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28 May 2011

Volume 134, Issue 20, Articles (20xxxx)

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J. Chem. Phys. 134, 204101 (2011); http://dx.doi.org/10.1063/1.3583816 (12 pages)

Petra Ruth Kaprálová-Žďánská
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Communication: Atomic and molecular Rydbergs from water

Jyoti Rajput and C. P. Safvan

J. Chem. Phys. 134, 201101 (2011); http://dx.doi.org/10.1063/1.3593199 (4 pages)

Online Publication Date: 23 May 2011

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We report the formation of energetic neutral Rydberg hydrogen atoms and transient Rydberg molecular ions, [(H2O)q+]☆ in ion-impact dissociation of isolated water molecules. The kinetic energy spectra of the neutral Rydberg H atoms are determined from the complete study of (H⋆, H+, O+) dissociation channel. This channel of water dissociation is suggested as a possible additional source of the energetic neutrals detected in upper atmospheres of extra solar planets, and of slow electrons which are known to play a major role in radiation induced damage to living cells.
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34.80.Ht Dissociation and dissociative attachment
31.50.Df Potential energy surfaces for excited electronic states
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Communication: Spectroscopic measurements for HfF+ of relevance to the investigation of fundamental constants

Beau J. Barker, Ivan O. Antonov, Vladimir E. Bondybey, and Michael C. Heaven

J. Chem. Phys. 134, 201102 (2011); http://dx.doi.org/10.1063/1.3595473 (4 pages) | Cited 2 times

Online Publication Date: 24 May 2011

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The properties of the HfF+ cation are thought to be well-suited for investigations of the electron electric dipole moment (eEDM) and temporal variations of the fine structure constant. Precision spectroscopic measurements involving the X1Σ+ and low-lying 3Δ1 states have been proposed to measure both. Due to the lack of data for HfF+, the design of these experiments has relied entirely on the predictions of electronic structure calculations. Spectroscopic characterizations of the X1Σ+, 3Δ1, 3Δ2 and 3Δ3 states are reported. The results further support the contention that HfF+ is a viable candidate for eEDM measurements. The spacings between adjacent X1Σ+ and 3Δ1 levels are found to be less favorable for the proposed studies of the fine structure constant.
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33.20.Ea Infrared spectra
33.20.Sn Rotational analysis
33.20.Tp Vibrational analysis
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.15.Pw Fine and hyperfine structure
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Communication: Equation of state of hard oblate ellipsoids by replica exchange Monte Carlo

G. Odriozola and F. de J. Guevara-Rodríguez

J. Chem. Phys. 134, 201103 (2011); http://dx.doi.org/10.1063/1.3596728 (4 pages) | Cited 2 times

Online Publication Date: 25 May 2011

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We implemented the replica exchange Monte Carlo technique to produce the equation of state of hard 1:5 aspect-ratio oblate ellipsoids for a wide density range. For this purpose, we considered the analytical approximation of the overlap distance given by Bern and Pechukas and the exact numerical solution given by Perram and Wertheim. For both cases we capture the expected isotropic-nematic transition at low densities and a nematic-crystal transition at larger densities. For the exact case, these transitions occur at the volume fraction 0.341, and in the interval 0.584 − 0.605, respectively.
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64.30.-t Equations of state of specific substances
64.70.mf Theory and modeling of specific liquid crystal transitions, including computer simulation
61.30.Cz Molecular and microscopic models and theories of liquid crystal structure
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Communication: Mode-specific photodissociation of vibrationally excited pyrrole

Michael Epshtein, Alexander Portnov, Salman Rosenwaks, and Ilana Bar

J. Chem. Phys. 134, 201104 (2011); http://dx.doi.org/10.1063/1.3596747 (4 pages) | Cited 1 time

Online Publication Date: 31 May 2011

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Laser-based spectroscopies coupled with molecular beam techniques facilitated the monitoring of H fragments released in ultraviolet photodissociation of pre-excited isoenergetic vibrational levels of pyrrole. Most noticeably, there was an order of magnitude larger reactivity for an eigenstate primarily consisting of two quanta of ring deformation than for another with one quantum of symmetric C–H stretch. The dynamics, the intramolecular interactions controlling the energy flow, and the mode-selectivity within a medium-sized, ten atom molecule, is discussed.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
33.20.Tp Vibrational analysis
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Communication: Rovibrationally selected study of the N2+(X; v+ = 1, N+ = 0−8) + Ar charge transfer reaction using the vacuum ultraviolet laser pulsed field ionization-photoion method

Yih Chung Chang, Hong Xu, Yuntao Xu, Zhou Lu, Yu-Hui Chiu, Dale J. Levandier, and C. Y. Ng

J. Chem. Phys. 134, 201105 (2011); http://dx.doi.org/10.1063/1.3596748 (4 pages)

Online Publication Date: 31 May 2011

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By employing an electric field pulsing scheme for vacuum ultraviolet laser pulsed field ionization-photoion (PFI-PI) measurements, we have been able to prepare a rovibrationally selected PFI-PI beam of N2+(v+ = 1, N+) with not only high intensity and high quantum state purity, but also high kinetic energy resolution, allowing absolute total cross sections [σ(v+ = 1, N+)] for the N2+(X; v+ = 1, N+) + Ar, N+ = 0–8 charge transfer reaction to be measured at center-of-mass collision energies (Ecm) down to thermal energies. The σ(v+ = 1, N+ = 0–8) values determined at Ecm = 0.04–10.00 eV are in good agreement with the theoretical predictions based on the Landau-Zener-Stückelberg formulism. Taking into account the experimental uncertainties, the σ(v+ = 1, N+), N+ = 0–8, measured at Ecm = 1.56 eV are found to be independent of N+.
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82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
33.20.Ni Vacuum ultraviolet spectra
33.20.Vq Vibration-rotation analysis
33.80.Eh Autoionization, photoionization, and photodetachment
82.20.Kh Potential energy surfaces for chemical reactions
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Communication: Electric dipole moment and hyperfine interaction of tungsten monocarbide, WC

Fang Wang and Timothy C. Steimle

J. Chem. Phys. 134, 201106 (2011); http://dx.doi.org/10.1063/1.3595469 (4 pages) | Cited 2 times

Online Publication Date: 31 May 2011

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The [17.6]2← X3Δ1(1,0) band of tungsten monocarbide, WC, was recorded using laser induced fluorescence both field-free and in the presence of a static electric field. The Stark induced shifts of the R(1) and Q(2) lines were analyzed to produce values for the permanent electric dipole moments, μ, of 3.90 ± 0.04 D and 2.57 ± 0.04 D for the X3Δ1(v = 0) and [17.6]2(v = 1) states, respectively. An upper limit to the Ω-doubling parameter, mathΔ, of 2 MHz is established. The 183W(I = 1/2) Fermi contact parameter, bF, for the X3Δ1(v = 0) is estimated to be 1363 MHz. The results are compared with theoretical predictions and a molecular orbital correlation description.
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31.30.Gs Hyperfine interactions and isotope effects
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
33.50.Dq Fluorescence and phosphorescence spectra
33.57.+c Magneto-optical and electro-optical spectra and effects
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A study of complex scaling transformation using the Wigner representation of wavefunctions

Petra Ruth Kaprálová-Žďánská

J. Chem. Phys. 134, 204101 (2011); http://dx.doi.org/10.1063/1.3583816 (12 pages)

Online Publication Date: 23 May 2011

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The complex scaling operator exp(−θmathmath/ℏ), being a foundation of the complex scaling method for resonances, is studied in the Wigner phase-space representation. It is shown that the complex scaling operator behaves similarly to the squeezing operator, rotating and amplifying Wigner quasi-probability distributions of the respective wavefunctions. It is disclosed that the distorting effect of the complex scaling transformation is correlated with increased numerical errors of computed resonance energies and widths. The behavior of the numerical error is demonstrated for a computation of CO2 + vibronic resonances.
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31.15.-p Calculations and mathematical techniques in atomic and molecular physics
02.50.Ng Distribution theory and Monte Carlo studies

Breakdown of the pseudopotential approximation for magnetizabilities and electric multipole moments: Test calculations for Au, AuF, and Snn cluster (n ⩽ 20)

Peter Schwerdtfeger, Behnam Assadollahzadeh, Urban Rohrmann, Rolf Schäfer, and James R. Cheeseman

J. Chem. Phys. 134, 204102 (2011); http://dx.doi.org/10.1063/1.3591338 (11 pages) | Cited 2 times

Online Publication Date: 23 May 2011

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The response of the electronic wavefunction to an external electric or magnetic field is widely considered to be a typical valence property and should, therefore, be adequately described by accurately adjusted pseudopotentials, especially if a small-core definition is used within this approximation. In this paper we show for atomic Au and Au+, as well as for the molecule AuF and tin clusters, that in contrast to the case of the static electric dipole polarizability or the electric dipole moment, core contributions to the static magnetizability are non-negligible, and can therefore lead to erroneous results within the pseudopotential approximation. This error increases with increasing size of the core chosen. For tin clusters, which are of interest in ongoing molecular beam experiments currently carried out by the Darmstadt group, the diamagnetic and paramagnetic isotropic components of the magnetizability tensor almost cancel out and large-core pseudopotentials do not even predict the correct sign for this property due to erroneous results in both the diamagnetic and (more importantly) the paramagnetic terms. Hence, all-electron calculations or pseudopotentials with very small cores are required to adequately predict magnetizabilities for atoms, molecules and the solid state, making it computationally more difficult to obtain this quantity for future investigations in heavy atom containing molecules or clusters. We also demonstrate for this property that all-electron density functional calculations are quite robust and give results close to wavefunction based methods for the atoms and molecules studied here.
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36.40.Cg Electronic and magnetic properties of clusters
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
32.30.-r Atomic spectra

Integrating steepest-descent reaction pathways for large molecules

Hrant P. Hratchian and Michael J. Frisch

J. Chem. Phys. 134, 204103 (2011); http://dx.doi.org/10.1063/1.3593456 (9 pages)

Online Publication Date: 23 May 2011

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Exploring potential energy surfaces of large molecular systems can be quite challenging due to the increased number of nuclear degrees of freedom. Many techniques that are well-suited for small and moderate size systems require diagonalization of the energy second-derivative matrix. Since the cost of this step scales as O(Natoms3) (where Natoms is the number of atomic centers), such methods quickly become infeasible and are eventually rendered cost prohibitive. In this work, the recently developed Euler-based predictor–corrector reaction path integration method [H. P. Hratchian, M. J. Frisch, and H. B. Schlegel, J. Chem. Phys. 133, 224101 (2010)]10.1063/1.3514202 is enhanced and proposed as a useful alternative to conventional reaction path following schemes in studies on very large systems. Because this integrator does not require Hessian diagonalization, the O(Natoms3) bottleneck afflicting other approaches is completely avoided. The effectiveness of the integrator in large system studies is demonstrated with an enzyme-catalyzed reaction employing an ONIOM (QM:MM) model chemistry and involving 5368 atomic centers.
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82.30.Vy Homogeneous catalysis in solution, polymers and zeolites
82.20.Wt Computational modeling; simulation
82.20.Kh Potential energy surfaces for chemical reactions
82.20.Hf Product distribution
82.20.Ej Quantum theory of reaction cross section

Adiabatic versus diabatic descriptions of the lowest Rydberg and valence 1Σ+ states of HCl

H. Lefebvre-Brion, H. P. Liebermann, and G. J. Vázquez

J. Chem. Phys. 134, 204104 (2011); http://dx.doi.org/10.1063/1.3590360 (7 pages) | Cited 1 time

Online Publication Date: 23 May 2011

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In this contribution we first report new ab initio self–consistent field configuration interaction calculations of the first excited adiabatic potential of 1Σ+ symmetry, the 21Σ+ or B1Σ+ state, which presents two minima and can thus be seen as made up of the Rydberg E1Σ+ and the valence V1Σ+ states. Based on the computed 21Σ+ potential, we devised a theoretical procedure to compute the vibronic structure in order to try to explain the energy levels observed in the region above 76 254.4 cm−1 which display an irregular vibrational structure, indicative of spectral perturbations. We try to find out which representation of the electronic states, the diabatic or the adiabatic one, is best suited to replicate the lowest observed vibronic levels of the E and V states. To this end, we deduce, from the 21Σ+ potential and its complementary adiabatic potential, two diabatic potentials. We then carry out a coupled equation treatment based on these diabatic potentials. The results of this treatment indicate that, in the present case, the adiabatic representation is better than the diabatic one to describe the observed vibronic levels. This is due, as expected, to the existence of a strong electrostatic interaction between the two diabatic potentials.
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31.15.am Relativistic configuration interaction (CI) and many-body perturbation calculations
31.15.vj Electron correlation calculations for atoms and ions: excited states
31.15.xr Self-consistent-field methods
33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions

Markov state models based on milestoning

Christof Schütte, Frank Noé, Jianfeng Lu, Marco Sarich, and Eric Vanden-Eijnden

J. Chem. Phys. 134, 204105 (2011); http://dx.doi.org/10.1063/1.3590108 (15 pages) | Cited 1 time

Online Publication Date: 24 May 2011

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Markov state models (MSMs) have become the tool of choice to analyze large amounts of molecular dynamics data by approximating them as a Markov jump process between suitably predefined states. Here we investigate “Core Set MSMs,” a new type of MSMs that build on metastable core sets acting as milestones for tracing the rare event kinetics. We present a thorough analysis of Core Set MSMs based on the existing milestoning framework, Bayesian estimation methods and Transition Path Theory (TPT). We show that Core Set MSMs can be used to extract phenomenological rate constants between the metastable sets of the system and to approximate the evolution of certain key observables. The performance of Core Set MSMs in comparison to standard MSMs is analyzed and illustrated on a toy example and in the context of the torsion angle dynamics of alanine dipeptide.
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87.15.B- Structure of biomolecules
87.15.ap Molecular dynamics simulation
36.20.Ey Conformation (statistics and dynamics)
33.20.Tp Vibrational analysis
33.15.Bh General molecular conformation and symmetry; stereochemistry
31.15.xv Molecular dynamics and other numerical methods

Fourth-order relativistic corrections to electrical first-order properties using direct perturbation theory

Stella Stopkowicz and Jürgen Gauss

J. Chem. Phys. 134, 204106 (2011); http://dx.doi.org/10.1063/1.3587633 (9 pages) | Cited 2 times

Online Publication Date: 25 May 2011

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In this work, we present relativistic corrections to first-order electrical properties obtained using fourth-order direct perturbation theory (DPT4) at the Hartree-Fock level. The considered properties, i.e., dipole moments and electrical-field gradients, have been calculated using numerical differentiation techniques based on a recently reported DPT4 code for energies [S. Stopkowicz and J. Gauss, J. Chem. Phys. 134, 064114 (2011)]10.1063/1.3522766. For the hydrogen halides HX, X=F, Cl, Br, I, and At, we study the convergence of the scalar-relativistic contributions by comparing the computed DPT corrections to results from spin-free Dirac-Hartree-Fock calculations. Furthermore, since in the DPT series spin-orbit contributions first appear at fourth order, we investigate their magnitude and judge the performance of the DPT4 treatment by means of Dirac-Hartree-Fock benchmark calculations. Finally, motivated by experimental investigations of the molecules CH2FBr, CHF2Br, and CH2FI, we present theoretical results for their halogen quadrupole-coupling tensors and give recommendations concerning the importance of higher-order scalar-relativistic and spin-orbit corrections.
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31.15.aj Relativistic corrections, spin-orbit effects, fine structure; hyperfine structure
31.15.xp Perturbation theory

Third-order corrections to random-phase approximation correlation energies

Andreas Heßelmann

J. Chem. Phys. 134, 204107 (2011); http://dx.doi.org/10.1063/1.3590916 (12 pages) | Cited 1 time

Online Publication Date: 26 May 2011

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Several random-phase approximation (RPA) correlation methods were compared in third order of perturbation theory. While all of the considered approaches are exact in second order of perturbation theory, it is found that their corresponding third-order correlation energy contributions strongly differ from the exact third-order correlation energy contribution due to missing interactions of the particle-particle−hole-hole type. Thus a simple correction method is derived which makes the different RPA methods also exact to third-order of perturbation theory. By studying the reaction energies of 16 chemical reactions for 21 small organic molecules and intermolecular interaction energies of 23 intermolecular complexes comprising weakly bound and hydrogen-bridged systems, it is found that the third-order correlation energy correction considerably improves the accuracy of RPA methods if compared to coupled-cluster singles doubles with perturbative triples as a reference.
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31.15.xp Perturbation theory

Configuration selection within vibrational multiconfiguration self-consistent field theory: Application to bridged lithium compounds

Sandra Heislbetz, Florian Pfeiffer, and Guntram Rauhut

J. Chem. Phys. 134, 204108 (2011); http://dx.doi.org/10.1063/1.3593714 (6 pages) | Cited 1 time

Online Publication Date: 26 May 2011

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A configuration selection scheme has been used to speed up vibrational multiconfiguration self-consistent field calculations. Deviations with respect to reference calculations were found to be negligible while yielding an acceleration of about two orders of magnitude. Its application to bridged lithium compounds (Li2H2, Li2F2, Li2O2, and Li3F3) based on high-level coupled-cluster potential energy surfaces provides accurate vibrational transitions for all fundamental modes. The explicit inclusion of 4-mode couplings was found to be important for Li2H2.
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33.20.Tp Vibrational analysis
33.15.Bh General molecular conformation and symmetry; stereochemistry
31.50.-x Potential energy surfaces
31.15.xr Self-consistent-field methods
31.15.bw Coupled-cluster theory

Electronic coupling calculation and pathway analysis of electron transfer reaction using ab initio fragment-based method. I. FMO–LCMO approach

Hirotaka Nishioka and Koji Ando

J. Chem. Phys. 134, 204109 (2011); http://dx.doi.org/10.1063/1.3594100 (12 pages)

Online Publication Date: 26 May 2011

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By making use of an ab initio fragment-based electronic structure method, fragment molecular orbital–linear combination of MOs of the fragments (FMO–LCMO), developed by Tsuneyuki et al. [Chem. Phys. Lett. 476, 104 (2009)]10.1016/j.cplett.2009.05.069, we propose a novel approach to describe long-distance electron transfer (ET) in large system. The FMO–LCMO method produces one-electron Hamiltonian of whole system using the output of the FMO calculation with computational cost much lower than conventional all-electron calculations. Diagonalizing the FMO–LCMO Hamiltonian matrix, the molecular orbitals (MOs) of the whole system can be described by the LCMOs. In our approach, electronic coupling TDA of ET is calculated from the energy splitting of the frontier MOs of whole system or perturbation method in terms of the FMO–LCMO Hamiltonian matrix. Moreover, taking into account only the valence MOs of the fragments, we can considerably reduce computational cost to evaluate TDA. Our approach was tested on four different kinds of model ET systems with non-covalent stacks of methane, non-covalent stacks of benzene, trans-alkanes, and alanine polypeptides as their bridge molecules, respectively. As a result, it reproduced reasonable TDA for all cases compared to the reference all-electron calculations. Furthermore, the tunneling pathway at fragment-based resolution was obtained from the tunneling current method with the FMO–LCMO Hamiltonian matrix.
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87.15.R- Reactions and kinetics
87.15.K- Molecular interactions; membrane-protein interactions
87.15.Pc Electronic and electrical properties
87.15.ht Ultrafast dynamics; charge transfer
82.39.Jn Charge (electron, proton) transfer in biological systems

A simple polarizable continuum solvation model for electrolyte solutions

Adrian W. Lange and John M. Herbert

J. Chem. Phys. 134, 204110 (2011); http://dx.doi.org/10.1063/1.3592372 (15 pages)

Online Publication Date: 26 May 2011

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We propose a Debye-Hückel-like screening model (DESMO) that generalizes the familiar conductor-like screening model (COSMO) to solvents with non-zero ionic strength and furthermore provides a numerical generalization of the Debye-Hückel model that is applicable to non-spherical solute cavities. The numerical implementation of DESMO is based upon the switching/Gaussian (SWIG) method for smooth cavity discretization, which we have recently introduced in the context of polarizable continuum models (PCMs). This approach guarantees that the potential energy is a smooth function of the solute geometry and analytic gradients for DESMO are reported here. The SWIG formalism also facilitates analytic implementation of two other PCMs that are based on a screened Coulomb potential: the “integral equation formalism” (IEF-PCM) and the “surface and simulation of volume polarization for electrostatics” [SS(V)PE] method. Fully analytic implementations of these screened PCMs are reported here for the first time. Numerical results, for model systems where an exact solution of the linearized Poisson-Boltzmann equation is available, demonstrate that these screened PCMs are highly accurate. In realistic test cases, they are as accurate as the best available three-dimensional finite-difference methods. In polar solvents, DESMO is nearly as accurate as more sophisticated screened PCMs, but is significantly simpler and more efficient.
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82.45.Gj Electrolytes
82.20.Uv Stochastic theories of rate constants
82.20.Kh Potential energy surfaces for chemical reactions

Pilot applications of internally contracted multireference coupled cluster theory, and how to choose the cluster operator properly

Matthias Hanauer and Andreas Köhn

J. Chem. Phys. 134, 204111 (2011); http://dx.doi.org/10.1063/1.3592786 (20 pages) | Cited 4 times

Online Publication Date: 26 May 2011

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The internally contracted multireference coupled cluster (icMRCC) method allows a highly accurate description of both static and dynamic correlation with a computational scaling similar to single reference coupled cluster theory. The authors show that the method can lose its orbital invariance and size consistency when no special care is taken in the elimination of redundant excitations. Using the BeH2 model system, four schemes are compared which differ in their treatment of linear dependencies between excitations of different rank (such as between singles and doubles). While the energy curves agree within tens of μEh when truncating the cluster operator at double excitations (icMRCCSD), inclusion of triple excitations (icMRCCSDT) leads to significant differences of more than 1 mEh. One scheme clearly yields the best results, while the others even turn out to be not size consistent. The former procedure uses genuine single and double excitations and discards those linear combinations of (spectator) double and triple excitations which have the same effect on the reference function. With this approach, the equilibrium structure and harmonic vibrational frequencies of ozone obtained with icMRCCSDT are in excellent agreement with CCSDTQ. The authors further apply icMRCC methods to potential energy surfaces of HF, LiF, N2, and to the singlet-triplet splitting of benzynes. In particular, the latter calculations have been made possible by implementing the method with the proper formal scaling using automated techniques.
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31.15.bw Coupled-cluster theory
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.50.-x Potential energy surfaces

Local control of non-adiabatic dissociation dynamics

L. Bomble, A. Chenel, C. Meier, and M. Desouter-Lecomte

J. Chem. Phys. 134, 204112 (2011); http://dx.doi.org/10.1063/1.3589911 (8 pages)

Online Publication Date: 27 May 2011

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We present a theoretical approach which consists of applying the strategy of local control to projectors based on asymptotic scattering states. This allows to optimize final state distributions upon laser excitation in cases where strong non-adiabatic effects are present. The approach, despite being based on a time-local formulation, can take non-adiabatic transitions that appear at later times fully into account and adopt a corresponding control strategy. As an example, we show various dissociation channels of HeH+, a system where the ultrafast dissociation dynamics is determined by strong non-Born-Oppenheimer effects.
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33.80.Gj Diffuse spectra; predissociation, photodissociation

A proof of Clausius’ theorem for time reversible deterministic microscopic dynamics

Denis J. Evans, Stephen R. Williams, and Debra J. Searles

J. Chem. Phys. 134, 204113 (2011); http://dx.doi.org/10.1063/1.3592531 (7 pages) | Cited 1 time

Online Publication Date: 27 May 2011

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In 1854 Clausius proved the famous theorem that bears his name by assuming the second “law” of thermodynamics. In the present paper we give a proof that requires no such assumption. Our proof rests on the laws of mechanics, a T-mixing property, an ergodic consistency condition, and on the axiom of causality. Our result relies on some recently derived theorems, such as the Evans-Searles and the Crooks fluctuation theorems and the recently discovered relaxation and dissipation theorems.
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05.70.Ce Thermodynamic functions and equations of state
05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion
05.20.-y Classical statistical mechanics

Many-body dissipative particle dynamics simulation of liquid/vapor and liquid/solid interactions

Marco Arienti, Wenxiao Pan, Xiaoyi Li, and George Karniadakis

J. Chem. Phys. 134, 204114 (2011); http://dx.doi.org/10.1063/1.3590376 (12 pages)

Online Publication Date: 27 May 2011

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The combination of short-range repulsive and long-range attractive forces in many-body dissipative particle dynamics (MDPD) is examined at a vapor/liquid and liquid/solid interface. Based on the radial distribution of the virial pressure in a drop at equilibrium, a systematic study is carried out to characterize the sensitivity of the surface tension coefficient with respect to the inter-particle interaction parameters. For the first time, the approximately cubic dependence of the surface tension coefficient on the bulk density of the fluid is evidenced. In capillary flow, MDPD solutions are shown to satisfy the condition on the wavelength of an axial disturbance leading to the pinch-off of a cylindrical liquid thread; correctly, no pinch-off occurs below the cutoff wavelength. Moreover, in an example that illustrates the cascade of fluid dynamics behaviors from potential to inertial-viscous to stochastic flow, the dynamics of the jet radius is consistent with the power law predictions of asymptotic analysis. To model interaction with a solid wall, MDPD is augmented by a set of bell-shaped weight functions; hydrophilic and hydrophobic behaviors, including the occurrence of slip in the latter, are reproduced using a modification in the weight function that avoids particle clustering. The dynamics of droplets entering an inverted Y-shaped fracture junction is shown to be correctly captured in simulations parametrized by the Bond number, confirming the flexibility of MDPD in modeling interface-dominated flows.
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68.03.Cd Surface tension and related phenomena
68.08.Bc Wetting
83.50.Lh Slip boundary effects (interfacial and free surface flows)
47.55.nb Capillary and thermocapillary flows
47.55.D- Drops and bubbles
47.45.Gx Slip flows and accommodation

Conical intersections in solution: Formulation, algorithm, and implementation with combined quantum mechanics/molecular mechanics method

Ganglong Cui and Weitao Yang

J. Chem. Phys. 134, 204115 (2011); http://dx.doi.org/10.1063/1.3593390 (12 pages)

Online Publication Date: 31 May 2011

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The significance of conical intersections in photophysics, photochemistry, and photodissociation of polyatomic molecules in gas phase has been demonstrated by numerous experimental and theoretical studies. Optimization of conical intersections of small- and medium-size molecules in gas phase has currently become a routine optimization process, as it has been implemented in many electronic structure packages. However, optimization of conical intersections of small- and medium-size molecules in solution or macromolecules remains inefficient, even poorly defined, due to large number of degrees of freedom and costly evaluations of gradient difference and nonadiabatic coupling vectors. In this work, based on the sequential quantum mechanics and molecular mechanics (QM/MM) and QM/MM-minimum free energy path methods, we have designed two conical intersection optimization methods for small- and medium-size molecules in solution or macromolecules. The first one is sequential QM conical intersection optimization and MM minimization for potential energy surfaces; the second one is sequential QM conical intersection optimization and MM sampling for potential of mean force surfaces, i.e., free energy surfaces. In such methods, the region where electronic structures change remarkably is placed into the QM subsystem, while the rest of the system is placed into the MM subsystem; thus, dimensionalities of gradient difference and nonadiabatic coupling vectors are decreased due to the relatively small QM subsystem. Furthermore, in comparison with the concurrent optimization scheme, sequential QM conical intersection optimization and MM minimization or sampling reduce the number of evaluations of gradient difference and nonadiabatic coupling vectors because these vectors need to be calculated only when the QM subsystem moves, independent of the MM minimization or sampling. Taken together, costly evaluations of gradient difference and nonadiabatic coupling vectors in solution or macromolecules can be reduced significantly. Test optimizations of conical intersections of cyclopropanone and acetaldehyde in aqueous solution have been carried out successfully.
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36.20.Kd Electronic structure and spectra
31.50.Gh Surface crossings, non-adiabatic couplings

Effective surface motion on a reactive cylinder of particles that perform intermittent bulk diffusion

Aleksei V. Chechkin, Irwin M. Zaid, Michael A. Lomholt, Igor M. Sokolov, and Ralf Metzler

J. Chem. Phys. 134, 204116 (2011); http://dx.doi.org/10.1063/1.3593198 (22 pages)

Online Publication Date: 31 May 2011

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In many biological and small scale technological applications particles may transiently bind to a cylindrical surface. In between two binding events the particles diffuse in the bulk, thus producing an effective translation on the cylindrical surface. We here derive the effective motion on the surface allowing for additional diffusion on the cylindrical surface itself. We find explicit solutions for the number of adsorbed particles at one given instant, the effective surface displacement, as well as the surface propagator. In particular sub- and superdiffusive regimes are found, as well as an effective stalling of diffusion visible as a plateau in the mean squared displacement. We also investigate the corresponding first passage problem.
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68.43.Jk Diffusion of adsorbates, kinetics of coarsening and aggregation
68.43.Mn Adsorption kinetics
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Dissociative excitation and fragmentation of S8 by electron impact

S. J. Brotton and J. W. McConkey

J. Chem. Phys. 134, 204301 (2011); http://dx.doi.org/10.1063/1.3582909 (9 pages)

Online Publication Date: 23 May 2011

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The vacuum-ultraviolet emission spectrum from 136 nm to 168 nm following the dissociative excitation of a predominantly S8 target by electron impact at 100 eV incident energy was measured. The relative cross sections for the dominant multiplets at 138.9, 142.9, 147.9, and 166.7 nm are presented. Excitation functions are shown for electron-impact energies from below threshold to 360 eV for the two most prominent emissions at 142.5 nm and 147.4 nm. Five thresholds are clearly apparent in both excitation functions. For the four highest energy channels, the energy separation between the adjacent thresholds is approximately constant and the cross sections reduce regularly as the threshold energies increase. We suggest possible fragmentation pathways of the dissociating S8 molecule that reproduce the energies of our observed thresholds.
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34.80.Gs Molecular excitation and ionization
34.80.Ht Dissociation and dissociative attachment
33.20.Ni Vacuum ultraviolet spectra

Photodissociation investigation of doubly charged ethanol clusters induced by inner-shell electron ionization

Y. Tamenori, K. Okada, K. Tabayashi, A. Hiraya, T. Gejo, and K. Honma

J. Chem. Phys. 134, 204302 (2011); http://dx.doi.org/10.1063/1.3590164 (9 pages)

Online Publication Date: 24 May 2011

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Fragmentation of doubly charged ethanol clusters [(C2H5OH)n] following the O 1s ionization has been investigated by means of the photoelectron-photoion-photoion coincidence (PEPIPICO) method. The dominant fission channel of (C2H5OH)n2+ was the formation of protonated cluster ion pairs [H(C2H5OH)l+/H(C2H5OH)m+]. The fragmentation mechanisms of these ion pairs were discussed based on the analysis of the PEPIPICO contour shape. It was clarified that the prominent fragmentation channel was a secondary decay mechanism, where neutral evaporation occurs after charge separation. On the other hand, the formation of small fragment ions was suppressed, excluding the formation of certain specific fragments (H3O+, C2H5+/COH+, and C2H4OH+). The formation of small fragment ions was suppressed due to the cooling effect caused by the neutral evaporation and the decrease in the electrostatic repulsive force caused by charge separation.
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36.40.Qv Stability and fragmentation of clusters
36.40.Wa Charged clusters
33.80.Gj Diffuse spectra; predissociation, photodissociation
33.80.Eh Autoionization, photoionization, and photodetachment
34.50.Gb Electronic excitation and ionization of molecules

Photoelectron imaging spectroscopy and theoretical investigation of ZrSi

K. Don Dasitha Gunaratne, Anirban Hazra, and A. W. Castleman, Jr.

J. Chem. Phys. 134, 204303 (2011); http://dx.doi.org/10.1063/1.3592371 (6 pages)

Online Publication Date: 25 May 2011

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The photoelectron spectrum of ZrSi− has been measured at two different photon energies: 2.33 eV and 3.49 eV, providing electron binding energy and photoelectron angular distribution information. The obtained vertical detachment energy of ZrSi− is 1.584(14) eV. The neutral ground and excited state terms are assigned based on experimental and theoretical results. The ground state of ZrSi is tentatively assigned as a 3Σ+ state with a configuration of 1σ2 1π4 1δ0 2σ1 3σ1. A low lying 3Πi neutral excited state is identified to be 0.238 eV (1919 cm−1) above the ground state. The anion ground state is designated as a 2Σ+ state with a 1σ2 1π4 1δ0 2σ2 3σ1 valence electron configuration. A Franck-Condon (FC) simulation of the photoelectron spectrum has been carried out. For the 3Σ+ ← 2Σ+ band, theoretically calculated bond lengths and frequencies are used in the FC calculation which give good agreement with experiment, while for the 3Πi ← 2Σ+ band, the ZrSi bond length is estimated from the FC spectrum. Comparisons are made with previously published theoretical studies and inconsistencies are pointed out. To the best of our knowledge, this study provides the first spectroscopic information on the transition metal-silicon diatomic, ZrSi.
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33.60.+q Photoelectron spectra
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
33.80.Eh Autoionization, photoionization, and photodetachment
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
33.15.Dj Interatomic distances and angles
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