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28 Jun 2010

Volume 132, Issue 24, Articles (24xxxx)

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J. Chem. Phys. 132, 244101 (2010); http://dx.doi.org/10.1063/1.3435332 (16 pages)

Cheng Zhang and Jianpeng Ma
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Communications: Evaluation of the nondiabaticity of quantum molecular dynamics with the dephasing representation of quantum fidelity

Tomáš Zimmermann and Jiří Vaníček

J. Chem. Phys. 132, 241101 (2010); http://dx.doi.org/10.1063/1.3451266 (4 pages) | Cited 6 times

Online Publication Date: 22 June 2010

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We propose an approximate method for evaluating the importance of non-Born–Oppenheimer effects on the quantum dynamics of nuclei. The method uses a generalization of the dephasing representation (DR) of quantum fidelity to several diabatic potential energy surfaces and its computational cost is the cost of dynamics of a classical phase space distribution. It can be implemented easily into any molecular dynamics program and also can utilize on-the-fly ab initio electronic structure information. We test the methodology on three model problems introduced by Tully and on the photodissociation of NaI. The results show that for dynamics close to the diabatic limit, the decay of fidelity due to nondiabatic effects is described accurately by the DR. In this regime, unlike the mixed quantum-classical methods such as surface hopping or Ehrenfest dynamics, the DR can capture more subtle quantum effects than the population transfer between potential energy surfaces. Hence we propose using the DR to estimate the dynamical importance of diabatic, spin-orbit, or other couplings between potential energy surfaces. The acquired information can help reduce the complexity of a studied system without affecting the accuracy of the quantum simulation.
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82.20.Kh Potential energy surfaces for chemical reactions
31.15.aj Relativistic corrections, spin-orbit effects, fine structure; hyperfine structure
31.50.-x Potential energy surfaces
33.80.Gj Diffuse spectra; predissociation, photodissociation
82.37.Vb Single molecule photochemistry
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Communications: A systematic method for locating transition structures of A+B→X type reactions

Satoshi Maeda and Keiji Morokuma

J. Chem. Phys. 132, 241102 (2010); http://dx.doi.org/10.1063/1.3457903 (4 pages) | Cited 7 times

Online Publication Date: 24 June 2010

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Search for transition structures (TSs) as first-order saddles is one of the most important tasks in theoretical study of chemical reaction. Although automated search has been established either by starting from a local minimum (MIN) or by connecting two MINs, there is no systematic method which can locate TSs of A+B→X(+Y) type reactions starting from separated reactants. We propose such an approach for the first time; it was demonstrated to work very well in the SN2, Diels–Alder, and Wittig reactions.
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82.20.Db Transition state theory and statistical theories of rate constants
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Communications: Chain and double-ring polymeric structures: Observation of AlnH3n+1− (n = 4–8) and Al4H14−

Xiang Li, Andrej Grubisic, Kit H. Bowen, Anil K. Kandalam, Boggavarapu Kiran, Gerd F. Gantefoer, and Puru Jena

J. Chem. Phys. 132, 241103 (2010); http://dx.doi.org/10.1063/1.3458912 (4 pages) | Cited 6 times

Online Publication Date: 24 June 2010

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A pulsed arc discharge source was used to prepare gas-phase, aluminum hydride cluster anions, AlnHm−, exhibiting enhanced hydrogen content. The maximum number of hydrogen atoms in AlnHm− species was m = 3n+1 for n = 5–8, i.e., AlnH3n+1−, and m = 3n+2 for n = 4, i.e., Al4H14−, as observed in their mass spectra. These are the most hydrogen-rich aluminum hydrides to be observed thus far, transcending the 3:1 hydrogen-to-aluminum ratio in alane. Even more striking, ion intensities for AlnHm− species with m = 3n+1 and m = 3n+2 hydrogen atoms were significantly higher than those of nearby AlnHm− mass peaks for which m<3n+1, i.e., the ion intensities for AlnH3n+1− and for Al4H14− deviated from the roughly bell-shaped ion intensity patterns seen for most AlnHm− species, in which m ranges from 1 to 3n. Calculations based on density functional theory showed that AlnH3n+1− clusters have chain and/or double-ring polymeric structures.
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36.20.Fz Constitution (chains and sequences)
36.20.Kd Electronic structure and spectra
31.15.E- Density-functional theory
33.15.Ta Mass spectra
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Communications: Ab initio second-order nonlinear optics in solids

Eleonora Luppi, Hannes Hübener, and Valérie Véniard

J. Chem. Phys. 132, 241104 (2010); http://dx.doi.org/10.1063/1.3457671 (4 pages) | Cited 2 times

Online Publication Date: 28 June 2010

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We present a first-principles theory for the calculation of the macroscopic second-order susceptibility χ(2), based on the time-dependent density-functional theory approach. Our method allows to include straightforwardly the many-body effects, such as crystal local fields and excitons. We apply the theory to the computation of the second-harmonic generation spectroscopy. In order to demonstrate the accuracy of this approach we present spectra for the cubic semiconductor GaAs for which we obtain a very good agreement with the experimental results. We point out that crystal local fields are not sufficient to reproduce the experimental results. Only when we account for the excitonic effects we obtain a very good agreement with the experimental second-harmonic generation spectrum.
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42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
71.70.Ch Crystal and ligand fields
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Communications: Signatures of quasiparticle entanglement in multidimensional nonlinear optical spectroscopy of aggregates

Shaul Mukamel

J. Chem. Phys. 132, 241105 (2010); http://dx.doi.org/10.1063/1.3454657 (4 pages) | Cited 10 times

Online Publication Date: 29 June 2010

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Excitons represent collective optical excitations in which the motions of electrons belonging to different chromophores are correlated. We discuss the utility of the notion of entanglement commonly used in quantum information processing, in the description of these excitations. A distinction is made between some apparent entanglement effects associated with the linear response that may be removed by a transformation of coordinates and can be handled classically, and genuine entanglement that is fundamentally quantum in nature and shows up only in the nonlinear optical response.
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42.65.-k Nonlinear optics
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Enhanced sampling and applications in protein folding in explicit solvent

Cheng Zhang and Jianpeng Ma

J. Chem. Phys. 132, 244101 (2010); http://dx.doi.org/10.1063/1.3435332 (16 pages) | Cited 14 times

Online Publication Date: 22 June 2010

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We report a single-copy tempering method for simulating large complex systems. In a generalized ensemble, the method uses runtime estimate of the thermal average energy computed from a novel integral identity to guide a continuous temperature-space random walk. We first validated the method in a two-dimensional Ising model and a Lennard-Jones liquid system. It was then applied to folding of three small proteins, trpzip2, trp-cage, and villin headpiece in explicit solvent. Within 0.5 ∼ 1 microsecond, all three systems were reversibly folded into atomic accuracy: the alpha carbon root mean square deviations of the best folded conformations from the native states were 0.2, 0.4, and 0.4 Å, for trpzip2, trp-cage, and villin headpiece, respectively.
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87.15.Cc Folding: thermodynamics, statistical mechanics, models, and pathways
87.15.kr Protein-solvent interactions
87.15.B- Structure of biomolecules

Non-Born–Oppenheimer quantum chemistry on the fly with continuous path branching due to nonadiabatic and intense optical interactions

Takehiro Yonehara and Kazuo Takatsuka

J. Chem. Phys. 132, 244102 (2010); http://dx.doi.org/10.1063/1.3439396 (17 pages) | Cited 8 times

Online Publication Date: 22 June 2010

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We extend our formerly proposed theory for non-Born–Oppenheimer electronic and nuclear wavepacket dynamics within on-the-fly scheme [ T. Yonehara, S. Takahashi, and K. Takatsuka, J. Chem. Phys. 130, 214113 (2009) ] to a case of nonadiabatic dynamics under an intense laser field: electron wavepacket in a molecule is propagated in attosecond time-scale along non-Born–Oppenheimer nuclear paths that smoothly branch due to nonadiabatic coupling and/or optical interactions. Such branching paths are determined consistently with the motion of the electron wavepackets. Furthermore, these nuclear paths are quantized in terms of Gaussian wavepackets (action decomposed function), which can be applied to nonclassical paths. Both electronic wavepacket dynamics and quantization of non-Born–Oppenheimer paths are generalized so as to include the direct effects of the classical vector potential of electromagnetic fields. In the second half of this paper, we perform numerical studies to explore nonadiabatic dynamics in a laser field by examining two cases: one is a two-state model system having an avoided crossing, and the other is two-state dynamics in HF molecule on the two low lying ab initio potential curves. Both are placed in laser fields. With the former system, we survey some basic properties of the coupling of nonadiabatic dynamics and laser interaction varying the relevant coupling parameters such as the laser timing with respect to the incident of nonadiabatic transition. This investigation will set a foundation for the future studies of control of electronic states in realistic multidimensional molecular systems. Application to the latter system shows that non-Born–Oppenheimer quantum chemistry in laser fields is indeed useful in the study of dynamics in ab initio level. Through the comparison with full quantum data, we verify that the formalism and methodology developed here work accurately. Furthermore, we attain some basic insight about the characteristics of molecules in laser fields.
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31.15.A- Ab initio calculations
31.50.Gh Surface crossings, non-adiabatic couplings
33.80.Be Level crossing and optical pumping

An efficient iterative grand canonical Monte Carlo algorithm to determine individual ionic chemical potentials in electrolytes

Attila Malasics and Dezső Boda

J. Chem. Phys. 132, 244103 (2010); http://dx.doi.org/10.1063/1.3443558 (10 pages) | Cited 10 times

Online Publication Date: 23 June 2010

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Two iterative procedures have been proposed recently to calculate the chemical potentials corresponding to prescribed concentrations from grand canonical Monte Carlo (GCMC) simulations. Both are based on repeated GCMC simulations with updated excess chemical potentials until the desired concentrations are established. In this paper, we propose combining our robust and fast converging iteration algorithm [ Malasics, Gillespie, and Boda, J. Chem. Phys. 128, 124102 (2008) ] with the suggestion of Lamperski [Mol. Simul. 33, 1193 (2007)] to average the chemical potentials in the iterations (instead of just using the chemical potentials obtained in the last iteration). We apply the unified method for various electrolyte solutions and show that our algorithm is more efficient if we use the averaging procedure. We discuss the convergence problems arising from violation of charge neutrality when inserting/deleting individual ions instead of neutral groups of ions (salts). We suggest a correction term to the iteration procedure that makes the algorithm efficient to determine the chemical potentials of individual ions too.
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82.60.Lf Thermodynamics of solutions
02.60.-x Numerical approximation and analysis
61.20.Ja Computer simulation of liquid structure
65.20.De General theory of thermodynamic properties of liquids, including computer simulation
82.45.Gj Electrolytes

Weak antiferromagnetic coupling in molecular ring is predicted correctly by density functional theory plus Hubbard U

Shruba Gangopadhyay, Artëm E. Masunov, Eliza Poalelungi, and Michael N. Leuenberger

J. Chem. Phys. 132, 244104 (2010); http://dx.doi.org/10.1063/1.3421645 (7 pages) | Cited 3 times

Online Publication Date: 23 June 2010

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We apply density functional theory with empirical Hubbard U parameter (DFT+U) to study Mn-based molecular magnets. Unlike most previous DFT+U studies, we calibrate U parameters for both metal and ligand atoms using five binuclear manganese complexes as the benchmarks. We note delocalization of the spin density onto acetate ligands due to π-back bonding, inverting spin polarization of the acetate oxygen atoms relative to that predicted from superexchange mechanism. This inversion may affect the performance of the models that assume strict localization of the spins on magnetic centers for the complexes with bridging acetate ligands. Next, we apply DFT+U methodology to Mn12 molecular wheel and find antiparallel spin alignment for the weakly interacting fragments Mn6, in agreement with experimental observations. Using the optimized geometry of the ground spin state instead of less accurate experimental geometry was found to be crucial for this good agreement. The protocol tested in this study can be applied for the rational design of single molecule magnets for molecular spintronics and quantum computing applications.
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75.25.-j Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.)
75.10.Lp Band and itinerant models
75.50.Xx Molecular magnets
75.50.Ee Antiferromagnetics
75.30.Et Exchange and superexchange interactions

Nonequilibrium numerical model of homogeneous condensation in argon and water vapor expansions

Ryan Jansen, Ingrid Wysong, Sergey Gimelshein, Michael Zeifman, and Udo Buck

J. Chem. Phys. 132, 244105 (2010); http://dx.doi.org/10.1063/1.3447379 (12 pages) | Cited 3 times

Online Publication Date: 24 June 2010

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A computational approach capable of modeling homogeneous condensation in nonequilibrium environments is presented. The approach is based on the direct simulation Monte Carlo (DSMC) method, extended as appropriate to include the most important processes of cluster nucleation and evolution at the microscopic level. The approach uses a recombination-reaction energy-dependent mechanism of the DSMC method for the characterization of dimer formation, and the RRK model for the cluster evaporation. Three-step testing and validation of the model is conducted by (i) comparison of clusterization rates in an equilibrium heat bath with theoretical predictions for argon and water vapor and adjustment of the model parameters, (ii) comparison of the nonequilibrium argon cluster size distributions with experimental data, and (iii) comparison of the nonequilibrium water cluster size distributions with experimental measurements. Reasonable agreement was observed for all three parts of the validation.
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64.70.fm Thermodynamics studies of evaporation and condensation
64.60.Q- Nucleation

Calculation of the first static hyperpolarizability tensor of three-dimensional periodic compounds with a local basis set: A comparison of LDA, PBE, PBE0, B3LYP, and HF results

Roberto Orlando, Valentina Lacivita, Radovan Bast, and Kenneth Ruud

J. Chem. Phys. 132, 244106 (2010); http://dx.doi.org/10.1063/1.3447387 (9 pages) | Cited 11 times

Online Publication Date: 24 June 2010

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The computational scheme for the evaluation of the second-order electric susceptibility tensor in periodic systems, recently implemented in the CRYSTAL code within the coupled perturbed Hartree–Fock (HF) scheme, has been extended to local-density, gradient-corrected, and hybrid density functionals (coupled-perturbed Kohn–Sham) and applied to a set of cubic and hexagonal semiconductors. The method is based on the use of local basis sets and analytical calculation of derivatives. The high-frequency dielectric tensor (ϵ∞) and second-harmonic generation susceptibility (d) have been calculated with hybrid functionals (PBE0 and B3LYP) and the HF approximation. Results are compared with the values of ϵ∞ and d obtained from previous plane-wave local density approximation or generalized gradient approximation calculations and from experiment. The agreement is in general good, although comparison with experiment is affected by a certain degree of uncertainty implicit in the experimental techniques.
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42.65.An Optical susceptibility, hyperpolarizability
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
78.20.Bh Theory, models, and numerical simulation

Efficient evaluation of the Coulomb force in the Gaussian and finite-element Coulomb method

Yuki Kurashige, Takahito Nakajima, Takeshi Sato, and Kimihiko Hirao

J. Chem. Phys. 132, 244107 (2010); http://dx.doi.org/10.1063/1.3457363 (7 pages)

Online Publication Date: 25 June 2010

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We propose an efficient method for evaluating the Coulomb force in the Gaussian and finite-element Coulomb (GFC) method, which is a linear-scaling approach for evaluating the Coulomb matrix and energy in large molecular systems. The efficient evaluation of the analytical gradient in the GFC is not straightforward as well as the evaluation of the energy because the SCF procedure with the Coulomb matrix does not give a variational solution for the Coulomb energy. Thus, an efficient approximate method is alternatively proposed, in which the Coulomb potential is expanded in the Gaussian and finite-element auxiliary functions as done in the GFC. To minimize the error in the gradient not just in the energy, the derived functions of the original auxiliary functions of the GFC are used additionally for the evaluation of the Coulomb gradient. In fact, the use of the derived functions significantly improves the accuracy of this approach. Although these additional auxiliary functions enlarge the size of the discretized Poisson equation and thereby increase the computational cost, it maintains the near linear scaling as the GFC and does not affects the overall efficiency of the GFC approach.
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31.15.xr Self-consistent-field methods
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions
02.50.-r Probability theory, stochastic processes, and statistics
02.60.-x Numerical approximation and analysis

Range-separated density-functional theory with random phase approximation applied to noncovalent intermolecular interactions

Wuming Zhu, Julien Toulouse, Andreas Savin, and János G. Ángyán

J. Chem. Phys. 132, 244108 (2010); http://dx.doi.org/10.1063/1.3431616 (9 pages) | Cited 23 times

Online Publication Date: 28 June 2010

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Range-separated methods combining a short-range density functional with long-range random phase approximations (RPAs) with or without exchange response kernel are tested on rare-gas dimers and the S22 benchmark set of weakly interacting complexes of Jurečka et al. [Phys. Chem. Chem. Phys. 8, 1985 (2006)] . The methods are also compared to full-range RPA approaches. Both range separation and inclusion of the Hartree–Fock exchange kernel largely improve the accuracy of intermolecular interaction energies. The best results are obtained with the method called RSH+RPAx, which yields interaction energies for the S22 set with an estimated mean absolute error of about 0.5–0.6 kcal/mol, corresponding to a mean absolute percentage error of about 7%–9% depending on the reference interaction energies used. In particular, the RSH+RPAx method is found to be overall more accurate than the range-separated method based on long-range second-order Møller–Plesset (MP2) perturbation theory (RSH+MP2).
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34.20.Gj Intermolecular and atom-molecule potentials and forces
31.15.eg Exchange-correlation functionals (in current density functional theory)
31.15.xr Self-consistent-field methods

Using fit functions in computational dielectric spectroscopy

Christian Schröder and Othmar Steinhauser

J. Chem. Phys. 132, 244109 (2010); http://dx.doi.org/10.1063/1.3432620 (16 pages) | Cited 10 times

Online Publication Date: 28 June 2010

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This work deals with the development of an appropriate set of fit functions for describing dielectric spectra based on simulated raw data. All these fit functions are of exponential character with properly chosen cofunctions. The type of the cofunctions is different for translation, rotation and their coupling. As an alternative to multiexponential fits we also discuss Kohlrausch–Williams–Watts functions. Since the corresponding Fourier–Laplace series for these stretched exponentials has severe convergence problems, we represent their Fourier–Laplace spectrum as a Havriliak–Negami expression with properly chosen parameters. A general relation between the parameter of the Kohlrausch–Williams–Watts and the Havriliak–Negami parameters is given. The set of fit functions is applied to the concrete simulation of the hydrated ionic liquid 1-ethyl-3-methyl-imidazolium triflate with H2O. The systematic variation of the water mole fraction permits to study the gradual transition from a neutral molecular liquid to molecular ionic liquids.
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61.20.Ja Computer simulation of liquid structure
61.25.-f Studies of specific liquid structures

Semiclassical initial value approximation for Green’s function

Kenneth G. Kay

J. Chem. Phys. 132, 244110 (2010); http://dx.doi.org/10.1063/1.3451076 (15 pages) | Cited 3 times

Online Publication Date: 28 June 2010

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A semiclassical initial value approximation is obtained for the energy-dependent Green’s function. For a system with f degrees of freedom the Green’s function expression has the form of a (2f−1)-dimensional integral over points on the energy surface and an integral over time along classical trajectories initiated from these points. This approximation is derived by requiring an integral ansatz for Green’s function to reduce to Gutzwiller’s semiclassical formula when the integrations are performed by the stationary phase method. A simpler approximation is also derived involving only an (f−1)-dimensional integral over momentum variables on a Poincaré surface and an integral over time. The relationship between the present expressions and an earlier initial value approximation for energy eigenfunctions is explored. Numerical tests for two-dimensional systems indicate that good accuracy can be obtained from the initial value Green’s function for calculations of autocorrelation spectra and time-independent wave functions. The relative advantages of initial value approximations for the energy-dependent Green’s function and the time-dependent propagator are discussed.
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02.30.Uu Integral transforms
02.10.Ud Linear algebra

Nonadiabatic dynamics with the help of multiconfigurational Ehrenfest method: Improved theory and fully quantum 24D simulation of pyrazine

Dmitrii V. Shalashilin

J. Chem. Phys. 132, 244111 (2010); http://dx.doi.org/10.1063/1.3442747 (11 pages) | Cited 10 times

Online Publication Date: 29 June 2010

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This article proposes an improved version of recently developed multiconfigurational Ehrenfest approach to quantum dynamics. The idea of the approach is to use frozen Gaussians (FG) guided by Ehrenfest trajectories as a basis set for fully quantum propagation. The method is applied to simulation of nonadiabatic dynamics of pyrazine and shows that nonadiabatic dynamics on two coupled electronic states S2 and S1, which determines pyrazine absorption spectrum, can be simulated with the help of a basis comprised of very small number of trajectory guided basis functions. For the 24 dimensional (24D) model, good results were obtained with the basis of only 250 trajectories guided FG per electronic state. The efficiency of the method makes it particularly suitable for future application together with direct dynamics, calculating potentials on the fly.
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31.15.xv Molecular dynamics and other numerical methods
33.20.Ea Infrared spectra
33.20.Kf Visible spectra
33.20.Lg Ultraviolet spectra

Matrix-free application of Hamiltonian operators in Coifman wavelet bases

Ramiro Acevedo, Richard Lombardini, and Bruce R. Johnson

J. Chem. Phys. 132, 244112 (2010); http://dx.doi.org/10.1063/1.3442746 (13 pages)

Online Publication Date: 29 June 2010

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A means of evaluating the action of Hamiltonian operators on functions expanded in orthogonal compact support wavelet bases is developed, avoiding the direct construction and storage of operator matrices that complicate extension to coupled multidimensional quantum applications. Application of a potential energy operator is accomplished by simple multiplication of the two sets of expansion coefficients without any convolution. The errors of this coefficient product approximation are quantified and lead to use of particular generalized coiflet bases, derived here, that maximize the number of moment conditions satisfied by the scaling function. This is at the expense of the number of vanishing moments of the wavelet function (approximation order), which appears to be a disadvantage but is shown surmountable. In particular, application of the kinetic energy operator, which is accomplished through the use of one-dimensional (1D) [or at most two-dimensional (2D)] differentiation filters, then degrades in accuracy if the standard choice is made. However, it is determined that use of high-order finite-difference filters yields strongly reduced absolute errors. Eigensolvers that ordinarily use only matrix-vector multiplications, such as the Lanczos algorithm, can then be used with this more efficient procedure. Applications are made to anharmonic vibrational problems: a 1D Morse oscillator, a 2D model of proton transfer, and three-dimensional vibrations of nitrosyl chloride on a global potential energy surface.
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31.30.jy Higher-order effective Hamiltonians
34.20.Cf Interatomic potentials and forces
02.10.Ud Linear algebra
02.70.Bf Finite-difference methods
33.20.Tp Vibrational analysis
31.50.-x Potential energy surfaces
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Isotope dependent, temperature regulated, energy repartitioning in a low-barrier, short-strong hydrogen bonded cluster

Xiaohu Li, Jos Oomens, John R. Eyler, David T. Moore, and Srinivasan S. Iyengar

J. Chem. Phys. 132, 244301 (2010); http://dx.doi.org/10.1063/1.3430525 (15 pages) | Cited 8 times

Online Publication Date: 22 June 2010

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We investigate and analyze the vibrational properties, including hydrogen/deuterium isotope effects, in a fundamental organic hydrogen bonded system using multiple experimental (infrared multiple photon dissociation and argon-tagged action spectroscopy) and computational techniques. We note a qualitative difference between the two experimental results discussed here and employ ab initio molecular dynamics simulations to explain these results. A deeper understanding of the differences between the isotopically labeled systems arises from an analysis of the simulated cluster spectroscopy and leads to a system-bath coupling interpretation. Specifically, when a few active modes, involving the shared hydrogen/deuterium stretch, are identified and labeled as “system,” with all other molecular vibrational modes being identified as “bath” modes, we find critical differences in the coupling between the system modes for the shared proton and shared deuteron cases. These differences affect the energy repartitioning between these modes resulting in a complex spectral evolution as a function of temperature. Furthermore, intensity borrowing across modes that are widely distributed in the frequency domain plays an important role on the simulated spectra.
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36.40.Mr Spectroscopy and geometrical structure of clusters
33.20.Tp Vibrational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.80.Gj Diffuse spectra; predissociation, photodissociation
33.15.Fm Bond strengths, dissociation energies
31.30.Gs Hyperfine interactions and isotope effects

First-principles studies for CO and O2 on gold nanocluster

Yao-Ping Xie and Xin-Gao Gong

J. Chem. Phys. 132, 244302 (2010); http://dx.doi.org/10.1063/1.3455714 (6 pages) | Cited 7 times

Online Publication Date: 22 June 2010

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First-principles calculations are performed to study the interaction of gold nanocluster Au55 with small molecules CO and O2. We find that the adsorption energy of CO on Au55 is among 0.5–0.7 eV at different sites and [CO+O2] can be coadsorbed on Au55. Comparisons between Au55 and Au32 show that the adsorption energy not only depends on the size of the cluster but also on the geometry of the cluster. Similar with smaller cluster (Au8 and Au32), the energy difference between [CO+O2] and [CO2+O] on Au55 is much larger than that in the free gas. Our calculations indicate that the nanocluster Au55 can enhance the reaction process, CO+O2→CO2+O, in which the reaction barrier is only about half electron volts.
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68.43.Mn Adsorption kinetics
68.43.Bc Ab initio calculations of adsorbate structure and reactions
36.40.Jn Reactivity of clusters
36.40.Mr Spectroscopy and geometrical structure of clusters
31.15.ap Polarizabilities and other atomic and molecular properties

A path-integral Monte Carlo study of a small cluster: The Ar trimer

R. Pérez de Tudela, M. Márquez-Mijares, T. González-Lezana, O. Roncero, S. Miret-Artés, G. Delgado-Barrio, and P. Villarreal

J. Chem. Phys. 132, 244303 (2010); http://dx.doi.org/10.1063/1.3445773 (10 pages) | Cited 5 times

Online Publication Date: 23 June 2010

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The Ar3 system has been studied between T = 0 K and T = 40 K by means of a path-integral Monte Carlo (PIMC) method. The behavior of the average energy in terms of the temperature has been explained by comparison with results obtained with the thermal averaged rovibrational spectra estimated via: (i) a quantum mechanical method based on distributed Gaussian functions for the interparticle distances and (ii) an analytical model which precisely accounts for the participation of the dissociative continua Ar2+Ar and Ar+Ar+Ar. Beyond T ∼ 20 K, the system explores floppier configurations than the rigid equilateral geometry, as linear and Ar–Ar2-like arrangements, and fragmentates around T ∼ 40 K. A careful investigation of the specific heat in terms of a confining radius in the PIMC calculation seems to discard a proper phase transition as in larger clusters, in apparent contradiction with previous reports of precise values for a liquid-gas transition. The onset of this noticeable change in the dynamics of the trimer occurs, however, at a remarkably low value of the temperature in comparison with Arn systems formed with more Ar atoms. Quantum mechanical effects are found of relevance at T ≤ 15 K, with both energies and radial distributions obtained with a quantum PIMC deviating from the corresponding classical results, thus precluding exclusively classical approaches for a precise description of the system at this low temperature range.
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36.40.Mr Spectroscopy and geometrical structure of clusters
31.15.-p Calculations and mathematical techniques in atomic and molecular physics
33.20.Vq Vibration-rotation analysis
33.15.Bh General molecular conformation and symmetry; stereochemistry
36.40.Ei Phase transitions in clusters

Photoionization of iodine atoms: Rydberg series which converge to the I+(1S0)←I(2P3/2) threshold

Marie Eypper, Fabrizio Innocenti, Alan Morris, Stefano Stranges, John B. West, George C. King, and John M. Dyke

J. Chem. Phys. 132, 244304 (2010); http://dx.doi.org/10.1063/1.3447382 (11 pages) | Cited 1 time

Online Publication Date: 24 June 2010

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Relative partial photoionization cross sections and angular distribution parameters β have been measured for the first and fourth (5p)−1 photoelectron (PE) bands of atomic iodine by performing angle-resolved constant-ionic-state (CIS) measurements on these PE bands between the 1D2 and 1S0 (5p)−1 ionic thresholds in the photon energy region of 12.9–14.1 eV. Rydberg series arising from the 5p→ns and 5p→nd excitations are observed in both the first PE band, I+(3P2)←I(2P3/2), and the fourth PE band, I+(1D2)←I(2P3/2), CIS spectra. For each Rydberg state, the resonance energy, quantum defect, linewidth, line shape, and photoelectron angular distribution parameter β have been determined. For the β-plots for each PE band, only resonances corresponding to 5p→nd excitations are observed; no resonances were seen at photon energies corresponding to the 5p→ns resonances in the CIS spectra. The β-plots are interpreted in terms of the parity unfavored channel with jt = 4 being the major contributor at the 5p→nd resonance positions, where jt is the quantum number for angular momentum transferred between the molecule, and the ion and photoelectron. Comparison of the results obtained with those published for bromine shows reasonably good agreement for the CIS spectra but poor agreement for the β-plots. It appears that parity unfavored channels are playing a greater role in the valence (np)−1 ionization of atomic iodine than in the corresponding ionization of atomic bromine.
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32.80.-t Photoionization and excitation

Long range intermolecular interactions between the alkali diatomics Na2, K2, and NaK

Warren T. Zemke, Jason N. Byrd, H. Harvey Michels, John A. Montgomery, Jr., and William C. Stwalley

J. Chem. Phys. 132, 244305 (2010); http://dx.doi.org/10.1063/1.3454656 (5 pages) | Cited 2 times

Online Publication Date: 24 June 2010

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Long range interactions between the ground state alkali diatomics Na2–Na2, K2–K2, Na2–K2, and NaK-NaK are examined. Interaction energies are first determined from ab initio calculations at the coupled-cluster with singles, doubles, and perturbative triples [CCSD(T)] level of theory, including counterpoise corrections. Long range energies calculated from diatomic molecular properties (polarizabilities and dipole and quadrupole moments) are then compared with the ab initio energies. A simple asymptotic model potential ELR = Eelec+Edisp+Eind is shown to accurately represent the intermolecular interactions for these systems at long range.
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34.20.Gj Intermolecular and atom-molecule potentials and forces
31.50.Bc Potential energy surfaces for ground electronic states
31.15.ap Polarizabilities and other atomic and molecular properties
31.15.bw Coupled-cluster theory
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility

The low-lying states of the scandium dimer

Cristopher Camacho, Henryk A. Witek, and Renzo Cimiraglia

J. Chem. Phys. 132, 244306 (2010); http://dx.doi.org/10.1063/1.3442374 (9 pages) | Cited 11 times

Online Publication Date: 25 June 2010

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A systematic investigation of low-lying states of Sc2 using multireference perturbation theory (NEVPT2 and NEVPT3) indicates that the ground state of this system is 5Σu− with re = 2.611 Å, ωe = 241.8 cm−1, and De = 1.78 eV. This state is closely followed by other low-lying states of Sc2: 3Σu−, 5Δu, 3Πg, 1Πg, and 1Σu−. Our energy ordering of the 5Σu− and 3Σu− states confirms the recent MRCI results of Kalemos et al. [J. Chem. Phys. 132, 024309 (2010)] and is at variance with the earlier diffusion Monte Carlo predictions of Matxain et al. [J. Chem. Phys. 128, 194315 (2008)] . An excellent agreement between the second- and third-order NEVPT results and between the computed and experimental values of ωe (241.8 versus 238.9 cm−1) for the 5Σu− state suggests high accuracy of our predictions.
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31.50.-x Potential energy surfaces

Insights into the ultraviolet spectrum of liquid water from model calculations

Paulo Cabral do Couto and Daniel M. Chipman

J. Chem. Phys. 132, 244307 (2010); http://dx.doi.org/10.1063/1.3453248 (11 pages) | Cited 10 times

Online Publication Date: 25 June 2010

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With a view toward a better molecular level understanding of the effects of hydrogen bonding on the ultraviolet absorption spectrum of liquid water, benchmark electronic structure calculations using high level wave function based methods and systematically enlarged basis sets are reported for excitation energies and oscillator strengths of valence excited states in the equilibrium water monomer and dimer and in a selection of liquid-like dimer structures. Analysis of the electron density redistribution associated with the two lowest valence excitations of the water dimer shows that these are usually localized on one or the other monomer, although valence hole delocalization can occur for certain relative orientations of the water molecules. The lowest excited state is mostly associated with the hydrogen bond donor and the significantly higher energy second excited state mostly with the acceptor. The magnitude of the lowest excitation energies is strongly dependent on where the valence hole is created, and only to a lesser degree on the perturbation of the excited electron density distribution by the neighboring water molecule. These results suggest that the lowest excitation energies in clusters and liquid water can be associated with broken acceptor hydrogen bonds, which provide energetically favorable locations for the formation of a valence hole. Higher valence excited states of the dimer typically involve delocalization of the valence hole and/or delocalization of the excited electron and/or charge transfer. Two of the higher valence excited states that involve delocalized valence holes always have particularly large oscillator strengths. Due to the pervasive delocalization and charge transfer, it is suggested that most condensed phase water valence excitations intimately involve more than one water molecule and, as a consequence, will not be adequately described by models based on perturbation of free water monomer states. The benchmark calculations are further used to evaluate a series of representative semilocal, global hybrid, and range separated hybrid functionals used in efficient time-dependent density functional methods. It is shown that such an evaluation is only meaningful when comparison is made at or near the complete basis set limit of the wave function based reference method. A functional is found that quantitatively describes the two lowest excitations of water dimer and also provides a semiquantitative description of the higher energy valence excited states. This functional is recommended for use in further studies on the absorption spectrum of large water clusters and of condensed phase water.
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61.50.Lt Crystal binding; cohesive energy
71.15.Mb Density functional theory, local density approximation, gradient and other corrections

Measurement of the differential cross section of the photoinitiated reactive collision of O(1D)+D2 using only one molecular beam: A study by three dimensional velocity mapping

S. Kauczok, C. Maul, A. I. Chichinin, and K.-H. Gericke

J. Chem. Phys. 132, 244308 (2010); http://dx.doi.org/10.1063/1.3427534 (12 pages) | Cited 2 times

Online Publication Date: 25 June 2010

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In order to measure the state selective double differential cross section of a reactive collision, the preparation of the reactants with defined initial velocities and quantum states in number densities high enough to achieve an acceptable count rate is most important. At the same time, secondary collisions have to be prevented in order to ensure that the nascent products are not thermalized. Usually, the best way to control the initial conditions is to use crossed molecular beams, but the number density decreases quadratically with the distance from the nozzle orifice which can be a problem, especially if a molecular product with a large number of populated states is to be analyzed state specifically by REMPI spectroscopy. In this contribution we would like to present a method for measuring the quantum state selective differential cross section of a photoinitiated reaction that combines the advantages of the PHOTOLOC technique (high reactant densities) and the parallel beams technique used by the groups of Kitsopoulos, Orr-Ewing, and Suits (defined relative velocity of the reactants). Moreover, an algorithm based on a Bayesian backward reconstruction developed by W. H. Richardson [J. Opt. Soc. Am. 62, 55 (1972)] has been derived. Both, one reactant and the precursor of the other reactant, are present in the same molecular beam and the center of mass velocity is selected by shifting the dissociation and the detection laser in time and space. Like in comparable methods, this produces a bias in the measured velocity distribution due to the fact that the reaction takes place in the whole volume surrounding the laser beams. This has been also reported by Toomes et al. in the case of the parallel beams technique and presents a general problem of probing reaction products by REMPI spectroscopy. To account for this, we develop a general approach that can be easily adapted to other conditions. The bias is removed in addition to deconvolution from the spread in reactant velocities. Using the benchmark system O(1D)+D2 with N2O as the precursor, we demonstrate that the technique is also applicable in a very general sense (i.e., also with a large spread in reactant velocities, products much faster than reactants) and therefore can be used also if such unfortunate conditions cannot be avoided. Since the resulting distribution of velocities in the laboratory frame is not cylindrically symmetric, three dimensional velocity mapping is the method of choice for the detection of the ionized products. For the reconstruction, the distance between the two laser beams is an important parameter. We have measured this distance using the photodissociation of HBr at 193 nm, detecting the H atoms near 243 nm. The collision energy resulting from the 193 nm photodissociation of N2O is 5.2±1.9 kcal/mol. Our results show a preference for backward scattered D atoms with the OH partner fragment in the high vibrational states (v = 4–6), in accord with previously published results claiming the growing importance of a linear abstraction mechanism for collision energies higher than 2.4 kcal/mol.
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34.50.Lf Chemical reactions
33.80.Gj Diffuse spectra; predissociation, photodissociation
37.20.+j Atomic and molecular beam sources and techniques
82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
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