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21 Oct 2007

Volume 127, Issue 15, Articles (15xxxx)

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Announcement: Elimination of page charges for 2008

Mark M. Cassar

J. Chem. Phys. 127, 150201 (2007); http://dx.doi.org/10.1063/1.2799056 (1 page)

Online Publication Date: 15 October 2007

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01.10.Cr Announcements, news, and awards
01.30.-y Physics literature and publications
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Product branching between reactive and nonreactive pathways in the collisional quenching of OH A 2Σ+ radicals by H2

Logan P. Dempsey, Craig Murray, and Marsha I. Lester

J. Chem. Phys. 127, 151101 (2007); http://dx.doi.org/10.1063/1.2800316 (4 pages) | Cited 9 times

Online Publication Date: 17 October 2007

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The collisional quenching of OH radicals in their excited A 2Σ+ electronic state by molecular hydrogen is examined to determine the partitioning between reactive and nonreactive pathways. This is achieved using a pump-probe laser technique to compare the population prepared in the excited OH A 2Σ+ state with that produced in the OH X 2Π ground state from nonreactive quenching. Only a small fraction of the products, less than 15%, arise from nonreactive quenching; reactive quenching is the dominant product channel. The branching between the product channels provides a new dynamical signature of the conical intersection region(s) that couple the excited state potential for OH A 2Σ++H2 with OH X 2Π+H2 and H2O+H products.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Hf Product distribution
82.20.Kh Potential energy surfaces for chemical reactions
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back to top Theoretical Methods and Algorithms

Avoiding unphysical kinetic traps in Monte Carlo simulations of strongly attractive particles

Stephen Whitelam and Phillip L. Geissler

J. Chem. Phys. 127, 154101 (2007); http://dx.doi.org/10.1063/1.2790421 (19 pages) | Cited 34 times

Online Publication Date: 15 October 2007

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We introduce a “virtual-move” Monte Carlo algorithm for systems of pairwise-interacting particles. This algorithm facilitates the simulation of particles possessing attractions of short range and arbitrary strength and geometry, an important realization being self-assembling particles endowed with strong, short-ranged, and angularly specific (“patchy”) attractions. Standard Monte Carlo techniques employ sequential updates of particles and can suffer from low acceptance rates when attractions are strong. In this event, collective motion can be strongly suppressed. Our algorithm avoids this problem by proposing simultaneous moves of collections (clusters) of particles according to gradients of interaction energies. One particle first executes a “virtual” trial move. We determine which of its neighbors move in a similar fashion by calculating individual bond energies before and after the proposed move. We iterate this procedure and update simultaneously the positions of all affected particles. Particles move according to an approximation of realistic dynamics without requiring the explicit computation of forces and without the step size restrictions required when integrating equations of motion. We employ a size- and shape-dependent damping of cluster movements, motivated by collective hydrodynamic effects neglected in simple implementations of Brownian dynamics. We discuss the virtual-move algorithm in the context of other Monte Carlo cluster-move schemes and demonstrate its utility by applying it to a model of biological self-assembly.
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05.40.Jc Brownian motion
87.16.A- Theory, modeling, and simulations
87.15.-v Biomolecules: structure and physical properties
05.10.Ln Monte Carlo methods
45.50.Tn Collisions

A comparison between plane wave and Gaussian-type orbital basis sets for hydrogen bonded systems: Formic acid as a test case

Sergio Tosoni, Christian Tuma, Joachim Sauer, Bartolomeo Civalleri, and Piero Ugliengo

J. Chem. Phys. 127, 154102 (2007); http://dx.doi.org/10.1063/1.2790019 (11 pages) | Cited 19 times

Online Publication Date: 16 October 2007

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The formic acid molecule, its dimers, and its molecular crystal are adopted as test systems to compare results obtained with plane wave (PW) basis sets and norm-conserving pseudopotentials to all-electron Gaussian-type orbital (GTO) calculations. The CPMD and CRYSTAL06 codes, respectively, are applied with the PBE, PW91, and BLYP density functionals. Hydrogen bonding is the leading interaction in the dimers and the crystal. In the latter, dispersive and weak C–H⋯O interactions are also relevant. Irrespective of the adopted functional, for all considered structures PW and GTO results converge smoothly as a function of the quality of the adopted basis sets to the same values for structures, energies of interaction, and harmonic vibrational features. To achieve a high level of mutual agreement the use of GTO basis sets of at least of triple-zeta quality including one set of polarization functions and PW basis sets with a kinetic energy cutoff higher than 110 Ry is recommended. Pros and cons of both approaches for studying molecular crystals are also discussed.
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33.15.Fm Bond strengths, dissociation energies
31.15.E- Density-functional theory
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Mt Rotation, vibration, and vibration-rotation constants

Unified view on multiconfigurational time propagation for systems consisting of identical particles

Ofir E. Alon, Alexej I. Streltsov, and Lorenz S. Cederbaum

J. Chem. Phys. 127, 154103 (2007); http://dx.doi.org/10.1063/1.2771159 (6 pages) | Cited 15 times

Online Publication Date: 16 October 2007

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We show that the successful and formally exact multiconfigurational time-dependent Hartree method (MCTDH) takes on a unified and compact form when specified for systems of identical particles (MCTDHF for fermions MCTDHB for bosons). In particular the equations of motion for the orbitals depend explicitly and solely on the reduced one- and two-body density matrices of the system’s many-particle wave function. We point out that this appealing representation of the equations of motion opens up further possibilities for approximate propagation schemes.
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05.30.Jp Boson systems
05.30.Fk Fermion systems and electron gas

Hyperspherical coupled channel calculations for the spectra and structure parameters of rare gas trimers NeAr2 and Ne2Ar

Hui-li Han, Yong Li, Xian-zhou Zhang, and Ting-yun Shi

J. Chem. Phys. 127, 154104 (2007); http://dx.doi.org/10.1063/1.2795714 (9 pages) | Cited 6 times

Online Publication Date: 16 October 2007

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We calculate the L = 0 vibration energies and rotational constants for the van der Waals trimers 20NeAr2, 20Ne2Ar, and their corresponding isotopologues within the framework of hyperspherical coordinates. The Schrödinger equation in hyperangular coordinates is solved at a series of fixed hyper-radii using B-splines and the resulting coupled hyper-radial equation is solved using the slow variable discretization method developed by Tolstikhin et al. [J. Phys. B 29, L389 (1996) ]. Using the special properties of B-splines, we make the knot distributions more precisely, characterizing the behavior of channel functions. Our method improves the convergence greatly. It turns out that our numerical tool works quite well in study of rare gas trimers. Calculations are performed on two kinds of pair potentials, the HFD-B and Tang-Toennies (TT) potentials, and the resultant rotational constants and their isotope shifts are compared with the experimental results obtained from high-resolution spectroscopy. The TT pair potentials give much better agreement with the experimental values for 20Ne2Ar and 22Ne2Ar trimers, while the HFD-B pair potentials give much better agreement with the experimental values for 20NeAr2 and 22NeAr2 trimers.
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31.15.xj Hyperspherical methods
33.20.Tp Vibrational analysis
33.20.Sn Rotational analysis
33.70.Jg Line and band widths, shapes, and shifts

Accelerated entropy estimates with accelerated dynamics

David D. L. Minh, Donald Hamelberg, and J. Andrew McCammon

J. Chem. Phys. 127, 154105 (2007); http://dx.doi.org/10.1063/1.2794754 (5 pages) | Cited 2 times

Online Publication Date: 16 October 2007

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Accelerated dynamics is applied to entropy calculations on a set of toy and molecular systems and is found to enhance the rate of convergence.
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05.70.Ce Thermodynamic functions and equations of state
02.50.Cw Probability theory
87.14.E- Proteins
87.10.-e General theory and mathematical aspects
87.15.-v Biomolecules: structure and physical properties

Selective sampling of transition paths

Xuebing Fu, Lijiang Yang, and Yi Qin Gao

J. Chem. Phys. 127, 154106 (2007); http://dx.doi.org/10.1063/1.2779325 (7 pages) | Cited 6 times

Online Publication Date: 17 October 2007

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In this short paper, we introduce an approximate method for the quick estimate of rate constants based on a simple sampling method of reactive transition paths over high energy barriers. It makes use of the previously introduced accelerated molecular dynamics (MD) simulation method to generate initial points for trajectory shooting. The accelerated MD simulations, although with the loss of real dynamics, lead to a quick calculation of thermodynamic properties and at the same time produce an ensemble of configurations with an enhanced sampling over the phase space that is more “reactive.” The forward/backward trajectory shooting as that used in the transition path sampling method is then initiated from the configurations obtained from accelerated MD simulations to generate transition paths on the original unbiased potential. This method selectively enhances sampling of successful trajectories and at the same time accelerates significantly the calculation of rate constants.
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82.20.Pm Rate constants, reaction cross sections, and activation energies
82.20.Wt Computational modeling; simulation
82.20.Fd Collision theories; trajectory models
82.20.Db Transition state theory and statistical theories of rate constants
82.60.-s Chemical thermodynamics

Anomalous lineshapes and aging effects in two-dimensional correlation spectroscopy

František Šanda and Shaul Mukamel

J. Chem. Phys. 127, 154107 (2007); http://dx.doi.org/10.1063/1.2793786 (12 pages) | Cited 2 times

Online Publication Date: 18 October 2007

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Multitime correlation functions provide useful probes for the ensembles of trajectories underlying the stochastic dynamics of complex systems. These can be obtained by measuring their optical response to sequences of ultrashort optical pulse. Using the continuous time random walk model for spectral diffusion, we analyze the signatures of anomalous relaxation in two-dimensional four wave mixing signals. Different models which share the same two point joint probability distribution show markedly different lineshapes and may be distinguished. Aging random walks corresponding to waiting time distributions with diverging first moment show dependence of 2D lineshapes on initial observation time, which persist for long times.
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42.65.Re Ultrafast processes; optical pulse generation and pulse compression
42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
05.40.Fb Random walks and Levy flights
02.50.Ey Stochastic processes

Exchange-hole dipole moment and the dispersion interaction revisited

Axel D. Becke and Erin R. Johnson

J. Chem. Phys. 127, 154108 (2007); http://dx.doi.org/10.1063/1.2795701 (6 pages) | Cited 39 times

Online Publication Date: 18 October 2007

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We have recently introduced a model of the dispersion interaction based on the position-dependent dipole moment of the exchange hole [ J. Chem. Phys. 122, 154104 (2005) ]. The original derivation, involving simple dipole-induced-dipole electrostatics, was somewhat heuristic, however, and lacking in rigor. Here we present a much more satisfying derivation founded on second-order perturbation theory in the closure approximation and a semiclassical evaluation of the relevant interaction integrals. Expressions for C6, C8, and C10 dispersion coefficients are obtained in a remarkably straightforward manner. Their values agree very well with ab initio reference data on dispersion coefficients between the atoms H, He, Ne, Ar, Kr, and Xe. We also highlight the importance of the exchange-hole contribution to the dispersion coefficients, especially to C6.
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34.20.Cf Interatomic potentials and forces
32.30.-r Atomic spectra
31.15.xp Perturbation theory

An improved long-range corrected hybrid exchange-correlation functional including a short-range Gaussian attenuation (LCgau-BOP)

Jong-Won Song, Seiken Tokura, Takeshi Sato, Mark A. Watson, and Kimihiko Hirao

J. Chem. Phys. 127, 154109 (2007); http://dx.doi.org/10.1063/1.2790017 (6 pages) | Cited 37 times

Online Publication Date: 18 October 2007

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A new hybrid exchange-correlation functional is presented based on the long-range correction (LC) scheme [ H. Iikura et al., J. Chem. Phys. 115, 3540 (2001) ; Tawada et al., J. Chem. Phys. 120, 8425 (2004) ], named LCgau-BOP. The key feature is the use of a two-parameter Gaussian correction to the Coulomb attenuation, which allows a more flexible description of exact exchange at short-range interelectronic separations. The new partitioning preserves 100% exact exchange in the long range, which is known to be important for the success of the LC scheme, with an asymptotic attenuation described by a standard error function with a parameter of 0.42. The LCgau partitioning was optimized for the reproduction of atomization energies over the G2 set and reaction barrier heights over Database/3, and produced results which are superior to B3LYP, CAM-BLYP, and the best LC functionals we are aware of. The results highlight the importance of including a substantial portion of exact exchange in the short range. Using the same parameters, the new functional was tested for the reproduction of geometries, as well as valence, Rydberg and charge-transfer excitations which are known challenges for conventional density functional theory. Our conclusion is that LCgau-BOP can provide a consistently more accurate description of thermochemistries, chemical reactions, and excitation energies than other existing long-range corrected functionals.
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31.15.E- Density-functional theory
31.30.-i Corrections to electronic structure
33.15.Fm Bond strengths, dissociation energies
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions

Laser-induced nonlinear response in photoassisted resonant electronic transport

Inés Urdaneta, Arne Keller, Osman Atabek, and Vladimiro Mujica

J. Chem. Phys. 127, 154110 (2007); http://dx.doi.org/10.1063/1.2787656 (9 pages) | Cited 5 times

Online Publication Date: 18 October 2007

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We consider an extension of our previous model for photoassisted electron transport in molecular and atomic junctions to the study of nonlinear processes, induced by a laser field, that are strongly influenced by a resonant level structure associated with the material structure between the two metal electrodes. Using a Floquet transformation to include the radiation field, we calculate the stationary Landauer current as a function of the intensity and frequency of the laser. The emphasis in this work is in the description of the interplay between the optical response of the junction and its transport properties. Our results may be then of importance in understanding the response of molecular junctions to multiphoton excitations, a regime where nonlinearities are dominant.
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72.40.+w Photoconduction and photovoltaic effects
72.10.Di Scattering by phonons, magnons, and other nonlocalized excitations
78.20.Jq Electro-optical effects

Ab initio density functional theory applied to quasidegenerate problems

Ireneusz Grabowski, Victor Lotrich, and Rodney J. Bartlett

J. Chem. Phys. 127, 154111 (2007); http://dx.doi.org/10.1063/1.2790013 (10 pages) | Cited 6 times

Online Publication Date: 18 October 2007

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Ab initio density functional theory (DFT), previously applied primarily at the second-order many-body perturbation theory (MBPT) level, is generalized to selected infinite-order effects by using a new coupled-cluster perturbation theory (CCPT). This is accomplished by redefining the unperturbed Hamiltonian in ab initio DFT to correspond to the CCPT2 orbital dependent functional. These methods are applied to the Be-isoelectronic systems as an example of a quasidegenerate system. The CCPT2 variant shows better convergence to the exact quantum Monte Carlo correlation potential for Be than any prior attempt. When using MBPT2, the semicanonical choice of unperturbed Hamiltonian, plays a critical role in determining the quality of the obtained correlation potentials and obtaining convergence, while the usual Kohn-Sham choice invariably diverges. However, without the additional infinite-order effects, introduced by CCPT2, the final potentials and energies are not sufficiently accurate. The issue of the effects of the single excitations on the divergence in ordinary OEP2 is addressed, and it is shown that, whereas their individual values are small, their infinite-order summation is essential to the good convergence of ab initio DFT.
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31.15.A- Ab initio calculations
31.15.E- Density-functional theory
31.15.bw Coupled-cluster theory
31.15.xp Perturbation theory

Umbrella sampling for nonequilibrium processes

Aryeh Warmflash, Prabhakar Bhimalapuram, and Aaron R. Dinner

J. Chem. Phys. 127, 154112 (2007); http://dx.doi.org/10.1063/1.2784118 (8 pages) | Cited 16 times

Online Publication Date: 18 October 2007

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The authors introduce an algorithm for determining the steady-state probability distribution of an ergodic system arbitrarily far from equilibrium. By enforcing equal sampling of different regions of phase space, as in umbrella sampling simulations of systems at equilibrium, low probability regions are explored to a much greater extent than in physically weighted simulations. The algorithm can be used to accumulate joint statistics for an arbitrary number of order parameters for a system governed by any stochastic dynamics. They demonstrate the efficiency of the algorithm by applying it to a model of a genetic toggle switch which evolves irreversibly according to a continuous time Monte Carlo procedure.
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05.70.Ln Nonequilibrium and irreversible thermodynamics
05.20.-y Classical statistical mechanics
05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion
05.10.Gg Stochastic analysis methods (Fokker-Planck, Langevin, etc.)
05.10.Ln Monte Carlo methods

Revisiting the Frenkel-Ladd method to compute the free energy of solids: The Einstein molecule approach

Carlos Vega and Eva G. Noya

J. Chem. Phys. 127, 154113 (2007); http://dx.doi.org/10.1063/1.2790426 (12 pages) | Cited 24 times

Online Publication Date: 18 October 2007

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In this paper a new method to evaluate the free energy of solids is proposed. The method can be regarded as a variant of the method proposed by Frenkel and Ladd [J. Chem. Phys. 81, 3188 (1984)] . The main equations of the method can be derived in a simple way. The method can be easily implemented within a Monte Carlo program. We have applied the method to determine the free energy of hard spheres in the solid phase for several system sizes. The obtained free energies agree within the numerical uncertainty with those obtained by Polson et al. [J. Chem. Phys. 112, 5339 (2000)] . The fluid-solid equilibria has been determined for several system sizes and compared to the values published previously by Wilding and Bruce [Phys. Rev. Lett. 85, 5138 (2000)] using the phase switch methodology. It is shown that both the free energies and the coexistence pressures present a strong size dependence and that the results obtained from free energy calculations agree with those obtained using the phase switch method, which constitutes a cross-check of both methodologies. From the results of this work we estimate the coexistence pressure of the fluid-solid transition of hard spheres in the thermodynamic limit to be p* = 11.54(4), which is slightly lower than the classical value of Hoover and Ree (p* = 11.70) [ J. Chem. Phys. 49, 3609 (1968) ]. Taking into account the strong size dependence of the free energy of the solid phase, we propose to introduce finite size corrections, which allow us to estimate approximately the free energy of the solid phase in the thermodynamic limit from the known value of the free energy of the solid phase with N molecules. We have also determined the free energy of a Lennard-Jones solid by using both the methodology of this work and the finite size correction. It is shown how a relatively good estimate of the free energy of the system in the thermodynamic limit is obtained even from the free energy of a relatively small system.
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65.40.G- Other thermodynamical quantities
65.40.Ba Heat capacity
64.70.D- Solid-liquid transitions

Real-time time-dependent density functional theory approach for frequency-dependent nonlinear optical response in photonic molecules

Y. Takimoto, F. D. Vila, and J. J. Rehr

J. Chem. Phys. 127, 154114 (2007); http://dx.doi.org/10.1063/1.2790014 (10 pages) | Cited 14 times

Online Publication Date: 19 October 2007

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We present ab initio calculations of frequency-dependent linear and nonlinear optical responses based on real-time time-dependent density functional theory for arbitrary photonic molecules. This approach is based on an extension of an approach previously implemented for a linear response using the electronic structure program SIESTA. Instead of calculating excited quantum states, which can be a bottleneck in frequency-space calculations, the response of large molecular systems to time-varying electric fields is calculated in real time. This method is based on the finite field approach generalized to the dynamic case. To speed the nonlinear calculations, our approach uses Gaussian enveloped quasimonochromatic external fields. We thereby obtain the frequency-dependent second harmonic generation β(−2ω;ω,ω), the dc nonlinear rectification β(0;−ω,ω), and the electro-optic effect β(−ω;ω,0). The method is applied to nanoscale photonic nonlinear optical molecules, including p-nitroaniline and the FTC chromophore, i.e., 2-[3-Cyano-4-(2-{5-[2-(4-diethylamino-phenyl)-vinyl] - thiophen-2-yl} - vinyl)-5,5-dimethyl-5H-furan-2-ylidene]- malononitrile, and yields results in good agreement with experiment.
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33.57.+c Magneto-optical and electro-optical spectra and effects
31.15.E- Density-functional theory
31.15.A- Ab initio calculations
31.15.vj Electron correlation calculations for atoms and ions: excited states

The appearance of an interval of energies that contain the whole diamagnetic contribution to NMR magnetic shieldings

Alejandro Maldonado and Gustavo A. Aucar

J. Chem. Phys. 127, 154115 (2007); http://dx.doi.org/10.1063/1.2787003 (8 pages) | Cited 10 times

Online Publication Date: 19 October 2007

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Working within relativistic polarization propagator approach, it was shown in a previous article that the electronic origin of diamagnetic contributions to NMR nuclear magnetic shielding, σd, are mostly excitations that fit in a well defined interval of energies such that 2mc2≦(εi−math)<4mc2. That interval of energies does not have, in principle, any physical reason to be so well defined, and gives a large amount of the total contribution to σd, e.g., close to 98% of it. Then a further study is given in this article, where we show some of the main characteristics of that interval of energy, such as its universal appearance and basis set independence. Our main result is the finding that σd is completely described by that interval of excitation energies, i.e., there is no contribution arising from outside of it. Most of the contributions belonging to that interval arise from virtual electronic energies larger than −3mc2. For heavier atoms, there are few contributions from states with virtual negative energies smaller than −3mc2. The model systems under study were noble gases, XH (X = Br, I, and At), XH2 (X = O, S, Se, Te, and Po), XH3 (X = N, P, As, Sb, and Bi); XH4 (X = Sn and Pb), and SnXH3 (X = Br and I). The pattern of contributions of occupied molecular orbitals (MOs) is also shown, where the 1s1/2 is the most important for excitations ending in the bottom half part of the above mentioned interval. On the other hand, the contribution of the other occupied MOs are more important than that of 1s1/2 for the other part of such interval. We also show that σd is electron correlation independent within both relativistic and nonrelativistic domain. In the case of σp, we find out a clear dependence of electron correlation effects with relativistic effects, which is of the order of 30% for Pb in PbH4.
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32.30.Dx Magnetic resonance spectra
33.25.+k Nuclear resonance and relaxation
31.30.J- Relativistic and quantum electrodynamic (QED) effects in atoms, molecules, and ions
31.15.V- Electron correlation calculations for atoms, ions and molecules

Double-hybrid density functional theory for excited electronic states of molecules

Stefan Grimme and Frank Neese

J. Chem. Phys. 127, 154116 (2007); http://dx.doi.org/10.1063/1.2772854 (18 pages) | Cited 60 times

Online Publication Date: 19 October 2007

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Double-hybrid density functionals are based on a mixing of standard generalized gradient approximations (GGAs) for exchange and correlation with Hartree-Fock (HF) exchange and a perturbative second-order correlation part (PT2) that is obtained from the Kohn-Sham (GGA) orbitals and eigenvalues. This virtual orbital-dependent functional (dubbed B2PLYP) contains only two empirical parameters that describe the mixture of HF and GGA exchange (ax) and of the PT2 and GGA correlation (ac), respectively. Extensive testing has recently demonstrated the outstanding accuracy of this approach for various ground state problems in general chemistry applications. The method is extended here without any further empirical adjustments to electronically excited states in the framework of time-dependent density functional theory (TD-DFT) or the closely related Tamm-Dancoff approximation (TDA-DFT). In complete analogy to the ground state treatment, a scaled second-order perturbation correction to configuration interaction with singles (CIS(D)) wave functions developed some years ago by Head-Gordon et al. [Chem. Phys. Lett. 219, 21 (1994)] is computed on the basis of density functional data and added to the TD(A)-DFT∕GGA excitation energy. The method is implemented by applying the resolution of the identity approximation and the efficiency of the code is discussed. Extensive tests for a wide variety of molecules and excited states (of singlet, triplet, and doublet multiplicities) including electronic spectra are presented. In general, rather accurate excitation energies (deviations from reference data typically <0.2 eV) are obtained that are mostly better than those from standard functionals. Still, systematic errors are obtained for Rydberg (too low on average by about 0.3 eV) and charge-transfer transitions but due to the relatively large ax parameter (0.53), B2PLYP outperforms most other functionals in this respect. Compared to conventional HF-based CIS(D), the method is more robust in electronically complex situations due to the implicit account of static correlation effects by the GGA parts. The (D) correction often works in the right direction and compensates for the overestimation of the transition energy at the TD level due to the elevated fraction of HF exchange in the hybrid GGA part. Finally, the limitations of the method are discussed for challenging systems such as transition metal complexes, cyanine dyes, and multireference cases.
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31.15.E- Density-functional theory
31.15.vj Electron correlation calculations for atoms and ions: excited states
31.15.ve Electron correlation calculations for atoms and ions: ground state
31.15.xp Perturbation theory

Performance of the correlation consistent composite approach for transition states: A comparison to G3B theory

Thomas V. Grimes, Angela K. Wilson, Nathan J. DeYonker, and Thomas R. Cundari

J. Chem. Phys. 127, 154117 (2007); http://dx.doi.org/10.1063/1.2790011 (8 pages) | Cited 14 times

Online Publication Date: 19 October 2007

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The correlation consistent composite approach (ccCA) was applied to the prediction of reaction barrier heights (i.e., transition state energy relative to reactants and products) for a standard benchmark set of reactions comprised of both hydrogen transfer reactions and nonhydrogen transfer reactions (i.e., heavy-atom transfer, SN2, and unimolecular reactions). The ccCA method was compared against G3B for the same set of reactions. Error metrics indicate that ccCA achieves “chemical accuracy” with a mean unsigned error (MUE) of 0.89 kcal/mol with respect to the benchmark data for barrier heights; G3B has a mean unsigned error of 1.94 kcal/mol. Further, the greater accuracy of ccCA for predicted reaction barriers is compared to other benchmarked literature methods, including density functional (BB1K, MUE = 1.16 kcal/mol) and wavefunction-based [QCISD(T), MUE = 1.10 kcal/mol] methods.
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82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
82.20.Db Transition state theory and statistical theories of rate constants
82.20.Pm Rate constants, reaction cross sections, and activation energies
82.20.Sb Correlation function theory of rate constants and its applications
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

On the excited electronic state dissociation of nitramine energetic materials and model systems

Y. Q. Guo, M. Greenfield, A. Bhattacharya, and E. R. Bernstein

J. Chem. Phys. 127, 154301 (2007); http://dx.doi.org/10.1063/1.2787587 (10 pages) | Cited 11 times

Online Publication Date: 15 October 2007

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In order to elucidate the difference between nitramine energetic materials, such as RDX (1,3,5-trinitro-1,3,5-triazacyclohexane), HMX (1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane), and CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane), and their nonenergetic model systems, including 1,4-dinitropiperazine, nitropiperidine, nitropyrrolidine, and dimethylnitramine, both nanosecond mass resolved excitation spectroscopy and femtosecond pump-probe spectroscopy in the UV spectral region have been employed to investigate the mechanisms and dynamics of the excited electronic state photodissociation of these materials. The NO molecule is an initial decomposition product of all systems. The NO molecule from the decomposition of energetic materials displays cold rotational and hot vibrational spectral structures. Conversely, the NO molecule from the decomposition of model systems shows relatively hot rotational and cold vibrational spectra. In addition, the intensity of the NO ion signal from energetic materials is proportional to the number of nitramine functional groups in the molecule. Based upon experimental observations and theoretical calculations of the potential energy surface for these systems, we suggest that energetic materials dissociate from ground electronic states after internal conversion from their first excited states, and model systems dissociate from their first excited states. In both cases a nitro-nitrite isomerization is suggested to be part of the decomposition mechanism. Parent ions of dimethylnitramine and nitropyrrolidine are observed in femtosecond experiments. All the other molecules generate NO as a decomposition product even in the femtosecond time regime. The dynamics of the formation of the NO product is faster than 180 fs, which is equivalent to the time duration of our laser pulse.
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82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.20.Hf Product distribution
82.20.Kh Potential energy surfaces for chemical reactions
82.53.Eb Pump probe studies of photodissociation
31.50.Df Potential energy surfaces for excited electronic states
33.20.Lg Ultraviolet spectra

Theoretical prediction of the ionization energies of the C4H7 radicals: 1-Methylallyl, 2-methylallyl, cyclopropylmethyl, and cyclobutyl radicals

Kai-Chung Lau, Wenxu Zheng, Ning-Bew Wong, and Wai-Kee Li

J. Chem. Phys. 127, 154302 (2007); http://dx.doi.org/10.1063/1.2774972 (12 pages) | Cited 3 times

Online Publication Date: 15 October 2007

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The ionization energies (IEs) for the 1-methylallyl, 2-methylallyl, cyclopropylmethyl, and cyclobutyl radicals have been calculated by the wave function based ab initio CCSD(T)/CBS approach, which involves the approximation to the complete basis set (CBS) limit at the coupled cluster level with single and double excitations plus quasiperturbative triple excitation [CCSD(T)]. The zero-point vibrational energy correction, the core-valence electronic correction, and the scalar relativistic effect correction are included in these calculations. The present CCSD(T)/CBS results are then compared with the IEs determined in the photoelectron experiment by Schultz et al. [J. Am. Chem. Soc. 106, 7336 (1984)] The predicted IE value (7.881 eV) of 2-methylallyl radical is found to compare very favorably with the experimental value of 7.90±0.02 eV. Two ionization transitions for cis-1-methylallyl and trans-1-methylallyl radicals have been considered here. The comparison between the predicted IE values and the previous measurements shows that the photoelectron peak observed by Schultz et al. likely corresponds to the adiabatic ionization transition for the trans-1-methylallyl radical to form trans-1-methylallyl cation. Although a precise IE value for the cyclopropylmethyl radical has not been directly determined, the experimental value deduced indirectly using other known energetic data is found to be in good accord with the present CCSD(T)/CBS prediction. We expect that the Franck-Condon factor for ionization transition of c-C4H7→bicyclobutonium is much less favorable than that for ionization transition of c-C4H7→planar-C4H7+, and the observed IE in the previous photoelectron experiment is likely due to the ionization transition for c-C4H7→planar-C4H7+. Based on our CCSD(T)/CBS prediction, the ionization transition of c-C4H7→bicyclobutonium with an IE value around 6.92 eV should be taken as the adiabatic ionization transition for the cyclobutyl radical. The present study provides support for the conclusion that the CCSD(T)/CBS approach with high-level energetic corrections can be used to provide reliable IE predictions for C4 hydrocarbon radicals with an uncertainty of ±22 meV. The CCSD(T)/CBS predictions to the heats of formation for the aforementioned radicals and cations are also presented.
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31.15.A- Ab initio calculations
31.15.bw Coupled-cluster theory
31.15.xp Perturbation theory
31.30.J- Relativistic and quantum electrodynamic (QED) effects in atoms, molecules, and ions
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy

Vibrational lifetimes of cyanide and carbon monoxide on noble and transition metal surfaces

Mattias Forsblom and Mats Persson

J. Chem. Phys. 127, 154303 (2007); http://dx.doi.org/10.1063/1.2794744 (5 pages) | Cited 4 times

Online Publication Date: 16 October 2007

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The electronic vibrational damping rates of the CN and CO internal stretch modes on the (111) surfaces of Ag, Cu, Au, and Pt were calculated using density functional theory calculations. Our calculated damping rates are in excellent agreement with experimental data obtained from pump-probe laser spectroscopy. The striking difference in trends and magnitudes between the internal stretch modes of CN and CO is in part rationalized in terms of the adsorbate-induced electronic structure within the framework of a simple Newns-Anderson model.
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68.43.Bc Ab initio calculations of adsorbate structure and reactions
68.35.Ja Surface and interface dynamics and vibrations
71.15.Mb Density functional theory, local density approximation, gradient and other corrections

An eight-degree-of-freedom, time-dependent quantum dynamics study for the H2+C2H reaction on a new modified potential energy surface

Dunyou Wang and Winifred M. Huo

J. Chem. Phys. 127, 154304 (2007); http://dx.doi.org/10.1063/1.2794757 (7 pages) | Cited 6 times

Online Publication Date: 16 October 2007

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An eight-dimensional time-dependent quantum dynamics wave packet approach is performed for the study of the H2+C2H→H+C2H2 reaction system on a new modified potential energy surface (PES) [ L.-P. Ju et al., Chem. Phys. Lett. 409, 249 (2005) ]. This new potential energy surface is obtained by modifying Wang and Bowman’s old PES [ J. Chem. Phys. 101, 8646 (1994) ] based on the new ab initio calculation. This new modified PES has a much lower transition state barrier height at 2.29 kcal/mol than Wang and Bowman’s old PES at 4.3 kcal/mol. This study shows that the reactivity for this diatom-triatom reaction system is enhanced by vibrational excitations of H2, whereas the vibrational excitations of C2H only have a small effect on the reactivity. Furthermore, the bending excitations of C2H, compared to the ground state reaction probability, hinder the reactivity. The comparison of the rate constant between this calculation and experimental results agrees with each other very well. This comparison indicates that the new modified PES corrects the large barrier height problem in Wang and Bowman’s old PES.
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82.20.Kh Potential energy surfaces for chemical reactions
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.40.-g Chemical kinetics and reactions: special regimes and techniques

Accounting for the dependence of P(E′,E) on the maximum impact parameter in classical trajectory calculations: Application to the H2O–H2O collisional relaxation

Raúl A. Bustos-Marún, Eduardo A. Coronado, and Juan C. Ferrero

J. Chem. Phys. 127, 154305 (2007); http://dx.doi.org/10.1063/1.2794760 (7 pages)

Online Publication Date: 16 October 2007

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In this work we report a novel methodology that is able to predict how energy transfer transition probability density functions [P(E′,E)] change with the maximum impact parameter (bmax) used in trajectory calculations (TC’s). The method assumes that P(E′,E) can be described by a sum of exponential functions and that all the trajectories with an initial impact parameter beyond a certain critical value will contribute only to the elastic peak [P(E′,E) for E′ = E]. This approach is applied to H2O–H2O collisions at different initial vibrational energies of the excited molecules and temperatures of bath gas. The results show that it is possible to reproduce with high accuracy the whole P(E′,E) obtained from a given bmax, using the results of TC’s performed at another bmax. The new methodology also leads us to propose a new criterion to choose the value of bmax.
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33.15.Mt Rotation, vibration, and vibration-rotation constants
34.10.+x General theories and models of atomic and molecular collisions and interactions (including statistical theories, transition state, stochastic and trajectory models, etc.)
34.50.-s Scattering of atoms and molecules
33.20.Tp Vibrational analysis

Observation of three weakly bound valence states of I2

Trevor Ridley, Kenneth P. Lawley, and Robert J. Donovan

J. Chem. Phys. 127, 154306 (2007); http://dx.doi.org/10.1063/1.2795722 (7 pages) | Cited 3 times

Online Publication Date: 16 October 2007

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Structured emission in the gas phase to two weakly bound valence states that correlate with the third dissociation limit, I*(2P1/2)+I*(2P1/2), designated as (bb), from two third tier ion-pair states of I2 correlating with I−(1S0)+I+(1D2), the 1g(1D2), and F′0u+(1D2) states, has been observed for the first time. The 1u(bb) state is shown to be bound by 377±2 cm−1 and molecular constants have been determined. Vibrational structure in the 0g+(bb) state could not be resolved but the spectrum is consistent with the state being bound by 435 cm−1. The relative integrated intensities of the emissions from both ion-pair states to various valence states have also been measured, and some aspects are rationalized in terms of the electronic configurations of the upper and lower states. Bound levels of a previously uncharacterized 1g(ab) valence state have also been observed in emission from the γ1u(3P2) ion-pair state. The lower state is shown to be bound by 270±2 cm−1 and molecular constants have been determined.
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33.15.Mt Rotation, vibration, and vibration-rotation constants
33.15.Fm Bond strengths, dissociation energies
33.70.Fd Absolute and relative line and band intensities
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