaction with a finite harmonic bath at zero temperature. In the case that the bath mimics a continuous Ohmic spectral density, in addition to the quantum classical transition induced by the interaction with the environment, we corroborate the existence of a blueshift due to the bath coupling, predicted by Pollak [Phys. Rev. A33, 4244 (1986)]. Furthermore, the decoherence dynamics of cat states is confirmed to be faster than that of single coherent states and we show that for a resonant bath the dissipation leads to an increase in the decoherence rate as compared to the low frequency bath.

Transition statefinding strategies for use with the growing string method
View Description Hide DescriptionEfficient identification of transition states is important for understanding reaction mechanisms. Most transition state search algorithms require long computational times and a good estimate of the transition state structure in order to converge, particularly for complex reaction systems. The growing string method (GSM) [B. Peters et al., J. Chem. Phys.120, 7877 (2004)] does not require an initial guess of the transition state; however, the calculation is still computationally intensive due to repeated calls to the quantum mechanics code. Recent modifications to the GSM [A. Goodrow et al., J. Chem. Phys.129, 174109 (2008)] have reduced the total computational time for converging to a transition state by a factor of 2 to 3. In this work, three transition statefinding strategies have been developed to complement the speedup of the modifiedGSM: (1) a hybrid strategy, (2) an energyweighted strategy, and (3) a substring strategy. The hybrid strategy initiates the string calculation at a low level of theory (HF/STO3G), which is then refined at a higher level of theory. The energyweighted strategy spaces points along the reaction pathway based on the energy at those points, leading to a higher density of points where the energy is highest and finer resolution of the transition state. The substring strategy is similar to the hybrid strategy, but only a portion of the lowlevel string is refined using a higher level of theory. These three strategies have been used with the modifiedGSM and are compared in three reactions: alanine dipeptideisomerization, Habstraction in methanol oxidation on catalysts, and C–H bond activation in the oxidative carbonylation of toluene to toluic acid on catalysts. In each of these examples, the substring strategy was proved most effective by obtaining a better estimate of the transition state structure and reducing the total computational time by a factor of 2 to 3 compared to the modifiedGSM. The applicability of the substring strategy has been extended to three additional examples: cyclopropane rearrangement to propylene, isomerization of methylcyclopropane to four different stereoisomers, and the bimolecular Diels–Alder condensation of 1,3butadiene and ethylene to cyclohexene. Thus, the substring strategy used in combination with the modifiedGSM has been demonstrated to be an efficient transition statefinding strategy for a wide range of types of reactions.

Phase equilibria of molecular fluids via hybrid Monte Carlo Wang–Landau simulations: Applications to benzene and alkanes
View Description Hide DescriptionIn recent years, powerful and accurate methods, based on a Wang–Landau sampling, have been developed to determine phase equilibria. However, while these methods have been extensively applied to study the phase behavior of model fluids, they have yet to be applied to molecular systems. In this work, we show how, by combining hybrid Monte Carlo simulations in the isothermalisobaric ensemble with the Wang–Landau sampling method, we determine the vaporliquid equilibria of various molecular fluids. More specifically, we present results obtained on rigid molecules, such as benzene, as well as on flexible chains of alkanes. The reliability of the method introduced in this work is assessed by demonstrating that our results are in excellent agreement with the results obtained in previous work on simple fluids, using either transition matrix or conventional Monte Carlo simulations with a Wang–Landau sampling, and on molecular fluids, using histogram reweighting or Gibbs ensemble Monte Carlo simulations.

Electronic structure and molecular dynamics of breaking the RO–NO_{2} bond
View Description Hide DescriptionDecomposition of energetic molecules such as pentaerythritol tetranitrate is accompanied by extensive changes in their electronic configuration and thus is challenging for ab initio Born–Oppenheimer molecular dynamics simulations. The performance of singledeterminant methods (in particular, densityfunctional theory) is validated on electronic structure and molecular dynamics simulations of bonddissociation in a smaller nitric ester, ethyl nitrate. Accurate description of dissociating molecule requires using unrestricted, spinsymmetrybroken orbitals. However, the iterative selfconsistent field procedure is prone to convergence failures in the bondbreaking region even if robust convergence algorithms are employed. As a result, molecular dynamics simulations of unimolecular decomposition need to be closely monitored and manually restarted to ensure seamless transition from the closedshell to openshell configuration.

Analytic dynamics of the Morse oscillator derived by semiclassical closures
View Description Hide DescriptionThe quantized Hamilton dynamics methodology [O. V. Prezhdo and Y. V. Pereverzev, J. Chem. Phys.113, 6557 (2000)] is applied to the dynamics of the Morse potential using the SU(2) ladder operators. A number of closed analytic approximations are derived in the Heisenberg representation by performing semiclassical closures and using both exact and approximate correspondence between the ladder and positionmomentum variables. In particular, analytic solutions are given for the exact classical dynamics of the Morse potential as well as a secondorder semiclassical approximation to the quantum dynamics. The analytic approximations are illustrated with the O–H stretch of water and a Xe–Xe dimer. The results are extended further to coupled Morse oscillators representing a linear triatomic molecule. The reported analytic expressions can be used to accelerate classical molecular dynamics simulations of systems containing Morse interactions and to capture quantummechanical effects.
 Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Threshold photoionization and density functional theory studies of bimetalliccarbide nanocrystals and fragments:
View Description Hide DescriptionGasphase bimetallic tantalumzirconiumcarbide clusters are generated using a constructed double ablation cluster source. The clusters are examined by photoionization efficiency spectroscopy to extract experimental ionization energies (IEs). The IE trend for the cluster series is reasonably similar to that of the cluster series [V. Dryza et al., J. Phys. Chem. A109, 11180 (2005)], although the IE reductions upon carbon addition are greater for the former. Complementary density functional theory calculations are performed for the various isomers constructed by attaching carbon atoms to the different faces of the tetrahedral cluster. The good agreement between the experimental IE trend and that calculated for these isomers support a face centered cubic nanocrystal structure for and nanocrystal fragment structures for the smaller clusters.

The dynamics of the reaction on an interpolated potential energy surface
View Description Hide DescriptionA potential energy surface that describes the title reaction has been constructed by interpolation of ab initio data. Classical trajectory studies on this surface show that the total reaction rate is close to that predicted by a Langevin model, although the mechanism is more complicated than simple ionmolecule capture. Only the product is observed classically. An estimate of the magnitude of rotational inelastic scattering is also reported.

On the accuracy of thermionic electron emission models. I. Electron detachment from
View Description Hide DescriptionDetailed statistical rate calculations combined with electron capturetheory and kinetic modeling for the electron attachment to and detachment from [Troe et al., J. Chem. Phys.127, 244303 (2007)] are used to test thermionic electron emission models. A new method to calculate the specific detachment rate constants and the electron energy distributions as functions of the total energy of the anion and the energy of the emitted electrons is presented, which is computationally simple but neglects fine structures in the detailed . Reduced electron energy distributions were found to be of the form with , whose shape corresponds to thermal distributions only to a limited extent. In contrast, the average energies can be roughly estimated within thermionic emission and finite heat bath concepts. An effective temperature is determined from the relation , where denotes the thermal internal energy of the detachment product at the temperature and EA is the electron affinity of . The average electron energy is then approximately given by , but dynamical details of the process are not accounted for by this approach. Simplified representations of in terms of from the literature are shown to lead to only semiquantitative agreement with the equally simple but more accurate calculations presented here. An effective “isokinetic” electron emission temperature does not appear to be useful for the electron detachment system considered because it neither provides advantages over a representation of as a function of , nor are recommended relations between and of sufficient accuracy.

Correction for dispersion and Coulombic interactions in molecular clusters with density functional derived methods: Application to polycyclic aromatic hydrocarbon clusters
View Description Hide DescriptionThe density functional based tight binding (DFTB) is a semiempirical method derived from the density functional theory(DFT). It inherits therefore its problems in treating van der Waals clusters. A major error comes from dispersion forces, which are poorly described by commonly used DFT functionals, but which can be accounted for by an a posteriori treatment DFTD. This correction is used for DFTB. The selfconsistent charge (SCC) DFTB is built on Mulliken charges which are known to give a poor representation of Coulombic intermolecular potential. We propose to calculate this potential using the class IV/charge model 3 definition of atomic charges. The selfconsistent calculation of these charges is introduced in the SCC procedure and corresponding nuclear forces are derived. Benzene dimer is then studied as a benchmark system with this corrected DFTB (cDFTBD) method, but also, for comparison, with the DFTD. Both methods give similar results and are in agreement with references calculations (CCSD(T) and symmetry adapted perturbation theory) calculations. As a first application, pyrene dimer is studied with the cDFTBD and DFTD methods. For coronene clusters, only the cDFTBD approach is used, which finds the sandwich configurations to be more stable than the Tshaped ones.

Ab initio study of methylbromide photodissociation in the band
View Description Hide DescriptionWe performed a theoretical study of the photodissociation dynamics of in the band using a wave packet propagation technique on coupled ab initio potential energy curves. The present model involves the and excited states which can be populated from the ground state by a perpendicular transition and which are correlated at large methylbromide distance to the ground bromide spinorbit state, as well as the and states which can be excited by a parallel and perpendicular transition (respectively) and both correlate to excited spinorbit state. The model provides absorption cross sections and branching ratios in excellent agreement with experimental results. Due to weak spinorbit interaction, the state is the dominant contributor to the absorption cross section, except for the red wing of the band where and states have significant absorption. However, spinorbit coupling is strong enough to induce nonadiabatic transitions between the and states during the dissociation process which should be experimentally detectable in the alignment properties of the fragments. Nonadiabatic transitions at the conical intersection between and are shown to play a minor role in this system.

Ab initio study of valence and Rydberg states of
View Description Hide DescriptionWe performed configuration interaction ab initio calculations on the valence and , , and Rydberg bands of the molecule as a function of the methylbromide distance for frozen geometries. The valence state potential energy curves are repulsive, the Rydberg state ones are similar to the one of the ion with a minimum at short distance. One state emerging from the band has valence and ionpair characters as distance increases and the corresponding potential curve has a second minimum at large distance. This state has a very strong parallel electric dipole transition moment with the ground state and plays a central role in UV photon absorption spectra. It is also responsible for the parallel character of the anisotropy parameters measured in ionpair production experiments. In each band, there is a single state, which has a nonnegligible transition moment with the ground state, corresponding to a transition perpendicular to the molecular axis of symmetry, except for the band where it is parallel. The perpendicular transition moments between ground and valence states increase sharply as methylbromide distance decreases due to a mixing between valence and Rydberg band at short distance. In each band, spin orbit interaction produces a pair of states, which have significant transition moments with the ground one. In the valence band, the mixing between singlet and triplet states is weak and the perpendicular transition to the state is dominant. In each Rydberg band, however, spinorbit interaction is larger than the exchange interaction and the two significant transition moments with the ground state have comparable strengths. The valence band has an additional state with significant parallel transition moment induced by spinorbit interaction with the ground state at large distance.

Theoretical investigation of the states of dissociating adiabatically up to
View Description Hide DescriptionA theoretical investigation of the electronic structure of the molecule, including spinorbit effects, has been performed. Potential energies have been calculated over a large range of up to for the states dissociating adiabatically into the limits up to . Equilibrium distances, transition energies, harmonic frequencies, as well as depths for wells and heights for barriers are reported for all of the bound states. Present ab initio calculations are shown to be able to reproduce quite accurately the small structures (wells and barrier) displayed at very longrange by the and purely longrange states. As the present data could help experimentalists, we make available extensive tables of energy values versus internuclear distances in our database at the web address http://wwwlasim.univlyon1.fr/spip.php?rubrique99.

The mechanism of the interstellar isomerization reaction catalyzed by : New Insights from the reaction electronic flux
View Description Hide DescriptionA theoretical study of the mechanism of the isomerization reaction is presented. The mechanism was studied in terms of reaction force, chemical potential, reaction electronic flux (REF), and bond orders. It has been found that the evolution of changes in REF along the intrinsic reaction coordinate can be explained in terms of bond orders. The energetic lowering of the hydrogen assisted (catalyzed) reaction has been identified as being due to the stabilization of the transition state complex and the stepwise bond dissociation and formation of the H–O and H–C bonds, respectively.