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21 Mar 2010

Volume 132, Issue 11, Articles (11xxxx)

Issue Cover Spotlight Figure

J. Chem. Phys. 132, 114106 (2010); http://dx.doi.org/10.1063/1.3328198 (12 pages)

Ji Feng, Roald Hoffmann, and N. W. Ashcroft
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Announcement: New Perspectives Section in The Journal of Chemical Physics

Marsha I. Lester

J. Chem. Phys. 132, 110201 (2010); http://dx.doi.org/10.1063/1.3372076 (1 page)

Online Publication Date: 16 March 2010

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01.10.Cr Announcements, news, and awards
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Cold and ultracold molecules: Spotlight on orbiting resonances

David W. Chandler

J. Chem. Phys. 132, 110901 (2010); http://dx.doi.org/10.1063/1.3357286 (5 pages) | Cited 10 times

Online Publication Date: 16 March 2010

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There is great interest in the production of cold molecules, at temperatures below 1 K, and ultracold molecules, at temperatures below 1 mK. Such molecules have potential applications in areas ranging from precision measurement to quantum information storage and processing, and quantum gases of ultracold polar molecules are expected to exhibit novel quantum phases. In addition, cold molecules open up a new domain for collision physics, dominated by long-range forces and scattering resonances. There have been major recent advances both in cooling molecules from room temperature and in forming molecules in ultracold atomic gases. As these techniques mature, and cold and ultracold samples are more accessible, collision studies at previously unavailable energies will be possible. This spotlight article will highlight some of the background and motivation for studying collisions at low energies and will direct readers to recent articles on the recent experimental advancements.
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34.50.-s Scattering of atoms and molecules
33.80.Gj Diffuse spectra; predissociation, photodissociation
33.15.Mt Rotation, vibration, and vibration-rotation constants
34.20.Gj Intermolecular and atom-molecule potentials and forces
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Frontiers in electronic structure theory

C. David Sherrill

J. Chem. Phys. 132, 110902 (2010); http://dx.doi.org/10.1063/1.3369628 (7 pages) | Cited 32 times

Online Publication Date: 18 March 2010

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Current and emerging research areas in electronic structure theory promise to greatly extend the scope and quality of quantum chemical computations. Two particularly challenging problems are the accurate description of electronic near-degeneracies (as occur in bond-breaking reactions, first-row transition elements, etc.) and the description of long-range dispersion interactions in density functional theory. Additionally, even with the emergence of reduced-scaling electronic structure methods and basis set extrapolation techniques, quantum chemical computations remain very time-consuming for large molecules or large basis sets. A variety of techniques, including density fitting and explicit correlation methods, are making rapid progress toward solving these challenges.
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31.15.E- Density-functional theory
31.15.eg Exchange-correlation functionals (in current density functional theory)
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Communications: Developing relationships between the local chemical reactivity of alloy catalysts and physical characteristics of constituent metal elements

Hongliang Xin, Neil Schweitzer, Eranda Nikolla, and Suljo Linic

J. Chem. Phys. 132, 111101 (2010); http://dx.doi.org/10.1063/1.3336015 (4 pages) | Cited 2 times

Online Publication Date: 15 March 2010

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We have used X-ray absorption spectroscopy and quantum chemical density functional theory calculations to identify critical features in the electronic structure of different sites in alloys that govern the local chemical reactivity. The measurements led to a simple model relating local geometric features of a site in an alloy to its electronic structure and chemical reactivity. The central feature of the model is that the formation of alloys does not lead to significant charge transfer between the constituent metal elements in the alloys, and that the local electronic structure and chemical reactivity can be predicted based on physical characteristics of constituent metal elements in their unalloyed form.
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82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
78.70.Dm X-ray absorption spectra
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Communications: Ab initio dynamics of rare thermally activated reactions

S. a Beccara, G. Garberoglio, P. Faccioli, and F. Pederiva

J. Chem. Phys. 132, 111102 (2010); http://dx.doi.org/10.1063/1.3355866 (4 pages) | Cited 6 times

Online Publication Date: 15 March 2010

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We introduce a framework to investigate ab initio the dynamics of rare thermally activated reactions, which cannot be studied using the existing techniques. The electronic degrees of freedom are described at the quantum-mechanical level in the Born–Oppenheimer approximation, while the nuclear degrees of freedom are coupled to a thermal bath, through a classical Langevin equation. This method is based on the path integral representation for the stochastic dynamics and yields the time evolution of both nuclear and electronic degrees of freedom, along the most probable reaction pathways, without spending computational time to explore metastable states. As a first illustrative application, we characterize the dominant pathway in the cyclobutene→butadiene reaction, using the semiempirical Parametrized Model 3 (PM3) approach.
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82.20.Wt Computational modeling; simulation
31.15.A- Ab initio calculations
31.50.Df Potential energy surfaces for excited electronic states
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Communications: On the linear response of mechanical systems with constraints

Carsten Hartmann, Christof Schütte, and Giovanni Ciccotti

J. Chem. Phys. 132, 111103 (2010); http://dx.doi.org/10.1063/1.3354126 (4 pages) | Cited 1 time

Online Publication Date: 16 March 2010

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We revisit the problem of the linear response of a constrained mechanical system. In doing so, we show that the standard expressions of Green and Kubo carry over to the constrained case without any alteration. The argument is based on the appropriate definition of constrained expectations by means of which Liouville’s theorem and the Green–Kubo relations naturally follow.
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46.32.+x Static buckling and instability
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Coupling different levels of resolution in molecular simulations

Simón Poblete, Matej Praprotnik, Kurt Kremer, and Luigi Delle Site

J. Chem. Phys. 132, 114101 (2010); http://dx.doi.org/10.1063/1.3357982 (7 pages) | Cited 8 times

Online Publication Date: 15 March 2010

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Simulation schemes for liquids or strongly fluctuating systems that allow to change the molecular representation in a subvolume of the simulation box while preserving the equilibrium with the surroundings introduce conceptual problems of thermodynamic consistency. In this work we present a general scheme based on thermodynamic arguments which ensures a thermodynamic equilibrium among molecules of different representations. The robustness of the algorithm is tested for two examples, namely, an adaptive resolution simulation, atomistic/coarse grained, for a liquid of tetrahedral molecules, and an adaptive resolution simulation of a binary mixture of tetrahedral molecules and spherical solutes.
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64.75.-g Phase equilibria
02.60.-x Numerical approximation and analysis
68.35.Md Surface thermodynamics, surface energies

The properties of dimers confined between two charged plates

Marius M. Hatlo, Klemen Bohinc, and Leo Lue

J. Chem. Phys. 132, 114102 (2010); http://dx.doi.org/10.1063/1.3354121 (10 pages) | Cited 4 times

Online Publication Date: 15 March 2010

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We consider two like-charged planar surfaces immersed in solution of oppositely charged dimer counterions with a bond length l. To analyze this system, we extend and employ a self-consistent field theory that has been shown to be accurate from the weak to the intermediate through to the strong coupling regimes. In the limit of very short dimers, the results converge to the results for pointlike divalent ions. Near the surfaces, the dimers lie parallel to the charged plates. In the intermediate coupling regime, the dimers are aligned perpendicularly to the surface when they are a distance l from a surface. In the weak coupling regime, the interactions are only repulsive. At slightly higher couplings, there is a minimum in the variation of the free energy with distance at approximately the bond length of the dimers, which arises from bridging conformations of the dimers. In the intermediate coupling regime, an additional minimum in the free energy is observed at much smaller distances, which is due to the correlations between the dimers. For large dimer bond lengths, this minimum is metastable with respect to the previous minimum. However, as the bond length decreases, this minimum becomes the stable, while the minimum associated with the dimer bond length becomes metastable and eventually disappears. For shorter dimer bond length the attractive interaction is the result of correlations between counterions and charges on the surfaces. We find that dimers can mediate attractive interaction between like-charged surfaces in the intermediate coupling regime. The analysis of orientations confirms the bridging mechanism for sufficiently long dimers, whereas at high electrostatic couplings charge correlations contribute to the attraction.
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31.15.xr Self-consistent-field methods
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
33.15.Dj Interatomic distances and angles
33.15.Bh General molecular conformation and symmetry; stereochemistry

Force field of para- and metabenzyne diradicals: A multireference coupled-cluster study

Xiangzhu Li and Josef Paldus

J. Chem. Phys. 132, 114103 (2010); http://dx.doi.org/10.1063/1.3354947 (10 pages) | Cited 10 times

Online Publication Date: 15 March 2010

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The reduced multireference (RMR) coupled-cluster (CC) method with singles and doubles (RMR CCSD) that exploits a modest size MR CISD wave function as an external source for a small subset of the most important (primary) triples and quadruples to account for quasidegeneracy due to a MR nature of the states considered, as well as its RMR CCSD(T) version corrected for the secondary triples, are employed to compute fundamental vibrational frequencies for the para- and metabenzene diradicals. A comparison is made with the available experimental data and with other methods, namely, the state selective or Mukherjee CCSD and completely renormalized, size-extensive version of CCSD(T), the so-called CR-CC(2,3), methods. Both the restricted Hartree–Fock and multiconfiguration self-consistent-field (MCSCF) molecular orbitals (MOs) are employed. The four-reference RMR CCSD using MCSCF MOs gives particularly satisfactory results. The breakdown of CCSD(T) and shortcomings of some other approaches are pointed out and the role of invariance with respect to MO rotation is analyzed. Correlation with triplet-singlet splittings is also pointed out.
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31.15.bw Coupled-cluster theory
31.15.xr Self-consistent-field methods
31.15.vj Electron correlation calculations for atoms and ions: excited states
33.15.Mt Rotation, vibration, and vibration-rotation constants

The chemical shift of deprotonated water dimer: Ab initio path integral simulation

Motoyuki Shiga, Kimichi Suzuki, and Masanori Tachikawa

J. Chem. Phys. 132, 114104 (2010); http://dx.doi.org/10.1063/1.3354948 (7 pages) | Cited 4 times

Online Publication Date: 15 March 2010

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The 1H NMR chemical shift in deprotonated water dimer H3O2− has been studied by ab initio path integral simulation. The simulation predicts that the isotropic shielding of hydrogen-bonded proton increases as a function of temperature by about 0.003 ppm/K. This change is about an order of magnitude larger than that of the nonhydrogen-bonded proton. It is concluded that this is caused by the significant difference in the quantum distribution of proton at high and low temperatures in the low barrier hydrogen bond.
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33.25.+k Nuclear resonance and relaxation
31.15.ae Electronic structure and bonding characteristics
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
33.15.Fm Bond strengths, dissociation energies
31.30.Gs Hyperfine interactions and isotope effects

Theoretical model for adhesive friction between elastomers and rough solid surfaces

Satoshi Momozono, Kenya Nakamura, and Keiji Kyogoku

J. Chem. Phys. 132, 114105 (2010); http://dx.doi.org/10.1063/1.3356220 (5 pages) | Cited 3 times

Online Publication Date: 15 March 2010

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A theoretical model for the adhesive friction between elastomers and rough solid surfaces is proposed on the basis of opening crack propagation processes at the boundary of the contact interfaces and the rate processes of formation of molecular bonds on the solid surface. This model, which is expressed as a product of the terms related to the two abovementioned processes, requires some measurable and fitted parameters such as the frictional shear strength expressed as a function of viscoelastic dissipation, rate-dependent elasticity, density of bonded molecular chains at a contact junction, critical velocity related to viscoelastic relaxation, and critical velocity related to the rate process of formation of molecular bonds on the solid surface. The friction-velocity relationship exhibits a remarkable fit to previously obtained experimental results for polymers such as engineering rubber, gels, and plastics (glassy polymers), and all fitting parameters are physically reasonable. The viscoelastic index “n” is also related to the “glass-to-rubber transition” of a nanometer-thick polymer layer for frictional behavior. Thus, from a practical viewpoint, this model can be used effectively for fitting the adhesive friction behavior of polymers.
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81.40.Pq Friction, lubrication, and wear
68.37.-d Microscopy of surfaces, interfaces, and thin films
62.20.Qp Friction, tribology, and hardness
61.50.Lt Crystal binding; cohesive energy
62.20.D- Elasticity
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.mt Cracks
81.40.Jj Elasticity and anelasticity, stress-strain relations

Double-diamond NaAl via pressure: Understanding structure through Jones zone activation

Ji Feng, Roald Hoffmann, and N. W. Ashcroft

J. Chem. Phys. 132, 114106 (2010); http://dx.doi.org/10.1063/1.3328198 (12 pages) | Cited 2 times

Online Publication Date: 15 March 2010

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Under normal conditions, sodium forms a 1:1 stoichiometric compound with indium, and also with thallium, both in the double-diamond structure. But sodium does not combine with aluminum at all. Could NaAl exist? If so, under what conditions and in which structural types? Instead of beginning with a purely computational and first-principles structure search, we are led to apply the early Brillouin and higher (Jones) zone ideas of the physics determining structural selection. We begin with a brief recapitulation of the higher zone concept as applied to the stability of metals and intermetallic compounds. We then discuss the extension of this concept to problems where density becomes a primary variable, within the second-order band structure approximation. An analysis of the range of applicability of pressure-induced Jones zone activation is presented. The simple NaAl compound serves us as a numerical laboratory for the application of this concept. Higher zone arguments and chemical intuition lead quite naturally to the suggestion that 1:1 compound formation between sodium and aluminum should be favored under pressure and specifically in the double-diamond structure. This is confirmed computationally by density functional theoretic methods within the generalized gradient approximation.
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71.20.Lp Intermetallic compounds
61.50.Lt Crystal binding; cohesive energy
71.15.Mb Density functional theory, local density approximation, gradient and other corrections

ONIOM-based QM:QM electronic embedding method using Löwdin atomic charges: Energies and analytic gradients

Nicholas J. Mayhall, Krishnan Raghavachari, and Hrant P. Hratchian

J. Chem. Phys. 132, 114107 (2010); http://dx.doi.org/10.1063/1.3315417 (6 pages) | Cited 7 times

Online Publication Date: 15 March 2010

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In this work, we report a new quantum mechanical:quantum mechanical (QM:QM) method which provides explicit electronic polarization of the high-level region by using the Löwdin atomic charges from the low-level region. This provides an embedding potential which naturally evolves with changes in nuclear geometry. However, this coupling of the high-level and low-level regions introduces complications in the energy gradient evaluation. Following previous work, we derive and implement efficient gradients where a single set of self-consistent field response equations is solved. We provide results for the calculation of deprotonation energies of a hydroxylated spherosiloxane cluster (Si8O12H7OH) and the dissociation energy of a water molecule from a [ZnIm3(H2O)]2+ complex. We find that the Löwdin charge embedding model provides results which are not only an improvement over mechanical embedding (no electronic embedding) but which are also resistant to large overpolarization effects which occur more often with Mulliken charge embedding. Finally, a scaled-Löwdin charge embedding method is also presented which provides a method for fine tuning the extent of electronic polarization.
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31.15.xr Self-consistent-field methods
33.15.Fm Bond strengths, dissociation energies
82.37.Np Single molecule reaction kinetics, dissociation, etc.

Thermodynamic properties of short-range attractive Yukawa fluid: Simulation and theory

Pedro Orea, Carlos Tapia-Medina, Davide Pini, and Albert Reiner

J. Chem. Phys. 132, 114108 (2010); http://dx.doi.org/10.1063/1.3357352 (7 pages) | Cited 7 times

Online Publication Date: 16 March 2010

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Coexistence properties of the hard-core attractive Yukawa potential with inverse-range parameter κ = 9, 10, 12, and 15 are calculated by applying canonical Monte Carlo simulation. As previously shown for longer ranges, we show that also for the ranges considered here the coexistence curves scaled by the critical density and temperature obey the law of corresponding states, and that a linear relationship between the critical density and the reciprocal of the critical temperature holds. The simulation results are compared to the predictions of the self-consistent Ornstein–Zernike approximation, and a good agreement is found for both the critical points and the coexistence curves, although some slight discrepancies are present.
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65.20.-w Thermal properties of liquids
64.70.F- Liquid-vapor transitions
61.20.Ja Computer simulation of liquid structure

Isotope shifts of the three lowest 1S states of the B+ ion calculated with a finite-nuclear-mass approach and with relativistic and quantum electrodynamics corrections

Sergiy Bubin, Jacek Komasa, Monika Stanke, and Ludwik Adamowicz

J. Chem. Phys. 132, 114109 (2010); http://dx.doi.org/10.1063/1.3358999 (6 pages) | Cited 2 times

Online Publication Date: 16 March 2010

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We present very accurate quantum mechanical calculations of the three lowest S-states [1s22s2(1S0), 1s22p2(1S0), and 1s22s3s(1S0)] of the two stable isotopes of the boron ion, 10B+ and 11B+. At the nonrelativistic level the calculations have been performed with the Hamiltonian that explicitly includes the finite mass of the nucleus as it was obtained by a rigorous separation of the center-of-mass motion from the laboratory frame Hamiltonian. The spatial part of the nonrelativistic wave function for each state was expanded in terms of 10 000 all-electron explicitly correlated Gaussian functions. The nonlinear parameters of the Gaussians were variationally optimized using a procedure involving the analytical energy gradient determined with respect to the nonlinear parameters. The nonrelativistic wave functions of the three states were subsequently used to calculate the leading α2 relativistic corrections (α is the fine structure constant; α = 1/c, where c is the speed of light) and the α3 quantum electrodynamics (QED) correction. We also estimated the α4 QED correction by calculating its dominant component. A comparison of the experimental transition frequencies with the frequencies obtained based on the energies calculated in this work shows an excellent agreement. The discrepancy is smaller than 0.4 cm−1.
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32.70.Jz Line shapes, widths, and shifts
31.30.J- Relativistic and quantum electrodynamic (QED) effects in atoms, molecules, and ions
31.30.jf QED calculations of level energies, transition frequencies, fine structure intervals (radiative corrections, self-energy, vacuum polarization, etc.)
32.30.Rj X-ray spectra

Continuous surface charge polarizable continuum models of solvation. I. General formalism

Giovanni Scalmani and Michael J. Frisch

J. Chem. Phys. 132, 114110 (2010); http://dx.doi.org/10.1063/1.3359469 (15 pages) | Cited 57 times

Online Publication Date: 17 March 2010

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Continuum solvation models are appealing because of the simplified yet accurate description they provide of the solvent effect on a solute, described either by quantum mechanical or classical methods. The polarizable continuum model (PCM) family of solvation models is among the most widely used, although their application has been hampered by discontinuities and singularities arising from the discretization of the integral equations at the solute-solvent interface. In this contribution we introduce a continuous surface charge (CSC) approach that leads to a smooth and robust formalism for the PCM models. We start from the scheme proposed over ten years ago by York and Karplus and we generalize it in various ways, including the extension to analytic second derivatives with respect to atomic positions. We propose an optimal discrete representation of the integral operators required for the determination of the apparent surface charge. We achieve a clear separation between “model” and “cavity” which, together with simple generalizations of modern integral codes, is all that is required for an extensible and efficient implementation of the PCM models. Following this approach we are now able to introduce solvent effects on energies, structures, and vibrational frequencies (analytical first and second derivatives with respect to atomic coordinates), magnetic properties (derivatives with respect of magnetic field using GIAOs), and in the calculation more complex properties like frequency-dependent Raman activities, vibrational circular dichroism, and Raman optical activity.
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82.20.Yn Solvent effects on reactivity
33.55.+b Optical activity and dichroism
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Tp Vibrational analysis
82.20.Db Transition state theory and statistical theories of rate constants
82.30.Nr Association, addition, insertion, cluster formation
33.20.Ea Infrared spectra
33.20.Fb Raman and Rayleigh spectra (including optical scattering)

MP2/CBS atomic and molecular benchmarks for H through Ar

Ericka C. Barnes and George A. Petersson

J. Chem. Phys. 132, 114111 (2010); http://dx.doi.org/10.1063/1.3317476 (9 pages) | Cited 3 times

Online Publication Date: 17 March 2010

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We extrapolate to the MP2/CBS limit with a sequence of optimized n-tuple-ζ augmented polarized basis sets (n = 4, 5, 6, and 7) for the entire set of 72 atoms, positive and negative atomic ions, homonuclear diatomic molecules, and hydrides representing the first two rows of the Periodic Table. The second-order correlation energies agree with accurate (±0.01 mEh) numerical values (He, Be, Ne, Mg, Ar, Zn+2, and Kr) to within ±0.1%. These MP2/CBS limits of the 72 species can now be used as benchmarks to calibrate more approximate calculations using smaller basis sets.
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31.15.xp Perturbation theory

Chemical verification of variational second-order density matrix based potential energy surfaces for the N2 isoelectronic series

Helen van Aggelen, Brecht Verstichel, Patrick Bultinck, Dimitri Van Neck, Paul W. Ayers, and David L. Cooper

J. Chem. Phys. 132, 114112 (2010); http://dx.doi.org/10.1063/1.3354910 (10 pages) | Cited 9 times

Online Publication Date: 17 March 2010

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A variational optimization of the second-order density matrix under the P-, Q-, and G-conditions was carried out for a set of diatomic 14-electron molecules, including N2, O22+, NO+, CO, and CN−. The dissociation of these molecules is studied by analyzing several chemical properties (dipole moments, population analysis, and bond indices) up to the dissociation limit (10 and 20 Å). Serious chemical flaws are observed for the heteronuclear diatomics in the dissociation limit. A careful examination of the chemical properties reveals that the origin of the dissociation problem lies in the flawed description of fractionally occupied species under the P-, Q-, and G-conditions. A novel constraint is introduced that imposes the correct dissociation and enforces size consistency. The effect of this constraint is illustrated with calculations on NO+, CO, CN−, N2, and O22+.
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31.50.-x Potential energy surfaces
31.15.xt Variational techniques
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
33.15.Fm Bond strengths, dissociation energies

Subsystem constraints in variational second order density matrix optimization: Curing the dissociative behavior

Brecht Verstichel, Helen van Aggelen, Dimitri Van Neck, Paul W. Ayers, and Patrick Bultinck

J. Chem. Phys. 132, 114113 (2010); http://dx.doi.org/10.1063/1.3354911 (6 pages) | Cited 11 times

Online Publication Date: 17 March 2010

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A previous study of diatomic molecules revealed that variational second-order density matrix theory has serious problems in the dissociation limit when the N-representability is imposed at the level of the usual two-index (P,Q,G) or even three-index (T1,T2) conditions [ H. Van Aggelen et al., Phys. Chem. Chem. Phys. 11, 5558 (2009) ]. Heteronuclear molecules tend to dissociate into fractionally charged atoms. In this paper we introduce a general class of N-representability conditions, called subsystem constraints, and show that they cure the dissociation problem at little additional computational cost. As a numerical example the singlet potential energy surface of Be B+ is studied. The extension to polyatomic molecules, where more subsystem choices can be identified, is also discussed.
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31.15.xt Variational techniques
31.50.Df Potential energy surfaces for excited electronic states
33.15.Fm Bond strengths, dissociation energies

Effect of confining conduit on effective viscosity of dilute colloidal suspension

Shahin Navardi and Sukalyan Bhattacharya

J. Chem. Phys. 132, 114114 (2010); http://dx.doi.org/10.1063/1.3315867 (10 pages) | Cited 3 times

Online Publication Date: 18 March 2010

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In this article, we discuss the effect of the bounding cylinder on the rheology of a dilute suspension. We consider a colloidal solution of spherical particles flowing through a cylinder under creeping motion assumption. For transport of such particulate fluid, the increase in the viscous loss due to the existence of suspended particles can be described in terms of enhanced effective viscosity ηeff for the medium. Einstein’s formula quantifies this increase in viscosity when the flow-domain is unbounded. For bounded domain, however, the increase in viscosity is not only dictated by the properties of the solutes but also affected by the geometry of the confinement. We illustrate this effect of geometry on the effective viscosity by accurately resolving the viscous interaction between a freely suspended sphere and a confining cylinder. First, we take into account a solution of equal spheres, and present the effective viscosity for different cylinder to sphere size ratio as well as for different excluded volume near the cylinder periphery for electrostatic interactions. Then, we also consider a variation in size distribution and determine the rheological properties for different means and variances of the distribution.
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83.80.Hj Suspensions, dispersions, pastes, slurries, colloids
83.60.Fg Shear rate dependent viscosity
83.50.Ha Flow in channels

A coupled cluster approach with a hybrid treatment of connected triple excitations for bond-breaking potential energy surfaces

Jun Shen, Enhua Xu, Zhuangfei Kou, and Shuhua Li

J. Chem. Phys. 132, 114115 (2010); http://dx.doi.org/10.1063/1.3359851 (12 pages) | Cited 8 times

Online Publication Date: 18 March 2010

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An approximate coupled cluster singles, doubles, and triples (CCSDT) method based on the unrestricted Hartree–Fock (UHF) reference, in which the contribution of triple excitations is approximately treated in a hybrid manner [denoted as CCSD(T)-h], is presented. In this approach, canonical UHF molecular orbitals are first transformed into corresponding orbitals so that each α-spin orbital is paired with only one β-spin orbital. Then, active orbitals (occupied or virtual) are automatically selected by setting a threshold for the overlap integrals of corresponding orbitals. With the concept of active orbitals, triple excitations can be divided into two subsets: (1) “active” triples involving at least one occupied active orbital and one virtual active orbital and (2) the remaining triples. The amplitudes of these two classes of triple excitations are obtained via two different approaches. When the present method is employed to study bond-breaking processes, it computationally scales as the seventh power of the system size, because the number of active orbitals involved in such processes is relatively small compared to the total number of the orbitals, and is usually independent on the system size. It has been applied to study the bond-breaking potential energy surfaces in the H8 model and five small molecules (HF, F2, CH4, H2O, and N2). For all systems under study, the overall performance of CCSD(T)-h is very competitive with that of CCSDT, and much better than that of the UHF-based CCSD(T).
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31.15.bw Coupled-cluster theory
31.50.-x Potential energy surfaces
31.15.xr Self-consistent-field methods

Coherent site-directed transport in complex molecular networks: An effective Hamiltonian approach

Shira Weissman and Uri Peskin

J. Chem. Phys. 132, 114116 (2010); http://dx.doi.org/10.1063/1.3355550 (9 pages)

Online Publication Date: 18 March 2010

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Defining the conditions for coherent site-directed transport from an electron donor to a specific acceptor through tunneling barriers in a network of multiple donor/acceptors sites is an important step toward controlling electronic processes in molecular networks. The required analysis is most challenging since the entire network in essentially involved in coherent transport. In this work we introduce an efficient approach for formulating an effective donor/acceptor coupling in terms of the entire network parameters. The approach is based on implementation of Feshbach projection operators to map the entire network Hamiltonian onto a subspace defined by two specific donor and acceptor sites. This nonperturbative approach enables to define regimes of network parameters in which the effective donor-acceptor coupling is optimal. This is demonstrated numerically for simple models of molecular networks.
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34.70.+e Charge transfer
31.15.X- Alternative approaches
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

A highly accurate potential energy curve for the mercury dimer

Elke Pahl, Detlev Figgen, Christian Thierfelder, Kirk A. Peterson, Florent Calvo, and Peter Schwerdtfeger

J. Chem. Phys. 132, 114301 (2010); http://dx.doi.org/10.1063/1.3354976 (6 pages) | Cited 11 times

Online Publication Date: 15 March 2010

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The potential energy curve of the electronic ground state of the mercury dimer based on CCSD(T) calculations at the complete basis set (CBS) limit, including corrections for the full triples ΔT and explicit spin-orbit (SO) interactions at the CCSD(T) level of theory, is presented. In the far long-range part, the potential energy curve is complemented by symmetry-adapted perturbation theory calculations. Potential curves of an analytically simple, extended Lennard-Jones form are obtained from very accurate fits to the CBS/CCSD(T)+SO and CBS/CCSD(T)+SO+ΔT data. The Hg2 potential curves yield dissociation energies of De = 424/392 cm−1 and equilibrium distances of re = 3.650/3.679 Å at the CBS/CCSD(T)+SO and CBS/CCSD(T)+SO+ΔT levels of theory, respectively. By including perturbative quadruple corrections in our coupled-cluster calculations and corrections from correlating the 4f-core, we arrive at a final dissociation energy of De = 405 cm−1, in excellent agreement with the experimentally estimated value of 407 cm−1 by Greif and Hensel. In addition, the rotational and vibrational spectroscopic constants as well as the second virial coefficient B(T) in dependence of the temperature T are calculated and validated against available experimental and theoretical data.
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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.bw Coupled-cluster theory
33.15.Fm Bond strengths, dissociation energies
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.15.aj Relativistic corrections, spin-orbit effects, fine structure; hyperfine structure

The math-math absorption of vinoxy radical revisited: Normal and Herzberg–Teller bands observed via cavity ringdown spectroscopy

Phillip S. Thomas, Rabi Chhantyal-Pun, Neal D. Kline, and Terry A. Miller

J. Chem. Phys. 132, 114302 (2010); http://dx.doi.org/10.1063/1.3352976 (10 pages) | Cited 1 time

Online Publication Date: 18 March 2010

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The math-math electronic absorption spectrum of vinoxy radical has been investigated using room temperature cavity ringdown spectroscopy. Analysis of the observed bands on the basis of computed vibrational frequencies and rotational envelopes reveals that two distinct types of features are present with comparable intensities. The first type corresponds to “normal” allowed electronic transitions to the origin and symmetric vibrations in the math state. The second type is interpreted in terms of excitations to asymmetric math state vibrations, which are only vibronically allowed by Herzberg–Teller coupling to the math state. Results of electronic structure calculations indicate that the magnitude of the Herzberg–Teller coupling is appropriate to produce vibronically induced transitions with intensities comparable to those of the normal bands.
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33.20.Ea Infrared spectra
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
31.15.bw Coupled-cluster theory
31.15.E- Density-functional theory
34.50.Gb Electronic excitation and ionization of molecules

Transient infrared absorption of t-CH3C(O)OO, c-CH3C(O)OO, and α-lactone recorded in gaseous reactions of CH3CO and O2

Sun-Yang Chen and Yuan-Pern Lee

J. Chem. Phys. 132, 114303 (2010); http://dx.doi.org/10.1063/1.3352315 (11 pages) | Cited 5 times

Online Publication Date: 18 March 2010

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A step-scan Fourier-transform infrared spectrometer coupled with a multipass absorption cell was utilized to monitor the transient species produced in gaseous reactions of CH3CO and O2; IR absorption spectra of CH3C(O)OO and α-lactone were observed. Absorption bands with origins at 1851±1, 1372±2, 1169±6, and 1102±3 cm−1 are attributed to t-CH3C(O)OO, and those at 1862±3, 1142±4, and 1078±6 cm−1 are assigned to c-CH3C(O)OO. A weak band near 1960 cm−1 is assigned to α-lactone, cyc-CH2C(O)O, a coproduct of OH. These observed rotational contours agree satisfactorily with simulated bands based on predicted rotational parameters and dipole derivatives, and observed vibrational wavenumbers agree with harmonic vibrational wavenumbers predicted with B3LYP/aug-cc-pVDZ density-functional theory. The observed relative intensities indicate that t-CH3C(O)OO is more stable than c-CH3C(O)OO by 3±2 kJ mol−1. Based on these observations, the branching ratio for the OH+α-lactone channel of the CH3CO+O2 reaction is estimated to be 0.04±0.01 under 100 Torr of O2 at 298 K. A simple kinetic model is employed to account for the decay of CH3C(O)OO.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
33.20.Sn Rotational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Tp Vibrational analysis
33.70.Fd Absolute and relative line and band intensities
82.20.Hf Product distribution
82.20.Db Transition state theory and statistical theories of rate constants
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