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21 Jan 2013

Volume 138, Issue 3, Articles (03xxxx)

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J. Chem. Phys. 138, 034901 (2013); http://dx.doi.org/10.1063/1.4773302 (11 pages)

Santosh Mogurampelly and Prabal K. Maiti
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Communication: Solute anisotropy effects in hydrated anion and neutral clusters

Hui Wen, Gao-Lei Hou, Shawn M. Kathmann, Marat Valiev, and Xue-Bin Wang

J. Chem. Phys. 138, 031101 (2013); http://dx.doi.org/10.1063/1.4776766 (4 pages)

Online Publication Date: 15 January 2013

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Specific ion effects in solvation processes are often rationalized in terms of spherically symmetric models involving an ion's size, charge, and polarizability. The effects of permanent charge anisotropy, related to the polyatomic nature of complex solutes, are expected to play a role in solvation but the extent of their importance remains unexplored. In this work, we provide compelling experimental and theoretical evidence that the anisotropic nature of complex polyoxyanion solutes can have a critical influence on the solvation process. Combined photoelectron spectroscopy and theoretical modeling results show that the electron binding energy of IO3−(H2O)n (n = 0–12) clusters is characterized by an anomalous drop at n = 10. Such behavior is unprecedented for rigid solute molecules and is related to the anisotropy of the neutral iodate radical that displays a strong selectivity to solvent configurations generated by the charged anion complex. These results highlight the significance of solute anisotropy and its potential impact on ion specificity and selectivity in aqueous environments.
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82.30.Nr Association, addition, insertion, cluster formation
33.60.+q Photoelectron spectra
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
36.40.Jn Reactivity of clusters
36.40.Mr Spectroscopy and geometrical structure of clusters
36.40.Wa Charged clusters
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Communication: Structure of magnetic lanthanide clusters from far-IR spectroscopy: Tb n+ (n = 5−9)

John Bowlan, Dan J. Harding, Jeroen Jalink, Andrei Kirilyuk, Gerard Meijer, and André Fielicke

J. Chem. Phys. 138, 031102 (2013); http://dx.doi.org/10.1063/1.4776768 (4 pages)

Online Publication Date: 15 January 2013

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Small lanthanide clusters have interesting magnetic properties, but their structures are unknown. We have identified the structures of small terbium cluster cations Tb n+ (n = 5−9) in the gas phase by analysis of their vibrational spectra. The spectra have been measured via IR multiple photon dissociation of their complexes with Ar atoms in the 50−250 cm−1 range with an infrared free electron laser. Density functional theory calculations using a 4f-in-core effective core potential (ECP) accurately reproduce the experimental far-IR spectra. The ECP corresponds to a 4f 85d16s2 trivalent configuration of terbium. The assigned structures are similar to those observed in several other transition metal systems. From this, we conclude that the bonding in Tb clusters is through the interactions between the 5d and 6s electrons, and that the 4f electrons have only an indirect effect on the cluster structures.
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36.40.Cg Electronic and magnetic properties of clusters
36.40.Mr Spectroscopy and geometrical structure of clusters
36.40.Vz Optical properties of clusters
36.40.Wa Charged clusters
31.15.E- Density-functional theory
33.80.Gj Diffuse spectra; predissociation, photodissociation
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back to top Theoretical Methods and Algorithms

Optimal scale-free network with a minimum scaling of transport efficiency for random walks with a perfect trap

Yihang Yang and Zhongzhi Zhang

J. Chem. Phys. 138, 034101 (2013); http://dx.doi.org/10.1063/1.4774269 (9 pages) | Cited 1 time

Online Publication Date: 15 January 2013

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Average trapping time (ATT) is central in the trapping problem since it is a key indicator characterizing the efficiency of the problem. Previous research has provided the scaling of a lower bound of the ATT for random walks in general networks with a deep trap. However, it is still not well understood in which networks this minimal scaling can be reached. Particularly, explicit quantitative results for ATT in such networks, even in a specific network, are lacking, in spite that such networks shed light on the design for optimal networks with the highest trapping efficiency. In this paper, we study the trapping problem taking place on a hierarchical scale-free network with a perfect trap. We focus on four representative cases with the immobile trap located at the root, a peripheral node, a neighbor of the root with a single connectivity, and a farthest node from the root, respectively. For all the four cases, we obtain the closed-form formulas for the ATT, as well as its leading scalings. We show that for all the four cases of trapping problems, the dominating scalings of ATT can reach the predicted minimum scalings. This work deepens the understanding of behavior of trapping in scale-free networks, and is helpful for designing networks with the most efficient transport process.
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05.40.Fb Random walks and Levy flights
05.60.Cd Classical transport
05.90.+m Other topics in statistical physics, thermodynamics, and nonlinear dynamical systems (restricted to new topics in section 05)
02.50.-r Probability theory, stochastic processes, and statistics

Reactive adsorption of ammonia and ammonia/water on CuBTC metal-organic framework: A ReaxFF molecular dynamics simulation

Liangliang Huang, Teresa Bandosz, Kaushik L. Joshi, Adri C. T. van Duin, and Keith E. Gubbins

J. Chem. Phys. 138, 034102 (2013); http://dx.doi.org/10.1063/1.4774332 (10 pages)

Online Publication Date: 15 January 2013

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We report ReaxFF molecular dynamics simulations for reactive adsorption of NH3 on dehydrated CuBTC metal-organic framework. If the temperature is moderate (up to 125 °C), the dehydrated CuBTC demonstrates a good hydrostatic stability for water concentrations up to 4.0 molecules per copper site. However, if the temperature increases to 550 K, the dehydrated CuBTC will collapse even at a small water concentration, 1.0 H2O molecule per copper site. When NH3 molecules are adsorbed in the channel and micropores of CuBTC, they prefer to chemisorb to the copper sites rather than forming a dimer with another NH3 molecule. The formation of equimolar Cu2(NH2)4 and (NH4)3BTC structures is observed at 348 K, which is in good agreement with previous experimental findings. The dehydrated CuBTC framework is partially collapsed upon NH3 adsorption, while the Cu–Cu dimer structure remains stable under the investigated conditions. Further calculations reveal that the stability of CuBTC is related to the ammonia concentration. The critical NH3 concentration after which the dehydrated CuBTC starts to collapse is determined to be 1.0 NH3 molecule per copper site. Depending on whether NH3 concentration is below or above the critical value, the dehydrated CuBTC can be stable to a higher temperature, 378 K, or can collapse at a lower temperature, 250 K. H2O/NH3 mixtures have also been studied, and we find that although water molecules do not demonstrate a strong interaction with the copper sites of CuBTC, the existence of water molecules can substantially prevent ammonia from interacting with CuBTC, and thus reduce the amount of chemisorbed NH3 molecules on CuBTC and stabilize the CuBTC framework to some extent.
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68.43.Mn Adsorption kinetics
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)

Multicanonical molecular dynamics by variable-temperature thermostats and variable-pressure barostats

Cheng Zhang and Michael W. Deem

J. Chem. Phys. 138, 034103 (2013); http://dx.doi.org/10.1063/1.4773435 (9 pages)

Online Publication Date: 15 January 2013

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Sampling from flat energy or density distributions has proven useful in equilibrating complex systems with large energy barriers. Several thermostats and barostats are presented to sample these flat distributions by molecular dynamics. These methods use a variable temperature or pressure that is updated on the fly in the thermodynamic controller. These methods are illustrated on a Lennard-Jones system and a structure-based model of proteins.
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87.15.ap Molecular dynamics simulation
34.20.Gj Intermolecular and atom-molecule potentials and forces
36.20.Kd Electronic structure and spectra
31.15.xv Molecular dynamics and other numerical methods
87.14.E- Proteins

Direct determination of exciton couplings from subsystem time-dependent density-functional theory within the Tamm–Dancoff approximation

Carolin König, Nicolas Schlüter, and Johannes Neugebauer

J. Chem. Phys. 138, 034104 (2013); http://dx.doi.org/10.1063/1.4774117 (15 pages) | Cited 1 time

Online Publication Date: 17 January 2013

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In subsystem time-dependent density functional theory (TDDFT) [J. Neugebauer, J. Chem. Phys. 126, 134116 (2007)10.1063/1.2713754] localized excitations are used to calculate delocalized excitations in large chromophore aggregates. We have extended this formalism to allow for the Tamm–Dancoff approximation (TDA). The resulting response equations have a form similar to a perturbative configuration interaction singles (CIS) approach. Thus, the inter-subsystem matrix elements in subsystem TDA can, in contrast to the full subsystem-TDDFT case, directly be interpreted as exciton coupling matrix elements. Here, we present the underlying theory of subsystem TDDFT within the TDA as well as first applications. Since for some classes of pigments, such as linear polyenes and carotenoids, TDA has been reported to perform better than full TDDFT, we also report applications of this formalism to exciton couplings in dimers of such pigments and in mixed bacteriochlorophyll–carotenoid systems. The improved description of the exciton couplings can be traced back to a more balanced description of the involved local excitations.
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71.35.-y Excitons and related phenomena
71.15.Mb Density functional theory, local density approximation, gradient and other corrections

Using fixed-node diffusion Monte Carlo to investigate the effects of rotation-vibration coupling in highly fluxional asymmetric top molecules: Application to H2D+

Andrew S. Petit, Bethany A. Wellen, and Anne B. McCoy

J. Chem. Phys. 138, 034105 (2013); http://dx.doi.org/10.1063/1.4774318 (11 pages)

Online Publication Date: 17 January 2013

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A fixed-node diffusion Monte Carlo approach for obtaining the energies and wave functions of the rotationally excited states of asymmetric top molecules that undergo large amplitude, zero-point vibrational motions is reported. The nodal surfaces required to introduce rotational excitation into the diffusion Monte Carlo calculations are obtained from the roots of the asymmetric top rigid rotor wave functions calculated using the system's zero-point, vibrationally averaged rotational constants. Using H2D+ as a model system, the overall accuracy of the methodology is tested by comparing to the results of converged variational calculations. The ability of the fixed-node diffusion Monte Carlo approach to provide insights into the nature and strength of the rotation-vibration coupling present in the rotationally excited states of highly fluxional asymmetric tops is discussed. Finally, the sensitivity of the methodology to the details of its implementation, such as the choice of embedding scheme, is explored.
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33.20.Vq Vibration-rotation analysis
33.20.Sn Rotational analysis
33.20.Tp Vibrational analysis

An efficient and near linear scaling pair natural orbital based local coupled cluster method

Christoph Riplinger and Frank Neese

J. Chem. Phys. 138, 034106 (2013); http://dx.doi.org/10.1063/1.4773581 (18 pages) | Cited 2 times

Online Publication Date: 17 January 2013

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In previous publications, it was shown that an efficient local coupled cluster method with single- and double excitations can be based on the concept of pair natural orbitals (PNOs) [F. Neese, A. Hansen, and D. G. Liakos, J. Chem. Phys. 131, 064103 (2009)10.1063/1.3173827]. The resulting local pair natural orbital-coupled-cluster single double (LPNO-CCSD) method has since been proven to be highly reliable and efficient. For large molecules, the number of amplitudes to be determined is reduced by a factor of 105–106 relative to a canonical CCSD calculation on the same system with the same basis set. In the original method, the PNOs were expanded in the set of canonical virtual orbitals and single excitations were not truncated. This led to a number of fifth order scaling steps that eventually rendered the method computationally expensive for large molecules (e.g., >100 atoms). In the present work, these limitations are overcome by a complete redesign of the LPNO-CCSD method. The new method is based on the combination of the concepts of PNOs and projected atomic orbitals (PAOs). Thus, each PNO is expanded in a set of PAOs that in turn belong to a given electron pair specific domain. In this way, it is possible to fully exploit locality while maintaining the extremely high compactness of the original LPNO-CCSD wavefunction. No terms are dropped from the CCSD equations and domains are chosen conservatively. The correlation energy loss due to the domains remains below <0.05%, which implies typically 15–20 but occasionally up to 30 atoms per domain on average. The new method has been given the acronym DLPNO-CCSD (“domain based LPNO-CCSD”). The method is nearly linear scaling with respect to system size. The original LPNO-CCSD method had three adjustable truncation thresholds that were chosen conservatively and do not need to be changed for actual applications. In the present treatment, no additional truncation parameters have been introduced. Any additional truncation is performed on the basis of the three original thresholds. There are no real-space cutoffs. Single excitations are truncated using singles-specific natural orbitals. Pairs are prescreened according to a multipole expansion of a pair correlation energy estimate based on local orbital specific virtual orbitals (LOSVs). Like its LPNO-CCSD predecessor, the method is completely of black box character and does not require any user adjustments. It is shown here that DLPNO-CCSD is as accurate as LPNO-CCSD while leading to computational savings exceeding one order of magnitude for larger systems. The largest calculations reported here featured >8800 basis functions and >450 atoms. In all larger test calculations done so far, the LPNO-CCSD step took less time than the preceding Hartree-Fock calculation, provided no approximations have been introduced in the latter. Thus, based on the present development reliable CCSD calculations on large molecules with unprecedented efficiency and accuracy are realized.
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31.15.bw Coupled-cluster theory

Molecular diffusion between walls with adsorption and desorption

Maximilien Levesque, Olivier Bénichou, and Benjamin Rotenberg

J. Chem. Phys. 138, 034107 (2013); http://dx.doi.org/10.1063/1.4775742 (3 pages)

Online Publication Date: 18 January 2013

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The time dependency of the diffusion coefficient of particles in porous media is an efficient probe of their geometry. The analysis of this quantity, measured, e.g., by nuclear magnetic resonance, can provide rich information pertaining to porosity, pore size distribution, permeability, and surface-to-volume ratio of porous materials. Nevertheless, in numerous if not all practical situations, transport is confined by walls where adsorption and desorption processes may occur. In this article, we derive explicitly the expression of the time-dependent diffusion coefficient between two confining walls in the presence of adsorption and desorption. We show that they strongly modify the time-dependency of the diffusion coefficient, even in this simple geometry. We finally propose several applications, from sorption rates measurements to the use as a reference for numerical implementations for more complex geometries.
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68.43.Mn Adsorption kinetics
76.60.-k Nuclear magnetic resonance and relaxation
68.43.Nr Desorption kinetics
61.43.Gt Powders, porous materials
68.43.Jk Diffusion of adsorbates, kinetics of coarsening and aggregation
back to top Advanced Experimental Techniques
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Multiple population-period transient spectroscopy (MUPPETS) in excitonic systems

Haorui Wu and Mark A. Berg

J. Chem. Phys. 138, 034201 (2013); http://dx.doi.org/10.1063/1.4773982 (16 pages)

Online Publication Date: 15 January 2013

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Time-resolved experiments with more than one period of incoherent time evolution are becoming increasingly accessible. When applied to a two-level system, these experiments separate homogeneous and heterogeneous contributions to kinetic dispersion, i.e., to nonexponential relaxation. Here, the theory of two-dimensional (2D) multiple population-period transient spectroscopy (MUPPETS) is extended to multilevel, excitonic systems. A nonorthogonal basis set is introduced to simplify pathway calculations in multilevel systems. Because the exciton and biexciton signals have different signs, 2D MUPPETS cleanly separates the exciton and biexciton decays. In addition to separating homogeneous and heterogeneous dispersion of the exciton, correlations between the exciton and biexciton decays are measurable. Such correlations indicate shared features in the two relaxation mechanisms. Examples are calculated as both 2D time decays and as 2D rate spectra. The effect of solvent heating (i.e., thermal gratings) is also calculated in multidimensional experiments on multilevel systems.
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82.20.Pm Rate constants, reaction cross sections, and activation energies
71.35.-y Excitons and related phenomena
back to top Atoms, Molecules, and Clusters

Attachment cross-sections of protonated and deprotonated water clusters

Sébastien Zamith, Grégoire de Tournadre, Pierre Labastie, and Jean-Marc L’Hermite

J. Chem. Phys. 138, 034301 (2013); http://dx.doi.org/10.1063/1.4775401 (4 pages)

Online Publication Date: 15 January 2013

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Attachment cross-sections of water molecules onto size selected protonated (H2O)nH+ and deprotonated (H2O)n − 1OH− water clusters have been measured in the size range n = 30–140 for 10 eV kinetic energy of the clusters in the laboratory frame. Within our experimental accuracy, the attachment cross-sections are found to have the same magnitude and size dependence for both species. It is shown that electrostatic interactions are likely to play a role even for the largest sizes investigated.
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36.40.Wa Charged clusters
36.40.Jn Reactivity of clusters

Velocity effects on the shape of pure H2O isolated lines: Complementary tests of the partially correlated speed-dependent Keilson-Storer model

H. Tran, N. H. Ngo, J.-M. Hartmann, R. R. Gamache, D. Mondelain, S. Kassi, A. Campargue, L. Gianfrani, A. Castrillo, E. Fasci, and F. Rohart

J. Chem. Phys. 138, 034302 (2013); http://dx.doi.org/10.1063/1.4774094 (8 pages)

Online Publication Date: 15 January 2013

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Complementary tests of the partially correlated speed-dependent Keilson-Storer (pCSDKS) model for the shape of isolated transition of pure water vapor [N. H. Ngo et al., J. Chem. Phys. 136, 154310 (2012)]10.1063/1.4704675 are made using new measurements. The latter have been recorded using a high sensitivity cavity ring down spectrometer, for seven self-broadened H2O lines in the 1.6 μm region at room temperature and for pressures from 0.5 to 15 Torr. Furthermore, the H218O spectra of [M. D. De Vizia et al., Phys. Rev. A 83, 052506 (2011)]10.1103/PhysRevA.83.052506 in the 1.38 μm region, measured at 273.15 K and for pressures from 0.3 to 3.75 Torr have also been used for comparison with the model. Recall that the pCSDKS model takes into account the collision-induced velocity changes, the speed dependences of the broadening and shifting coefficients as well as the partial correlation between velocity and rotational-state changes. All parameters of the model have been fixed at values previously determined, except for a scaling factor applied to the input speed-dependent line broadening. Comparisons between predictions and experiments have been made by looking at the results obtained when fitting the calculated and measured spectra by Voigt profiles. The good agreement obtained for all considered lines, at different temperature and pressure conditions, confirms the consistency and the robustness of the model. Limiting cases of the model have been then derived, showing the influence of different contributions to the line shape.
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33.70.Jg Line and band widths, shapes, and shifts
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors

Hybrid chromophore/template nanostructures: A customizable platform material for solar energy storage and conversion

Alexie M. Kolpak and Jeffrey C. Grossman

J. Chem. Phys. 138, 034303 (2013); http://dx.doi.org/10.1063/1.4773306 (6 pages)

Online Publication Date: 15 January 2013

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Challenges with cost, cyclability, and/or low energy density have largely prevented the development of solar thermal fuels, a potentially attractive alternative energy technology based on molecules that can capture and store solar energy as latent heat in a closed cycle. In this paper, we present a set of novel hybrid photoisomer/template solar thermal fuels that can potentially circumvent these challenges. Using first-principles computations, we demonstrate that these fuels, composed of organic photoisomers bound to inexpensive carbon-based templates, can reversibly store solar energy at densities comparable to Li-ion batteries. Furthermore, we show that variation of the template material in combination with the photoisomer can be used to optimize many of the key performance metrics of the fuel—i.e., the energy density, the storage lifetime, the temperature of the output heat, and the efficiency of the solar-to-heat conversion. Our work suggests that the solar thermal fuels concept can be translated into a practical and highly customizable energy storage and conversion technology.
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88.40.-j Solar energy
84.60.-h Direct energy conversion and storage

Heat capacities of mass selected deprotonated water clusters

Sébastien Zamith, Pierre Labastie, and Jean-Marc L’Hermite

J. Chem. Phys. 138, 034304 (2013); http://dx.doi.org/10.1063/1.4774408 (6 pages)

Online Publication Date: 15 January 2013

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Heat capacities of mass selected deprotonated water clusters (H2O)n−1OH− have been measured in the size range n = 48–118, as a function of temperature. We have found that they undergo a melting-like transition in the range 110–130 K. The transition temperature is size dependent with a strong correlation with the dissociation energy around the shell closure at n = 55.
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65.20.Jk Studies of thermodynamic properties of specific liquids
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
64.70.dj Melting of specific substances

A simple but accurate potential for the naphthalene-argon complex: Applications to collisional energy transfer and matrix isolated IR spectroscopy

F. Calvo, Cyril Falvo, and Pascal Parneix

J. Chem. Phys. 138, 034305 (2013); http://dx.doi.org/10.1063/1.4773469 (10 pages)

Online Publication Date: 16 January 2013

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An explicit polarizable potential for the naphthalene-argon complex has been derived assuming only atomic contributions, aiming at large scale simulations of naphthalene under argon environment. The potential was parametrized from dedicated quantum chemical calculations at the CCSD(T) level, and satisfactorily reproduces available structural and energetic properties. Combining this potential with a tight-binding model for naphthalene, collisional energy transfer is studied by means of dedicated molecular dynamics simulations, nuclear quantum effects being accounted for in the path-integral framework. Except at low target temperature, nuclear quantum effects do not alter the average energies transferred by the collision or the collision duration. However, the distribution of energy transferred is much broader in the quantum case due to the significant zero-point energy and the higher density of states. Using an ab initio potential for the Ar-Ar interaction, the IR absorption spectrum of naphthalene solvated by argon clusters or an entire Ar matrix is computed via classical and centroid molecular dynamics. The classical spectra exhibit variations with growing argon environment that are absent from quantum spectra. This is interpreted by the greater fluxional character experienced by the argon atoms due to vibrational delocalization.
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31.50.-x Potential energy surfaces
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Ea Infrared spectra
31.15.aq Strongly correlated electron systems: generalized tight-binding method
31.15.bw Coupled-cluster theory
31.15.xv Molecular dynamics and other numerical methods

Non-equivalent carbon atoms in the resonant inelastic soft X-ray scattering map of cysteine

F. Meyer, L. Weinhardt, M. Blum, M. Bär, R. G. Wilks, W. Yang, C. Heske, and F. Reinert

J. Chem. Phys. 138, 034306 (2013); http://dx.doi.org/10.1063/1.4774059 (5 pages)

Online Publication Date: 17 January 2013

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Resonant inelastic soft x-ray scattering (RIXS) was used to study the electronic structure of solid cysteine films. A RIXS map approach, i.e., plotting the x-ray emission intensity as a function of excitation and emission energy, allows us to separate the contributions of the three chemically non-equivalent carbon atoms in cysteine. In particular, we can identify orbitals localized near the photoexcited atoms, as well as orbitals that are delocalized over the entire molecule.
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87.15.M- Spectra of biomolecules
33.80.-b Photon interactions with molecules

Ratchet effect and amplitude dependence of phase locking in a two-dimensional Frenkel-Kontorova model

Cang-Long Wang, Jasmina Tekić, Wen-Shan Duan, Zhi-Gang Shao, and Lei Yang

J. Chem. Phys. 138, 034307 (2013); http://dx.doi.org/10.1063/1.4776226 (5 pages)

Online Publication Date: 17 January 2013

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We demonstrate the ratchet and phase locking effects in a two-dimensional overdamped Frenkel-Kontorova model with a square symmetric periodic substrate when both a longitudinal dc drive and a circular ac drive are applied. Besides the harmonic steps, the large half integer steps can also clearly be seen in the longitudinal (x) direction. These half integer steps are directly correlated to the appearance of positive and negative ratchet effects in the transverse (y) direction due to the symmetry breaking in the combination of the dc and ac drives. The angle between the net displacement and the longitudinal direction is analytically obtained in a single period of the ac drive. In the examination of the amplitude dependence of the ac drive, the maxima decrease monotonically with the amplitude, while the anomalies occur for the critical depinning force and the harmonic steps due to the spatial symmetry breaking of orbits in the presence of the ac drive.
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05.60.-k Transport processes
45.50.Jf Few- and many-body systems
02.30.Px Abstract harmonic analysis
05.50.+q Lattice theory and statistics (Ising, Potts, etc.)

A photoelectron spectroscopy and density functional study of di-tantalum boride clusters: Ta2Bx− (x = 2–5)

Lu Xie, Wei-Li Li, Constantin Romanescu, Xin Huang, and Lai-Sheng Wang

J. Chem. Phys. 138, 034308 (2013); http://dx.doi.org/10.1063/1.4776769 (11 pages)

Online Publication Date: 17 January 2013

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The structural and electronic properties for di-tantalum boride clusters Ta2Bx− (x = 2–5) were investigated using photoelectron spectroscopy and density functional calculations. The photoelectron spectra for Ta2Bx− (x = 2–5) are obtained at several photon energies with rich spectral features. Density functional theory calculations are performed at the BP86 level to search for the global minima of both the anionic and neutral clusters. The calculated vertical electron detachment energies for the global minimum and low-lying isomers are compared with the experimental data. Strong boron-boron bonding is found to dominate the lowest energy structures of Ta2Bx− and Ta2Bx (x = 2–5), which are shown to be bipyramidal with the boron atoms forming an equatorial belt around the Ta–Ta dimer. Strong Ta–Ta bonding is observed in Ta2Bx− and Ta2Bx for x = 2–4, whereas the Ta–Ta distance is increased significantly in Ta2B5− and Ta2B5.
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36.40.Cg Electronic and magnetic properties of clusters
36.40.Mr Spectroscopy and geometrical structure of clusters
31.15.E- Density-functional theory
33.60.+q Photoelectron spectra
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
33.80.Eh Autoionization, photoionization, and photodetachment

Collection efficiency of photoelectrons injected into near- and supercritical argon gas

A. F. Borghesani and P. Lamp

J. Chem. Phys. 138, 034309 (2013); http://dx.doi.org/10.1063/1.4775594 (13 pages)

Online Publication Date: 17 January 2013

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Injection of photoelectrons into gaseous or liquid dielectrics is a widely used technique to produce cold plasmas in weakly ionized systems for investigating the transport properties of electrons. We report measurements of the collection efficiency of photoelectrons injected into dense argon gas for T = 152.7  K, close to the critical temperature Tc ≈ 150.9  K, and for T = 200.0  K. The high-field data agree with the Young-Bradbury model and with previous measurements below Tc and at an intermediate temperature above Tc. The effective, density-dependent electron-atom momentum transfer scattering cross section can be deduced. However, the weak-field data near Tc show large deviations from the theoretical model. We show that the electron behavior at weak field is influenced by electrostriction effects that are only important near the critical point.
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51.50.+v Electrical properties (ionization, breakdown, electron and ion mobility, etc.)
52.20.Fs Electron collisions
52.20.Hv Atomic, molecular, ion, and heavy-particle collisions
52.25.Mq Dielectric properties
52.50.-b Plasma production and heating
34.80.Dp Atomic excitation and ionization
back to top Liquids, Glasses, and Crystals

Structural study of Al2O3-Na2O-CaO-P2O5 bioactive glasses as a function of aluminium content

J. M. Smith, S. P. King, E. R. Barney, J. V. Hanna, R. J. Newport, and D. M. Pickup

J. Chem. Phys. 138, 034501 (2013); http://dx.doi.org/10.1063/1.4774330 (7 pages)

Online Publication Date: 15 January 2013

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Calcium phosphate based biomaterials are extensively used in the context of tissue engineering: small changes in composition can lead to significant changes in properties allowing their use in a wide range of applications. Samples of composition (Al2O3)x(Na2O)0.11-x(CaO)0.445(P2O5)0.445, where x = 0, 0.03, 0.05, and 0.08, were prepared by melt quenching. The atomic-scale structure has been studied using neutron diffraction and solid state 27Al MAS NMR, and these data have been rationalised with the determined density of the final glass product. With increasing aluminium concentration the density increases initially, but beyond about 3 mol. % Al2O3 the density starts to decrease. Neutron diffraction data show a concomitant change in the aluminium speciation, which is confirmed by 27Al MAS NMR studies. The NMR data reveal that aluminium is present in 4, 5, and 6-fold coordination and that the relative concentrations of these environments change with increasing aluminium concentration. Materials containing aluminium in 6-fold coordination tend to have higher densities than analogous materials with the aluminium found in 4-fold coordination. Thus, the density changes may readily be explained in terms of an increase in the relative concentration of 4-coordinated aluminium at the expense of 6-fold aluminium as the Al2O3 content is increased beyond 3 mol. %.
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61.43.Fs Glasses
76.60.-k Nuclear magnetic resonance and relaxation
81.20.-n Methods of materials synthesis and materials processing
81.40.Gh Other heat and thermomechanical treatments

Landau-Placzek ratio for heat density dynamics and its application to heat capacity of liquids

Taras Bryk, Giancarlo Ruocco, and Tullio Scopigno

J. Chem. Phys. 138, 034502 (2013); http://dx.doi.org/10.1063/1.4774406 (5 pages) | Cited 1 time

Online Publication Date: 15 January 2013

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Exact relation for contributions to heat capacity of liquids is obtained from hydrodynamic theory. It is shown from analysis of the long-wavelength limit of heat density autocorrelation functions that the heat capacity of simple liquids is represented as a sum of two contributions due to “phonon-like” collective excitations and heat relaxation. The ratio of both contributions being the analogy of Landau-Placzek ratio for heat processes depends on the specific heats ratio. The theory of heat density autocorrelation functions in liquids is verified by computer simulations. Molecular dynamics simulations for six liquids having the ratio of specific heats γ in the range 1.1–2.3, were used for evaluation of the heat density autocorrelation functions and predicted Landau-Placzek ratio for heat processes. The dependence of contributions from collective excitations and heat relaxation process to specific heat on γ is shown to be in excellent agreement with the theory.
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65.20.-w Thermal properties of liquids
61.20.Ja Computer simulation of liquid structure

Generalized extended Navier-Stokes theory: Correlations in molecular fluids with intrinsic angular momentum

J. S. Hansen, Peter J. Daivis, Jeppe C. Dyre, B. D. Todd, and Henrik Bruus

J. Chem. Phys. 138, 034503 (2013); http://dx.doi.org/10.1063/1.4774095 (9 pages)

Online Publication Date: 16 January 2013

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The extended Navier-Stokes theory accounts for the coupling between the translational and rotational molecular degrees of freedom. In this paper, we generalize this theory to non-zero frequencies and wavevectors, which enables a new study of spatio-temporal correlation phenomena present in molecular fluids. To discuss these phenomena in detail, molecular dynamics simulations of molecular chlorine are performed for three different state points. In general, the theory captures the behavior for small wavevector and frequencies as expected. For example, in the hydrodynamic regime and for molecular fluids with small moment of inertia like chlorine, the theory predicts that the longitudinal and transverse intrinsic angular velocity correlation functions are almost identical, which is also seen in the molecular dynamics simulations. However, the theory fails at large wavevector and frequencies. To account for the correlations at these scales, we derive a phenomenological expression for the frequency dependent rotational viscosity and wavevector and frequency dependent longitudinal spin viscosity. From this we observe a significant coupling enhancement between the molecular angular velocity and translational velocity for large frequencies in the gas phase; this is not observed for the supercritical fluid and liquid state points.
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47.10.ad Navier-Stokes equations
47.11.Mn Molecular dynamics methods
47.32.Ef Rotating and swirling flows
66.20.Cy Theory and modeling of viscosity and rheological properties, including computer simulation

Lattice summations for spread out particles: Applications to neutral and charged systems

D. M. Heyes and A. C. Brańka

J. Chem. Phys. 138, 034504 (2013); http://dx.doi.org/10.1063/1.4775367 (13 pages)

Online Publication Date: 17 January 2013

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This work is concerned with the lattice energy of periodic assemblies of mass and charge distributions of the form, exp (−αp2), where α is an adjustable positive variable and math is the vector from the lattice site or average position. The energy of interaction between two distributions is the density-weighted integral of the interactions between the volume elements of each distribution. Reciprocal space lattice summation formulas derived for particles represented by gaussian smeared-out density distributions are applied to the gaussian potential and a bounded version of the soft-sphere potential for a range of exponents. Two types of spatial broadening are considered, continuous or physical broadening (PB) and broadening resulting from the time average of point particle positions, so-called “time” broadening (TB). For neutral mass distributions a reciprocal space lattice summation formula is derived which is applied to the bounded soft-sphere potential. For the charged systems, the methodology described in Heyes [J. Chem. Phys. 74, 1924 (1981)10.1063/1.441285] is used, which for the PB case gives the Ewald-like formulas derived by Gingrich and Wilson [Chem. Phys. Lett. 500, 178 (2010)10.1016/j.cplett.2010.10.010] using a different method. Another expression for the lattice energy of the spread out charge distributions is derived which is cast entirely in terms of a summation over the reciprocal lattice vectors, without the arbitrary charge spreading function used in the Ewald method. The effects of charge spreading on a generalized definition of the Madelung constant (M) for a selection of crystal lattices are shown to be insignificant for route mean square displacements up to values typical of melting of an ionic crystal. When the length scale of the charge distribution becomes comparable to or greater than the mean inter particle spacing, however, the effects of charge broadening on the lattice energy are shown to be significant. In the PB case, M → 0 for the uniform charge density or α → 0 limit, and M ultimately becomes negative in the TB case for a large enough root mean square displacement (or small enough α).
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61.50.Lt Crystal binding; cohesive energy

Free energy surface of ST2 water near the liquid-liquid phase transition

Peter H. Poole, Richard K. Bowles, Ivan Saika-Voivod, and Francesco Sciortino

J. Chem. Phys. 138, 034505 (2013); http://dx.doi.org/10.1063/1.4775738 (7 pages) | Cited 1 time

Online Publication Date: 17 January 2013

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We carry out umbrella sampling Monte Carlo simulations to evaluate the free energy surface of the ST2 model of water as a function of two order parameters, the density and a bond-orientational order parameter. We approximate the long-range electrostatic interactions of the ST2 model using the reaction-field method. We focus on state points in the vicinity of the liquid-liquid critical point proposed for this model in earlier work. At temperatures below the predicted critical temperature we find two basins in the free energy surface, both of which have liquid-like bond orientational order, but differing in density. The pressure and temperature dependence of the shape of the free energy surface is consistent with the assignment of these two basins to the distinct low density and high density liquid phases previously predicted to occur in ST2 water.
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68.03.Cd Surface tension and related phenomena
65.20.De General theory of thermodynamic properties of liquids, including computer simulation
64.70.Ja Liquid-liquid transitions

Distribution of melting times and critical droplet in kinetic Monte Carlo and molecular dynamics

C. A. Lemarchand

J. Chem. Phys. 138, 034506 (2013); http://dx.doi.org/10.1063/1.4775773 (10 pages)

Online Publication Date: 18 January 2013

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A kinetic Monte Carlo model on a lattice, based on a reaction-like mechanism, is used to investigate the microscopic properties of the homogeneous melting of a metastable crystal. The kinetic Monte Carlo model relies on nearest-neighbors interactions and a few relevant dynamical parameters. To examine the reliability of the model, careful comparison with molecular dynamics simulations of a hard sphere crystal is drawn. A criterion on the critical nature of a microscopic configuration is deduced from the bimodal character of the probability density function of melting time. For kinetic Monte Carlo simulations with dynamical parameter values which fit the molecular dynamics results, the number of liquid sites of the critical droplet is found to be smaller than 300 and the ability of the critical droplet to invade the entire system is shown to be independent of the droplet shape as long as this droplet remains compact. In kinetic Monte Carlo simulations, the size of the critical droplet is independent of the system size. Molecular dynamics evidences a more complex dependence of melting time on system size, which reveals non-trivial finite size effects.
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61.20.Ja Computer simulation of liquid structure
64.70.dj Melting of specific substances
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