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14 Aug 2010

Volume 133, Issue 6, Articles (06xxxx)

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J. Chem. Phys. 133, 064504 (2010); http://dx.doi.org/10.1063/1.3455889 (10 pages)

H. E. Maynard-Casely, C. L. Bull, M. Guthrie, I. Loa, M. I. McMahon, E. Gregoryanz, R. J. Nelmes, and J. S. Loveday
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Communication: Ionization and Coulomb explosion of xenon clusters by intense, few-cycle laser pulses

D. Mathur and F. A. Rajgara

J. Chem. Phys. 133, 061101 (2010); http://dx.doi.org/10.1063/1.3469821 (4 pages) | Cited 5 times

Online Publication Date: 9 August 2010

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Intense, ultrashort pulses of 800 nm laser light (12 fs, ∼ 4 optical cycles) of peak intensity 5×1014 W cm−2 have been used to irradiate gas-phase Xen clusters (n = 500–25 000) so as to induce multiple ionization and subsequent Coulomb explosion. Energy distributions of exploding ions are measured in the few-cycle domain that does not allow sufficient time for the cluster to undergo expansion due to Coulombic and hydrodynamic pressures. This results in overall dynamics that appear to be significantly different to those in the many-cycle regime. One manifestation is that the maximum ion energies are measured to be much lower than those obtained when longer pulses of the same intensity are used. Ion yields are cluster-size independent but polarization dependent in that they are significantly larger when the polarization is perpendicular to the detection axis than along it. This unexpected behavior is qualitatively rationalized in terms of a spatially anisotropic shielding effect induced by the electronic charge cloud within the cluster.
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36.40.Qv Stability and fragmentation of clusters
33.80.Eh Autoionization, photoionization, and photodetachment
33.80.Gj Diffuse spectra; predissociation, photodissociation
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Communication: Conical intersections using constrained density functional theory–configuration interaction

Benjamin Kaduk and Troy Van Voorhis

J. Chem. Phys. 133, 061102 (2010); http://dx.doi.org/10.1063/1.3470106 (4 pages) | Cited 3 times

Online Publication Date: 13 August 2010

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The constrained density functional theory–configuration interaction (CDFT-CI) method has previously been used to calculate ground-state energies and barrier heights. In this work, it is examined for use in computing electronic excited states, for the challenging case of conical intersections. Conical intersections are a prevalent feature of excited electronic surfaces, but conventional time-dependent density functional theory calculations are found to be entirely unsatisfactory at describing them, for two small systems. CDFT-CI calculations on those systems are found to be in qualitative agreement with reference CAS surfaces. These results suggest that with a suitable definition of atomic populations and a careful choice of constrained states, CDFT-CI could be the basis for a seamless description of electronic degeneracy.
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31.15.es Applications of density-functional theory (e.g., to electronic structure and stability; defect formation; dielectric properties, susceptibilities; viscoelastic coefficients; Rydberg transition frequencies)
31.15.vj Electron correlation calculations for atoms and ions: excited states
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Communication: Molecular dynamics simulations of the interfacial structure of alkali metal fluoride solutions

Haijun Feng, Jian Zhou, Xiaohua Lu, and Kristen A. Fichthorn

J. Chem. Phys. 133, 061103 (2010); http://dx.doi.org/10.1063/1.3478520 (4 pages) | Cited 1 time

Online Publication Date: 13 August 2010

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Molecular dynamics simulations are carried out to study the interfacial profiles of alkali metal fluoride solutions (NaF, KF, RbF, and CsF) at 1 atm and 300 K. For these solutions, we find that the occupancy of the cations in the interfacial region is comparable to or greater than that of the F− anion. Cations that have weaker hydration abilities have higher concentrations at the interface. The order of enhanced concentrations of cations at the interface is Na+<K+<Rb+<Cs+. The partitioning mechanism can be understood in terms of ionic hydration theory, which shows that the interfacial behavior of ions is related to hydration interactions. This work provides new insight into the interfacial structure of electrolyte solutions and enriches the theory of electrolyte interfaces.
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68.08.De Liquid-solid interface structure: measurements and simulations
82.30.Nr Association, addition, insertion, cluster formation
82.45.Gj Electrolytes
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
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back to top Theoretical Methods and Algorithms

A density functional theory study of the zero-field splitting in high-spin nitrenes

Eugenii Ya. Misochko, Denis V. Korchagin, Konstantin V. Bozhenko, Sergei V. Chapyshev, and Sergei M. Aldoshin

J. Chem. Phys. 133, 064101 (2010); http://dx.doi.org/10.1063/1.3474574 (10 pages)

Online Publication Date: 9 August 2010

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This work presents a detailed evaluation of the performance of density functional theory (DFT) for the prediction of zero-field splittings (ZFSs) in high-spin nitrenes. A number of well experimentally characterized triplet mononitrenes, quartet nitrenoradicals, quintet dinitrenes, and septet trinitrenes have been considered. Several DFT-based approaches for the prediction of ZFSs have been compared. It is shown that the unrestricted Kohn–Sham and the Pederson–Khanna approaches are the most successful for the estimation of the direct spin-spin (SS) interaction and the spin-orbit coupling (SOC) parts, respectively, to the final ZFS parameters. The most accurate theoretical predictions (within 10%) are achieved by using the PBE density functional in combination with the DZ, EPR-II, and TZV basis sets. For high-spin nitrenes constituted from light atoms, the contribution of the SOC part to ZFS parameters is quite small (7%–12%). By contrast, for chlorine-substituted septet trinitrenes, the contribution of the SOC part is small only to D value but, in the case of E value, it is as large as the SS part and has opposite sign. Due to this partial cancellation of two different contributions, SS and SOC, the resulting values of E in heavy molecules are almost two times smaller than those predicted by analysis of the widely used semiempirical one-center spin-spin interaction model. The decomposition of DSS into n-center (n = 1–4) interactions shows that the major contribution to DSS results from the one-center spin-spin interactions. This fact indicates that the semiempirical SS interaction model accurately predicts the ZFS parameters for all types of high-spin nitrenes with total spin S = 2 and 3, if their molecules are constructed from the first-row atoms.
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33.15.Pw Fine and hyperfine structure
31.30.Gs Hyperfine interactions and isotope effects
31.15.ej Spin-density functionals

Quantum Monte Carlo ground state energies for the singly charged ions from Li through Ar

P. Maldonado, A. Sarsa, E. Buendía, and F. J. Gálvez

J. Chem. Phys. 133, 064102 (2010); http://dx.doi.org/10.1063/1.3467879 (7 pages) | Cited 1 time

Online Publication Date: 10 August 2010

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Nonrelativistic frozen nucleus all-electron Quantum Monte Carlo ground state energies of positive and negative ions Li+ to Ar+ and Li− to Cl−, respectively, are reported. Explicitly correlated wave functions with a single configuration model function times a Jastrow factor are employed for all of the systems studied. The accuracy obtained for the ions in the third period is similar to that reached for the ions in the second one. For those ions with a stronger multiconfiguration nature a restricted multiconfiguration expansion has been employed. The ground state energy here obtained for the charged species shows a similar quality to that reached for neutral atoms. Starting from those results, ionization potentials and electron affinities are calculated.
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31.15.xv Molecular dynamics and other numerical methods
32.50.+d Fluorescence, phosphorescence (including quenching)
02.70.Ss Quantum Monte Carlo methods

Free energy calculations using dual-level Born–Oppenheimer molecular dynamics

Marius Retegan, Marilia Martins-Costa, and Manuel F. Ruiz-López

J. Chem. Phys. 133, 064103 (2010); http://dx.doi.org/10.1063/1.3466767 (5 pages) | Cited 3 times

Online Publication Date: 10 August 2010

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We describe an efficient and accurate method to compute free energy changes in complex chemical systems that cannot be described through classical molecular dynamics simulations, examples of which are chemical and photochemical reactions in solution, enzymes, interfaces, etc. It is based on the use of dual-level Born–Oppenheimer molecular dynamics simulations. A low-level quantum mechanical method is employed to calculate the potential of mean force through the umbrella sampling technique. Then, a high-level quantum mechanical method is used to estimate a free energy correction on selected points of the reaction coordinate using perturbation theory. The precision of the results is comparable to that of ab initio molecular dynamics methods such as the Car–Parrinello approach but the computational cost is much lower, roughly by two to three orders of magnitude. The method is illustrated by discussing the association free energy of simple organometallic compounds, although the field of application is very broad.
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82.60.-s Chemical thermodynamics
82.30.Nr Association, addition, insertion, cluster formation
82.20.Wt Computational modeling; simulation
82.50.-m Photochemistry

Spin-component-scaling second-order Møller–Plesset theory and its variants for economical correlation energies: Unified theoretical interpretation and use for quartet N3

A. J. C. Varandas

J. Chem. Phys. 133, 064104 (2010); http://dx.doi.org/10.1063/1.3465551 (15 pages) | Cited 3 times

Online Publication Date: 11 August 2010

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The spin-component-scaling second-order Møller–Plesset theory proposed by Grimme, the scaled opposite-spin variant of Head-Gordon and co-workers, and other variants of the theory to treat the electron correlation energy are examined. A refinement of scaled opposite-spin theory for strong chemical interactions is suggested where the scaled correlation contribution is chosen such as to mimic closely the one obtained by more sophisticated methods of the coupled cluster type. With the scaling factor chosen to vary in a simple statistical manner with the number of opposite-spin electron pairs of the system, the parameters have been calibrated from standard coupled cluster type calculations for a chosen ab initio test data set. The new approach, termed as variable-scaling opposite spin, aims to be applicable at any regions of the molecule configuration space where second-order Møller–Plesset perturbation theory converges. It thus benefits of all advantages inherent to the original theory, which makes it an attractive approach on a computational cost basis. Because the method in one of its formats fails size-extensivity, the consequences and remedies of this are analyzed. Illustrations are presented for many molecules utilizing Dunning-type basis sets, in particular, for a detailed analysis of N3 in its lowest quartet state, which does not belong to the test set. Extrapolations of the calculated raw energies to the complete one-electron basis set limit are also reported, giving the most reliable estimates available thus far of the energetics for the N(4S)+N2 exchange reaction. All spin-component-scaling schemes are known to show difficulties in dealing with weak interactions of the van der Waals type, which has justified the design of specific variants of the theory according to the property and regime of interactions. Several variants of the theory are then examined using a second test set of molecules, and shown to be linked via a coordinate that evolves gradually between two known extreme regimes. It is further shown that such a coordinate can be specified via a constrained Feenberg-type scaling approach, a theory whose merits are also explored.
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31.15.vq Electron correlation calculations for polyatomic molecules
34.20.Gj Intermolecular and atom-molecule potentials and forces
31.15.xp Perturbation theory
33.15.Bh General molecular conformation and symmetry; stereochemistry
31.15.A- Ab initio calculations
31.15.bw Coupled-cluster theory

Linear complex polarization propagator in a four-component Kohn–Sham framework

Sebastien Villaume, Trond Saue, and Patrick Norman

J. Chem. Phys. 133, 064105 (2010); http://dx.doi.org/10.1063/1.3461163 (10 pages) | Cited 5 times

Online Publication Date: 12 August 2010

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An algorithm for the solution of the linear response equation in the random phase approximation is presented. All entities including frequency arguments, matrices, and vectors, are assumed to be complex, and it represents the core equation solver needed in complex polarization propagator approaches where nonstimulated relaxation channels are taken into account. Stability and robustness of the algorithm are demonstrated in applications regarding visible, ultraviolet, and x-ray spectroscopies. An implementation of the algorithm at the level of four-component relativistic, noncollinear, density functional theory for imaginary (but not complex) frequency arguments has been achieved and is used to determine the electric dipole dispersion interaction coefficients for the rubidium and cesium dimers. Our best estimates for the C6 coefficients of Rb2 and Cs2 are equal to 14.0×103 and 21.9×103 a.u., respectively.
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31.15.E- Density-functional theory
31.10.+z Theory of electronic structure, electronic transitions, and chemical binding

Spin-adapted open-shell random phase approximation and time-dependent density functional theory. I. Theory

Zhendong Li and Wenjian Liu

J. Chem. Phys. 133, 064106 (2010); http://dx.doi.org/10.1063/1.3463799 (22 pages) | Cited 8 times

Online Publication Date: 13 August 2010

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The spin-adaptation of single-reference quantum chemical methods for excited states of open-shell systems has been nontrivial. The primary reason is that the configuration space, generated by a truncated rank of excitations from only one component of a reference multiplet, is spin-incomplete. Those “missing” configurations are of higher ranks and can, in principle, be recaptured by a particular class of excitation operators. However, the resulting formalisms are then quite involved and there are situations [e.g., time-dependent density functional theory (TD-DFT) under the adiabatic approximation] that prevent one from doing so. To solve this issue, we propose here a tensor-coupling scheme that invokes all the components of a reference multiplet (i.e., a tensor reference) rather than increases the excitation ranks. A minimal spin-adapted n-tuply excited configuration space can readily be constructed by tensor products between the n-tuple tensor excitation operators and the chosen tensor reference. Further combined with the tensor equation-of-motion formalism, very compact expressions for excitation energies can be obtained. As a first application of this general idea, a spin-adapted open-shell random phase approximation is first developed. The so-called “translation rule” is then adopted to formulate a spin-adapted, restricted open-shell Kohn–Sham (ROKS)-based TD-DFT (ROKS-TD-DFT). Here, a particular symmetry structure has to be imposed on the exchange-correlation kernel. While the standard ROKS-TD-DFT can access only excited states due to singlet-coupled single excitations, i.e., only some of the singly excited states of the same spin (Si) as the reference, the new scheme can capture all the excited states of spin Si−1, Si, or Si+1 due to both singlet- and triplet-coupled single excitations. The actual implementation and computation are very much like the (spin-contaminated) unrestricted Kohn–Sham-based TD-DFT. It is also shown that spin-contaminated spin-flip configuration interaction approaches can easily be spin-adapted via the tensor-coupling scheme.
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31.15.ee Time-dependent density functional theory
31.15.eg Exchange-correlation functionals (in current density functional theory)
31.15.vj Electron correlation calculations for atoms and ions: excited states

Conical intersections of free energy surfaces in solution: Effect of electron correlation on a protonated Schiff base in methanol solution

Toshifumi Mori, Katsuhiro Nakano, and Shigeki Kato

J. Chem. Phys. 133, 064107 (2010); http://dx.doi.org/10.1063/1.3472033 (11 pages) | Cited 3 times

Online Publication Date: 13 August 2010

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The minimum energy conical intersection (MECI) optimization method with taking account of the dynamic electron correlation effect [ T. Mori and S. Kato, Chem. Phys. Lett. 476, 97 (2009) ] is extended to locate the MECI of nonequilibrium free energy surfaces in solution. A multistate electronic perturbation theory is introduced into the nonequilibrium free energy formula, which is defined as a function of solute and solvation coordinates. The analytical free energy gradient and interstate coupling vectors are derived, and are applied to locate MECIs in solution. The present method is applied to study the cis-trans photoisomerization reaction of a protonated Schiff base molecule (PSB3) in methanol (MeOH) solution. It is found that the effect of dynamic electron correlation largely lowers the energy of S1 state. We also show that the solvation effect strongly stabilizes the MECI obtained by twisting the terminal CN bond to become accessible in MeOH solution, whereas the conical intersection is found to be unstable in gas phase. The present study indicates that both electron correlation and solvation effects are important in the photoisomerization reaction of PSB3. The effect of counterion is also examined, and seems to be rather small in solution. The structures of free energy surfaces around MECIs are also discussed.
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82.60.Lf Thermodynamics of solutions
82.30.Nr Association, addition, insertion, cluster formation
82.50.-m Photochemistry
82.30.Qt Isomerization and rearrangement
82.20.-w Chemical kinetics and dynamics

An efficient matrix-matrix multiplication based antisymmetric tensor contraction engine for general order coupled cluster

Michael Hanrath and Anna Engels-Putzka

J. Chem. Phys. 133, 064108 (2010); http://dx.doi.org/10.1063/1.3467878 (11 pages) | Cited 4 times

Online Publication Date: 13 August 2010

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In this paper, we present an efficient implementation of general tensor contractions, which is part of a new coupled-cluster program. The tensor contractions, used to evaluate the residuals in each coupled-cluster iteration are particularly important for the performance of the program. We developed a generic procedure, which carries out contractions of two tensors irrespective of their explicit structure. It can handle coupled-cluster-type expressions of arbitrary excitation level. To make the contraction efficient without loosing flexibility, we use a three-step procedure. First, the data contained in the tensors are rearranged into matrices, then a matrix-matrix multiplication is performed, and finally the result is backtransformed to a tensor. The current implementation is significantly more efficient than previous ones capable of treating arbitrary high excitations.
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31.15.bw Coupled-cluster theory
02.10.Ud Linear algebra

The multiscale coarse-graining method: Assessing its accuracy and introducing density dependent coarse-grain potentials

Sergei Izvekov, Peter W. Chung, and Betsy M. Rice

J. Chem. Phys. 133, 064109 (2010); http://dx.doi.org/10.1063/1.3464776 (16 pages) | Cited 7 times

Online Publication Date: 13 August 2010

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The ability of particle-based coarse-grain potentials, derived using the recently proposed multiscale coarse-graining (MS-CG) methodology [ S. Izvekov and G. A. Voth, J. Phys. Chem. B 109, 2469 (2005) ; J. Chem. Phys. 123, 134105 (2005) ] to reconstruct atomistic free-energy surfaces in coarse-grain coordinates is discussed. The MS-CG method is based on force-matching generalized forces associated with the coarse-grain coordinates. In this work, we show that the MS-CG method recovers only part of the atomistic free-energy landscape in the coarse-grain coordinates (termed the potential of mean force contribution). The portion of the atomistic free-energy landscape that is left out in the MS-CG procedure contributes to a pressure difference between atomistic and coarse-grain ensembles. Employing one- and two-site coarse-graining of nitromethane as worked examples, we discuss the virial and compressibility constraints to incorporate a pressure correction interaction into the MS-CG potentials and improve performance at different densities. The nature of the pressure correction interaction is elucidated and compared with those used in structure-based coarse-graining. As pairwise approximations to the atomistic free-energy, the MS-CG potentials naturally depend on the variables describing a thermodynamic state, such as temperature and density. Such dependencies limit state-point transferability. For nitromethane, the one- and two-site MS-CG potentials appear to be transferable across a broad range of temperatures. In particular, the two-site models, which are matched to low and ambient temperature liquid states, perform well in simulations of the ambient crystal structure. In contrast, the transferability of the MS-CG models of nitromethane across different densities is found to be problematic. To achieve better state-point transferability, density dependent MS-CG potentials are introduced and their performance is examined in simulations of nitromethane under various thermodynamic conditions, including shocked states.
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65.20.Jk Studies of thermodynamic properties of specific liquids
61.66.Hq Organic compounds
61.20.-p Structure of liquids
61.20.Gy Theory and models of liquid structure
65.40.-b Thermal properties of crystalline solids
61.25.Em Molecular liquids
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Lifetimes of heavy-Rydberg ion-pair states formed through Rydberg electron transfer

M. Cannon, C. H. Wang, F. B. Dunning, and C. O. Reinhold

J. Chem. Phys. 133, 064301 (2010); http://dx.doi.org/10.1063/1.3466924 (7 pages) | Cited 2 times

Online Publication Date: 11 August 2010

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The lifetimes of K+∙∙Cl−, K+∙∙CN−, and K+∙∙SF6− heavy-Rydberg ion-pair states produced through Rydberg electron transfer reactions are measured directly as a function of binding energy using electric field induced detachment and the ion-pair decay channels discussed. The data are interpreted using a Monte Carlo collision code that models the detailed kinematics of electron transfer reactions. The lifetimes of K+∙∙Cl− ion-pair states are observed to be very long, >100 μs, and independent of binding energy. The lifetimes of strongly bound (>30 meV) K+∙∙CN− ion pairs are found to be similarly long but begin to decrease markedly as the binding energy is reduced below this value. This behavior is attributed to conversion of rotational energy in the CN− ion into translational energy of the ion pair. No long-lived K+∙∙SF6− ion pairs are observed, their lifetimes decreasing with increasing binding energy. This behavior suggests that ion-pair loss is associated with mutual neutralization as a result of charge transfer.
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34.50.Gb Electronic excitation and ionization of molecules
34.50.Ez Rotational and vibrational energy transfer
33.15.Mt Rotation, vibration, and vibration-rotation constants
34.70.+e Charge transfer
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
32.80.Ee Rydberg states

Ultrafast deactivation processes in the 2-aminopyridine dimer and the adenine-thymine base pair: Similarities and differences

Yue-Jie Ai, Feng Zhang, Gang-Long Cui, Yi Luo, and Wei-Hai Fang

J. Chem. Phys. 133, 064302 (2010); http://dx.doi.org/10.1063/1.3464485 (7 pages) | Cited 1 time

Online Publication Date: 11 August 2010

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2-aminopyridine dimer has frequently been used as a model system for studying photochemistry of DNA base pairs. We examine here the relevance of 2-aminopyridine dimer for a Watson–Crick adenine-thymine base pair by studying UV-light induced photodynamics along two main hydrogen bridges after the excitation to the localized 1ππ∗ excited-state. The respective two-dimensional potential-energy surfaces have been determined by time-dependent density functional theory with Coulomb-attenuated hybrid exchange-correlation functional (CAM-B3LYP). Different mechanistic aspects of the deactivation pathway have been analyzed and compared in detail for both systems, while the related reaction rates have also be obtained from Monte Carlo kinetic simulations. The limitations of the 2-aminopyridine dimer as a model system for the adenine-thymine base pair are discussed.
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87.15.ht Ultrafast dynamics; charge transfer
82.39.-k Chemical kinetics in biological systems
87.15.ak Monte Carlo simulations
82.20.Kh Potential energy surfaces for chemical reactions
82.50.Hp Processes caused by visible and UV light
87.14.gk DNA

Imaging the radical channel in acetaldehyde photodissociation: Competing mechanisms at energies close to the triplet exit barrier

G. A. Amaral, A. Arregui, L. Rubio-Lago, J. D. Rodríguez, and L. Bañares

J. Chem. Phys. 133, 064303 (2010); http://dx.doi.org/10.1063/1.3474993 (8 pages) | Cited 2 times

Online Publication Date: 11 August 2010

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The photodissociation of acetaldehyde in the radical channel has been studied at wavelengths between 315 and 325 nm using the velocity-map imaging technique. Upon one-photon absorption at 315 nm, the molecule is excited to the first singlet excited state S1, which, in turn, undergoes intersystem crossing to the first excited triplet state T1. On the triplet surface, the molecule dissociates into CH3 and HCO radicals with large kinetic energy release (KER), in accordance with the well characterized exit barrier on T1. However, at longer wavelengths (>320 nm), which correspond to excitation energies just below the triplet barrier, a sudden change in KER is observed. At these photolysis wavelengths, there is not enough energy to surpass the exit barrier on the triplet state, which leaves the possibility of unimolecular dissociation on S0 after internal conversion from S1. We have characterized the fragments’ KER at these wavelengths, as well as determined the energy partitioning for the radical fragments. A new accurate estimate of the barrier height on T1 is presented.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
31.50.Df Potential energy surfaces for excited electronic states
82.50.Pt Multiphoton processes
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)

Giant Renner–Teller vibronic coupling in the BF2 radical: An ab initio study of the math 2A1 and math 2Π electronic states

Riccardo Tarroni and Dennis J. Clouthier

J. Chem. Phys. 133, 064304 (2010); http://dx.doi.org/10.1063/1.3477765 (11 pages) | Cited 3 times

Online Publication Date: 12 August 2010

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The potential energy surfaces (PESs) of the ground math 2A1 and the first excited math 2Π (1 2B1,2 2A1) electronic states of the BF2 radical have been studied ab initio, using a large basis set and CCSD(T) and EOM-CCSD techniques. The calculated PESs were used to variationally calculate the energy levels up to ≈ 36 000 cm−1 above the ground state. The Renner–Teller splitting parameter (ε = 0.928) found for the math 2Π state of this radical is very large which results in an unusual excited state energy level structure.
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33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions
31.15.ae Electronic structure and bonding characteristics
31.50.Bc Potential energy surfaces for ground electronic states
31.50.Df Potential energy surfaces for excited electronic states
31.15.bw Coupled-cluster theory

The electronic structure of the triiodide ion from relativistic correlated calculations: A comparison of different methodologies

André Severo Pereira Gomes, Lucas Visscher, Hélène Bolvin, Trond Saue, Stefan Knecht, Timo Fleig, and Ephraim Eliav

J. Chem. Phys. 133, 064305 (2010); http://dx.doi.org/10.1063/1.3474571 (12 pages) | Cited 3 times

Online Publication Date: 12 August 2010

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The triiodide ion I3− exhibits a complex photodissociation behavior, the dynamics of which are not yet fully understood. As a first step toward determining the full potential energy surfaces of this species for subsequent simulations of its dissociation processes, we investigate the performance of different electronic structure methods [time-dependent density functional theory, complete active space perturbation theory to second order (CASPT2), Fock-space coupled cluster and multireference configuration interaction] in describing the ground and excited states of the triiodide ion along the symmetrical dissociation path. All methods apart from CASPT2 include scalar relativity and spin-orbit coupling in the orbital optimization, providing useful benchmark data for the more common two-step approaches in which spin-orbit coupling is introduced in the configuration interaction. Time-dependent density functional theory with the statistical averaging of model orbital potential functional is off the mark for this system. Another choice of functional may improve performance with respect to vertical excitation energies and spectroscopic constants, but all functionals are likely to face instability problems away from the equilibrium region. The Fock-space coupled cluster method was shown to perform clearly best in regions not too far from equilibrium but is plagued by convergence problems toward the dissociation limit due to intruder states. CASPT2 shows good performance at significantly lower computational cost, but is quite sensitive to symmetry breaking. We furthermore observe spikes in the CASPT2 potential curves away from equilibrium, signaling intruder state problems that we were unable to curb through the use of level shifts. Multireference configuration interaction is, in principle, a viable option, but its computational cost in the present case prohibits use other than for benchmarking purposes.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
31.50.-x Potential energy surfaces
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
31.15.ee Time-dependent density functional theory
31.15.bw Coupled-cluster theory
back to top Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

Recoupling of native homonuclear dipolar couplings in magic-angle-spinning solid-state NMR by the double-oscillating field technique

Lasse Arnt Straasø and Niels Chr. Nielsen

J. Chem. Phys. 133, 064501 (2010); http://dx.doi.org/10.1063/1.3464334 (9 pages) | Cited 1 time

Online Publication Date: 9 August 2010

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A new solid-state NMR method, the double-oscillating field technique (DUO), that under magic-angle-spinning conditions produces an effective Hamiltonian proportional to the native high-field homonuclear dipole-dipole coupling operator is presented. The method exploits one part of the radio frequency (rf) field to recouple the dipolar coupling interaction with a relatively high scaling factor and to eliminate offset effects over a reasonable bandwidth while in the recoupling frame, the other part gives rise to a sufficiently large longitudinal component of the residual rf field that averages nonsecular terms and in addition ensures stability toward rf inhomogeneity and rf miscalibration. The capability of the DUO experiment to mediate transfer of polarization is described theoretically and compared numerically and experimentally with finite pulse rf driven recoupling and experimentally with dipolar-assisted rotational resonance. Two-dimensional recoupling experiments were performed on antiparallel amyloid fibrils of the decapeptide SNNFGAILSS with the FGAIL fragment uniformly labeled with 13C and 15N.
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76.70.Fz Double nuclear magnetic resonance (DNMR), dynamical nuclear polarization
75.10.Dg Crystal-field theory and spin Hamiltonians

Monte Carlo study of structural ordering of Lennard-Jones fluids confined in nanochannels

Hamideh Abtahinia and Fatemeh Ebrahimi

J. Chem. Phys. 133, 064502 (2010); http://dx.doi.org/10.1063/1.3469771 (9 pages)

Online Publication Date: 9 August 2010

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We investigate quantitatively the ordering of Lennard-Jones fluids confined in a thin and infinitely long nanochannel with square cross section. The most probable spatial configurations of the atoms were examined by Monte Carlo simulations, and the order parameter was calculated. The effect of the various parameters, such as the wall-fluid attractive interaction, the size of constriction, and the temperature, was studied. The results indicate that for strong wall-fluid interactions and small constrictions, the ordering of the fluid particles is almost perfect. Geometrical mismatch, as well as increasing the system’s temperature, deteriorates the ordering phenomenon, even for very small openings. We observe a nontrivial trend in the dependence of the order parameter on the size of the opening of the channel with a linear size smaller than five atomic layers. We also examined the rearrangements of the fluid’s atoms in more symmetrical pores—slitlike pores and cylindrical nanopores—and discuss their similarities and differences with the square channels.
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61.20.Ja Computer simulation of liquid structure

A generalized reactive force field for nonlinear hydrogen bonds: Hydrogen dynamics and transfer in malonaldehyde

Yonggang Yang and Markus Meuwly

J. Chem. Phys. 133, 064503 (2010); http://dx.doi.org/10.1063/1.3447701 (12 pages) | Cited 2 times

Online Publication Date: 9 August 2010

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Using molecular dynamics (MD) simulations, the spectroscopy and dynamics of malonaldehyde is investigated. To this end, the recently proposed molecular mechanics with proton transfer (MMPT) potential is generalized to nonlinear hydrogen bonds. The calculated properties for malonaldehyde in both gas and condensed phases, including equilibrium geometries, infrared spectra, tunneling splittings, and hydrogen transfer rates, compare well with previous experimental and computational works. In particular, by using a harmonic bath averaged (HBA) Hamiltonian, which is based on a reaction path Hamiltonian, it is possible to estimate the tunneling splitting in an efficient manner. It is found that a zero point corrected barrier of 6.7 kcal/mol and effective masses of 1.234 (i.e., 23.4% larger than the mass of a physical H-atom) and 1.117 (for the physical D-atom) are consistent with the measured splittings of 21.6 and 2.9 cm−1, respectively. The HBA Hamiltonian also yields a pair of hydrogen transfer fundamentals at 1573 and 1267 cm−1, similar to results obtained with a reaction surface Hamiltonian on a MP2/6-31G∗∗ potential energy surface. This amounts to a substantial redshift of more than 1000 cm−1 which can be rationalized by comparison with weakly (HCO+: rare gas) and strongly (H2OH+OH2) proton-bound systems. Hydrogen transfer rates in vacuum and water were determined from the validated MMPT potential and it is found that the solvent enhances the rate by a factor of 5 at 300 K. The rates of 2.4/ns and 10/ns are commensurate with previous density functional tight binding ab initio MD studies.
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82.20.Kh Potential energy surfaces for chemical reactions
02.70.Ns Molecular dynamics and particle methods

The distorted close-packed crystal structure of methane A

H. E. Maynard-Casely, C. L. Bull, M. Guthrie, I. Loa, M. I. McMahon, E. Gregoryanz, R. J. Nelmes, and J. S. Loveday

J. Chem. Phys. 133, 064504 (2010); http://dx.doi.org/10.1063/1.3455889 (10 pages) | Cited 1 time

Online Publication Date: 9 August 2010

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We have determined the full crystal structure of the high-pressure phase methane A. X-ray single-crystal diffraction data were used to determine the carbon-atom arrangement, and neutron powder diffraction data from a deuterated sample allowed the deuterium atoms to be located. It was then possible to refine all the hydrogen positions from the single-crystal x-ray data. The structure has 21 molecules in a rhombohedral unit cell, and is quite strongly distorted from the cubic close-packed structure of methane I, although some structural similarities remain. Full knowledge of this structure is important for modeling of methane at higher pressures, including in relation to the mineralogy of the outer solar system. We discuss interesting structural parallels with the carbon tetrahalides.
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61.66.Hq Organic compounds

Why are para-hydrogen clusters superfluid? A quantum theorem of corresponding states study

Mikhail B. Sevryuk, J. Peter Toennies, and David M. Ceperley

J. Chem. Phys. 133, 064505 (2010); http://dx.doi.org/10.1063/1.3458640 (13 pages) | Cited 5 times

Online Publication Date: 10 August 2010

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The quantum theorem of corresponding states is applied to N = 13 and N = 26 cold quantum fluid clusters to establish where para-hydrogen clusters lie in relation to more and less quantum delocalized systems. Path integral Monte Carlo calculations of the energies, densities, radial and pair distributions, and superfluid fractions are reported at T = 0.5 K for a Lennard-Jones (LJ) (12,6) potential using six different de Boer parameters including the accepted value for hydrogen. The results indicate that the hydrogen clusters are on the borderline to being a nonsuperfluid solid but that the molecules are sufficiently delocalized to be superfluid. A general phase diagram for the total and kinetic energies of LJ (12,6) clusters encompassing all sizes from N = 2 to N = ∞ and for the entire range of de Boer parameters is presented. Finally the limiting de Boer parameters for quantum delocalization induced unbinding (“quantum unbinding”) are estimated and the new results are found to agree with previous calculations for the bulk and smaller clusters.
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67.10.-j Quantum fluids: general properties
81.30.Dz Phase diagrams of other materials
71.15.Nc Total energy and cohesive energy calculations
61.20.Ja Computer simulation of liquid structure

Three pulse UV photon echo studies of molecules in solution: Effect of the chirp

A. Ajdarzadeh Oskouei, A. Tortschanoff, O. Bräm, F. van Mourik, A. Cannizzo, and M. Chergui

J. Chem. Phys. 133, 064506 (2010); http://dx.doi.org/10.1063/1.3463448 (7 pages)

Online Publication Date: 11 August 2010

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We report on the electronic dephasing times of the nonpolar chromophore diphenylacetylene (DPA) in ethanol and in cyclohexane (polar and nonpolar solvents respectively) by photon echo measurements in the ultraviolet. Contrary to previous reports, we observed sub-100-fs electronic dephasing times for DPA in both solvents. We identify fast dynamics of τ = 40±10 fs on the photon echo peak shift (PEPS) traces of DPA in ethanol. In addition, we observed a dependence of the PEPS asymptotic value on the temporal chirp of the pulses. We propose a model to describe it in terms of phase-matching condition and beam geometry.
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33.20.Lg Ultraviolet spectra
42.50.Md Optical transient phenomena: quantum beats, photon echo, free-induction decay, dephasings and revivals, optical nutation, and self-induced transparency

Determining the three-phase coexistence line in methane hydrates using computer simulations

M. M. Conde and C. Vega

J. Chem. Phys. 133, 064507 (2010); http://dx.doi.org/10.1063/1.3466751 (12 pages) | Cited 8 times

Online Publication Date: 12 August 2010

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Molecular dynamics simulations have been performed to estimate the three-phase (solid hydrate-liquid water-gaseous methane) coexistence line for the water-methane binary mixture. The temperature at which the three phases are in equilibrium was determined for three different pressures, namely, 40, 100, and 400 bar by using direct coexistence simulations. In the simulations water was described by using either TIP4P, TIP4P/2005, or TIP4P/Ice models and methane was described as simple Lennard-Jones interaction site. Lorentz–Berthelot combining rules were used to obtain the parameters of the cross interactions. For the TIP4P/2005 model positive deviations from the energetic Lorentz–Berthelot rule were also considered to indirectly account for the polarization of methane when introduced in liquid water. To locate the three-phase coexistence point, two different global compositions were used, which yielded (to within statistical uncertainty) the same predictions for the three-phase coexistence temperatures, although with a somewhat different time evolution. The three-phase coexistence temperatures obtained at different pressures when using the TIP4P/Ice model of water were in agreement with the experimental results. The main reason for this is that the TIP4P/Ice model reproduces the melting point of ice Ih.
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81.30.Dz Phase diagrams of other materials
61.20.Ja Computer simulation of liquid structure

Theory and simulation of the dynamic heat capacity of the east Ising model

Jonathan R. Brown, John D. McCoy, and Brian Borchers

J. Chem. Phys. 133, 064508 (2010); http://dx.doi.org/10.1063/1.3469767 (8 pages)

Online Publication Date: 13 August 2010

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A recently developed methodology for the calculation of the dynamic heat capacity from simulation is applied to the east Ising model. Results show stretched exponential relaxation with the stretching exponent, β, decreasing with decreasing temperature. For low temperatures, the logarithm of the relaxation time is approximately proportional to the inverse of the temperature squared, which is the theoretical limiting behavior predicted by theories of facilitated dynamics. In addition, an analytical approach is employed where the overall relaxation is a composite of relaxation processes of subdomains, each with their own characteristic time. Using a Markov chain method, these times are computed both numerically and in closed form. The Markov chain results are seen to match the simulations at low temperatures and high frequencies. The dynamics of the east model are tracked very well by this analytic procedure, and it is possible to associate features of the spectrum of the dynamic heat capacity with specific domain relaxation events.
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65.40.Ba Heat capacity
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