Volume 132, Issue 4, 28 January 2010

The asymmetric stretching vibration of the amphiphilic trifluoroacetate ion and its isotopologue in were investigated with infrared spectroscopy (FTIR), ultrafast infrared pump probe, and two dimensional vibrational photon echo techniques and simulations. Trifluoroacetate ions have a nonexponential depopulation of the first vibrational excited state, which is well described by a kinetic mechanism involving a temperature dependent solvent assisted relaxation to the symmetric stretch mode. The vibrational spectrum of the asymmetric stretch of the isotopologue presents an unusual spectral shape. The frequencyfrequency autocorrelation function shows a static term not present in the form, which is caused by an accidental degeneracy with a combinational mode. A newly developed frequency map for carboxylate is used to characterize the processes and dynamics observed in the frequency fluctuations of the carboxylate asymmetric stretch mode in aqueous solution. An assignment of the molecular processes that govern the frequency fluctuations is suggested from an analysis of the solvation shell configurations obtained from molecular dynamics simulations.
 COMMUNICATIONS


Communications: Spatial separation of enantiomers by coherent optical means
View Description Hide DescriptionWe show that it is possible to spatially separate a (“racemic”) mixture of lefthanded and righthanded chiral molecules using optically induced forces. The separation is affected by the irradiation of the racemic mixture with three spatiallyinhomogeneous and partially overlapping cw laser fields, giving rise to enantiomeric and vibrational dependent lightinduced forces. Two experimental arrangements, one consisting of placing the racemate in 1 mK optical trap, the second composed of a tightly skimmed molecular beam of the racemic mixture, forming one arm of a foursided cavity, are numerically tested by performing quantum wave packet propagation and classical trajectories. Both the classical and the quantum simulations show an essentially perfect spatial separation between enantiomers of several vibrational states.

Communications: Can one identify nonequilibrium in a threestate system by analyzing twostate trajectories?
View Description Hide DescriptionFor a threestate Markov system in a stationary state, we discuss whether, on the basis of data obtained from effective twostate (or onoff) trajectories, it is possible to discriminate between an equilibrium state and a nonequilibrium steady state. By calculating the full phase diagram we identify a large region where such data will be consistent only with nonequilibrium conditions. This regime is considerably larger than the region with oscillatory relaxation, which has previously been identified as a sufficient criterion for nonequilibrium.

Communications: Survival of the fittest: Accelerating convergence in full configurationinteraction quantum Monte Carlo
View Description Hide DescriptionWe provide a very simple adaptation of our recently published quantum Monte Carlo algorithm in full configurationinteraction (Slater determinant) spaces which dramatically reduces the number of walkers required to achieve convergence. A survival criterion is imposed for newly spawned walkers. We define a set of initiator determinants such that progeny of walkers spawned from such determinants onto unoccupied determinants are able to survive, while the progeny of walkers not in this set can survive only if they are spawned onto determinants which are already occupied. The set of initiators is originally defined to be all determinants constructable from a subset of orbitals, in analogy with completeactive spaces. This set is dynamically updated so that if a noninitiator determinant reaches an occupation larger than a preset limit, it becomes an initiator. The new algorithm allows signcoherent sampling of the FCI space to be achieved with relatively few walkers. Using the molecule as an illustration, we show that rather small initiator spaces and numbers of walkers can converge with submilliHartree accuracy to the known full configurationinteraction (FCI) energy (in the ccpVDZ basis), in both the equilibrium geometry and the multiconfigurational stretched case. We use the same method to compute the energy with ccpVTZ and ccpVQZ basis sets, the latter having an FCI space of over with very modest computational resources.

Communications: Comparison of activation barriers for the Johari–Goldstein and alpha relaxations and its implications
View Description Hide DescriptionThe range of activation barrier heights for the Johari–Goldstein (JG) relaxation in glasses is shown to overlap the range for the main (alpha) relaxation, but to be on the average somewhat lower. This suggests the JG relaxation, like the alpha, involve transitions between megabasins in the energy landscape, and that the original conjecture by Johari and this author that the JG relaxation is an intrabasin one cannot be correct. A further possibility is that there is a closer connection of the JG relaxation to the phenomenon of dynamic heterogeneity in supercooled liquids than so far assumed.
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 ARTICLES

 Theoretical Methods and Algorithms

Correlation energy functional and potential from timedependent exactexchange theory
View Description Hide DescriptionIn this work we studied a new functional for the correlation energy obtained from the exactexchange (EXX) approximation within timedependent density functional theory.Correlation energies have been calculated for a number of different atoms showing excellent agreement with results from more sophisticated methods. These results lose little accuracy by approximating the EXX kernel by its static value, a procedure which enormously simplifies the calculations. The correlation potential, obtained by taking the functional derivative with respect to the density, turns out to be remarkably accurate for all atoms studied. This potential has been used to calculate ionization potentials, static polarizabilities, and van der Waals coefficients with results in close agreement with experiment.

A partialpropensity variant of the compositionrejection stochastic simulation algorithm for chemical reaction networks
View Description Hide DescriptionWe present the partialpropensity stochastic simulation algorithm with compositionrejection sampling (PSSACR). It is an exact formulation of the stochastic simulation algorithm (SSA) for wellstirred systems of coupled chemical reactions. The new formulation is a partialpropensity variant [R. Ramaswamy, N. GonzálezSegredo, and I. F. Sbalzarini, J. Chem. Phys.130, 244104 (2009)] of the composition rejection SSA [A. Slepoy, A. P. Thompson, and S. J. Plimpton, J. Chem. Phys.128, 205101 (2008)]. The computational cost of this new formulation is bounded by a constant for weakly coupled reaction networks, and it increases at most linearly with the number of chemical species for strongly coupled reaction networks. PSSACR thus combines the advantages of partialpropensity methods and the compositionrejection SSA, providing favorable scaling of the computational cost for all classes of reaction networks.

Extrapolated highorder propagators for path integral Monte Carlo simulations
View Description Hide DescriptionWe present a new class of highorder imaginary time propagators for path integral Monte Carlo simulations that require no higher order derivatives of the potential nor explicit quadratures of Gaussian trajectories. Higher orders are achieved by an extrapolation of the primitive secondorder propagator involving subtractions. By requiring all terms of the extrapolated propagator to have the same Gaussian trajectory, the subtraction only affects the potential part of the path integral. The resulting violation of positivity has surprisingly little effects on the accuracy of the algorithms at practical time steps. Thus in principle, arbitrarily high order algorithms can be devised for path integral Monte Carlo simulations. We verified the fourth, sixth, and eighth order convergences of these algorithms by solving for the ground state energy and pair distribution function of liquid , which is representative of a dense, and strongly interacting, quantum manybody system.

Role of the triplet state in the green emission peak of polyfluorene films: A time evolution study
View Description Hide DescriptionThe blue emission of ethylhexyl substituted polyfluorene (PF2/6) films is accompanied by a low energy green emission peak around 500 nm in inert atmosphere. The intensity of this 500 nm peak is large in electroluminescence(EL) compared to photoluminescence(PL) measurements. Furthermore, the green emission intensity reduces dramatically in the presence of molecular oxygen. To understand this, we have modeled various nonradiative processes by time dependent quantum many body methods. These are (i) intersystem crossing to study conversion of excited singlets to triplets leading to a phosphorescence emission, (ii) electronhole recombination (ehR) process in the presence of a paramagnetic impurity to follow the yield of triplets in a polyene system doped with paramagnetic metal atom, and (iii) quenching of excited triplet states in the presence of oxygen molecules to understand the low intensity of EL emission in ambient atmosphere, when compared with that in nitrogen atmosphere. We have employed the Pariser–Parr–Pople Hamiltonian to model the molecules and have invoked electronelectron repulsions beyond zero differential approximation while treating interactions between the organic molecule and the rest of the system. Our time evolution methods show that there is a large cross section for triplet formation in the ehR process in the presence of paramagnetic impurity with degenerate orbitals. The triplet yield through ehR process far exceeds that in the intersystem crossing pathway, clearly pointing to the large intensity of the 500 nm peak in EL compared to PL measurements. We have also modeled the triplet quenching process by a paramagnetic oxygen molecule which shows a sizable quenching cross section especially for systems with large sizes. These studies show that the most probable origin of the experimentally observed low energy EL emission is the triplets.

Jensen–Tsallis divergence and atomic dissimilarity for position and momentum space electron densities
View Description Hide DescriptionQuantifying the dissimilarity among two or more manyelectron systems by means of their oneparticle densities is a hot topic within the physical applications of the information theory. This is a relevant achievement of the socalled “divergence measures,” for which several definitions have been considered, each one with its own advantages and difficulties. Nevertheless, all of them are considered in order to disclose the differences among the involved systems, neutral atoms in the present work, according to their densities in the position and momentum spaces. The pioneering Jensen–Shannon divergence (JSD) constitutes a particular case of the oneparameter Jensen–Tsallis divergence (JTD). The analysis here provided for the JTD of atomic systems generalizes and improves some previous results on the JSD one. Such an improvement mainly arises from the capability of JTD to modify, by means of its characteristic parameter, the relative contribution of relevant specific regions of the atomic densities in both conjugated spaces.

A wavefunction based approach for polarizable charge model: Systematic comparison of polarization effects on protic, aprotic, and ionic liquids
View Description Hide DescriptionWe first describe a wavefunction based formalism of polarizable charge model by starting from the Hartree product ansatz for the total wave function and making the secondorder expansion of individual molecular energies with the use of partial charge operators. The resulting model is shown to be formally equivalent to the charge response kernel model that starts from the linearresponse approximation to partial charges, and also closely related to a family of fluctuating charge models that are based on the electronegativity equalization principle. We then apply the above model to a systematic comparison of polarization effects on qualitatively different liquids, namely, protic solvents (water and methanol), an aprotic polar solvent (acetonitrile), and imidazoliumbased ionic liquids. Electronic polarization is known to decelerate molecular motions in conventional solvents while it accelerates them in ionic liquids. To obtain more insights into these phenomena, we consider an effective decomposition of total polarization energy into molecular contributions, and show that their statistical distribution is wellcorrelated with the acceleration/deceleration of molecular motions. In addition, we perform effective nonpolarizable simulations based on mean polarized charges, and compare them with fully polarizable simulations. The result shows that the former can reproduce structural properties of conventional solvents rather accurately, while they fail qualitatively to reproduce acceleration of molecular motions in ionic liquids.

Firstorder nonadiabatic couplings from timedependent hybrid density functional response theory: Consistent formalism, implementation, and performance
View Description Hide DescriptionFirstorder nonadiabatic coupling matrix elements (NACMEs) are key for phenomena such as nonradiative transitions and excitedstate decay, yet a consistent and practical first principles treatment has been elusive for molecules with more than a few heavy atoms. Here we present theory, implementation using Gaussian basis sets, and benchmarks of firstorder NACMEs between ground and excited states in the framework of timedependent hybrid density functional theory (TDDFT). A timedependent response approach to NACMEs which avoids explicit computation of excitedstatewave functions is outlined. In contrast to previous approaches, the present treatment produces exact analytical derivative couplings between timedependent Kohn–Sham (TDKS) determinants in a finite atomcentered basis set. As in analytical gradient theory, derivative molecular orbital coefficients can be eliminated, making the computational cost independent of the number of nuclear degrees of freedom. Our expression reduces to the exact Chernyak–Mukamel formula for firstorder NACMEs in the complete basisset limit, but greatly improves basisset convergence in finite atomcentered basis sets due to additional Pulay type terms. The Chernyak–Mukamel formula is shown to be equivalent to the Hellmann–Feynman contribution in analytical gradient theory. Our formalism may be implemented in TDDFT analytical excitedstate gradient codes with minor modifications. Tests for systems with up to 147 atoms show that evaluation of firstorder NACMEs causes total computation times to increase by an insignificant 10% on average. The resolutionoftheidentity approximation for the Coulomb energy reduces the computational cost by an order of magnitude for nonhybrid functionals, while errors are insignificant with standard auxiliary basis sets. We compare the computed NACMEs to full configuration interaction (FCI) in benchmark results for diatomic molecules; hybrid TDDFT and FCI are found to be in agreement for regions of the potential energy curve where the Kohn–Sham groundstate reference is stable and the character of the excitation is properly captured by the present functionals. With these developments, nonadiabatic molecular dynamics simulations of molecular systems in the 100 atoms regime are within reach.

Ewaldtype formulas for Gaussianbasis Bloch states in onedimensionally periodic systems
View Description Hide DescriptionExpressions for integrals involving general Gaussian (s, p, d, …) basis Bloch functions are presented. Applying the Poissontransformation and the Ewaldtype partitioning scheme, all lattice sums appearing in these expressions lead to fast convergence in both direct and Fourier spaces. Numerical results produced for selected test cases show that a limited number of terms in the lattice sums are necessary to get convergence in the two spaces.

A firstprinciples study of weakly bound molecules using exact exchange and the random phase approximation
View Description Hide DescriptionWe present a study of the binding energy (BE) curves of rare gas and alkalineearth dimers using an energy functional that includes exact exchange (EXX) and correlationenergies within the random phase approximation (RPA). Our results for the equilibrium positions and long range behavior of the potential energy curves show great improvements over those obtained at the density functional theory level, within local and semilocal approximations. BEs are improved as well in the case of rare gas dimers. For Ar and Kr, the accuracy of our results is comparable to that of socalled van der Waals density functionals, although EXX/RPA yields BE curves that agree better with experiment for large separation distances, as expected. We also discuss shortcomings of the EXX/RPA perturbative approach and analyze possible sources of error in the description of the potential energy curve of alkalineearth dimers, in particular, , exhibiting an unphysical maximum at large separations. We suggest that the lack of selfconsistency in current EXX/RPA approaches might be largely responsible for most of the observed shortcomings. Finally, we present a tightbinding approach to obtain the eigenvalues of the dielectric matrix entering the calculation of the RPA correlationenergy that greatly improves the efficiency of EXX/RPA calculations.

On the Cholesky decomposition for electron propagator methods: General aspects and application on
View Description Hide DescriptionTo treat the electronic structure of large molecules by electron propagator methods we developed a parallel computer program called . The program exploits the sparsity of the twoelectron integral matrix by using Cholesky decomposition techniques. The advantage of these techniques is that the error introduced is controlled only by one parameter, which can be chosen as small as needed. We verify the tolerance of electron propagator methods to the Cholesky decomposition threshold and demonstrate the power of the program for a representative example . All decomposition schemes addressed in literature are investigated. Even with moderate thresholds the maximal error encountered in the calculated electron affinities and ionization potentials amount to a few meV only, and the error becomes negligible for small thresholds.

Firstprinciples analysis of the C–N bond scission of methylamine on Mobased model catalysts
View Description Hide DescriptionThe C–N bond breaking of methylamine on clean, carbon (nitrogen, oxygen)modified Mo(100) [denoted as Mo(100) and Mo(100)–C(N,O), respectively], , MoN(100), and Pt(100) surfaces has been investigated by the firstprinciples density functional theory (DFT) calculations. The results show that the reaction barriers of the C–N bond breaking in on Mo(100)–C(N,O) are higher than that on clean Mo(100). The calculated energy barrier can be correlated linearly with the density of Mo states at the Fermi level after the adsorption of for those surfaces. Moreover, the DFT results show that the subsurface atom, e.g., carbon, can reduce the reaction barrier. In addition, We noticed that the activation energies for the C–N bond breaking on and MoN(100) are similar to that on Pt(100), suggesting that the catalytic properties of the transition metal carbides and nitrides for C–N bond scission of might be very similar to the expensive Ptgroup metals.

Universal correction for the Becke–Johnson exchange potential
View Description Hide DescriptionThe Becke–Johnson exchange potential [A. D. Becke and E. R. Johnson, J. Chem. Phys.124, 221101 (2006)] has been successfully used in electronic structure calculations within densityfunctional theory. However, in its original form, the potential may dramatically fail in systems with nonCoulombic external potentials, or in the presence of external magnetic or electric fields. Here, we provide a systemindependent correction to the Becke–Johnson approximation by (i) enforcing its gaugeinvariance and (ii) making it exact for any singleelectron system. The resulting approximation is then better designed to deal with currentcarrying states and recovers the correct asymptotic behavior for systems with any number of electrons. Tests of the resulting corrected exchange potential show very good results for a hydrogen chain in an electric field and for a fourelectron harmonium in a magnetic field.

Enhancement and deenhancement effects in vibrational resonance Raman optical activity
View Description Hide DescriptionIn this study, we investigate interference between several excited electronic states in resonance enhanced vibrational Raman optical activity (RROA) spectra. A gradient Franck–Condon model for the excitedstate potential energy surface is applied in order to include vibronic effects in the description of the RROA intensities. Both sumoverstates and timedependent expressions for the RROA intensities in case of closelying excited states are given. As an example, we compare the calculated RROA and resonance Raman spectra of naproxen to the experimental ones. Subsequently, we examine the excitation profiles of naproxen and study the vibration at in more detail in order to demonstrate how the consideration of a second excited electronic state can lead to significant changes in the RROA intensities.

The weak covalent bond in NgAuF (, Kr, Xe): A challenge for subsystem density functional theory
View Description Hide DescriptionWe have assessed the accuracy of a representative set of currently available approximate kineticenergy functionals used within the frozendensity embedding scheme for the NgAuF (, Kr, Xe) molecules, which we partitioned into a Ng and a AuF subsystem. Although it is weak, there is a covalent interaction between these subsystems which represents a challenge for this subsystem density functional theory approach. We analyzed the effectiveembedding potentials and resulting electron density distributions and provide a quantitative analysis of the latter from dipole moment differences and rootmeansquare errors in the density with respect to the supermolecular Kohn–Sham density functional theory reference calculation. Our results lead to the conclusion that none of the tested approximate kineticenergy functionals performs well enough to describe the bond between the noble gas and gold adequately. This observation contributes to the growing evidence that the current procedure to obtain approximate kineticenergy functionals by reparametrizing functionals obtained via the “conjointness” hypothesis of Lee, Lee, and Parr [Phys. Rev. A44, 768 (1991)] is insufficient to treat metalligand interactions with covalent character.
 Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Transition probabilities for the Au (, , and ) with reaction
View Description Hide DescriptionTransition probabilities on the interaction of the ground and the lowest excited states of goldAu (, , and ) with silane are studied through ab initio Hartree–Fock selfconsistent field calculations, where the atom’s core is represented by relativistic effective core potentials. These calculations are followed by a multiconfigurational selfconsistent field study. The correlation energy is accounted for through extensive variational and perturbative second order multireference Moller–Plesset configuration interaction analysis of selected perturbations obtained by iterative process calculations using the CIPSI program package. It is found that the Au atom in the state inserts in the Si–H bond. In this interaction its corresponding potential energy surface is initially attractive and only becomes repulsive after encountering an avoided crossing with the initially repulsive surface linked to the fragments. The A, B, and curves derived from the atom interaction with silane are initially repulsive, each one of them showing two avoided crossings, while the curve goes sharply downwards until it meets the curve interacting adiabatically, which is linked with the moieties. The curve becomes repulsive after the avoided crossing with the , curve. The lowestlying potential leads to the intermediate molecule. This intermediate molecule, diabatically correlated with the system which lies 3.34 kcal/mol above the ground state reactants, has been carefully characterized as have the dissociation channels leading to the and products. These products are reached from the intermediate without any activation barrier. The calculation results are successfully compared to experiment. Landau–Zener theory of avoided crossings is applied to these interactions considering the angle instead of the distance as the reaction coordinate.

Photoelectron imaging of small silicon cluster anions,
View Description Hide DescriptionPhotoelectron imaging experiments were conducted on small silicon cluster anions, , acquired at a photon energy of 3.49 eV (355 nm). Electronic transitions arising from the anion ground states are observed, and the evaluated vertical detachment energies agree well with previous measurements and theoretical calculations. The anisotropy parameters have also been determined for each unique feature appearing in the photoelectron angular distributions at the employed photon energy. Separate calculations using density functional theory are also undertaken to determine the relative atomic orbital contributions constructing the interrogated highest occupied and lowlying molecular orbitals of a specific cluster. A method to interpret the observed cluster angular distributions, term the wave approach, is then implemented which provides quantitative predictions of the anisotropy parameter for partial wave emission from molecular orbitals partitioned by varying contributions of atomic orbital angular momenta. Highlighted in the wave analysis is the ability of discriminating between disparate molecular orbitals from two nearly isoenergetic structural isomers of opposing point group symmetry for the and cluster ions, respectively.