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21 Jan 2006

Volume 124, Issue 3, Articles (03xxxx)

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Exact quantum dynamics of N(math)+H2→NH+H reaction: Cross-sections, rate constants, and dependence on reactant rotation

Shi Ying Lin and Hua Guo

J. Chem. Phys. 124, 031101 (2006); http://dx.doi.org/10.1063/1.2163871 (3 pages) | Cited 20 times

Online Publication Date: 17 January 2006

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Using an exact Chebyshev wave packet method, initial state-specified (vi = 0, ji = 0,2) integral cross-sections and rate constants are obtained for the title reaction on the latest ab initio potential energy surface. Reaction probabilities up to J = 29 are dependent on the reactant rotation and show mild oscillations superimposed on a broad background. Due to a barrier in the entrance channel, the cross sections increase with energy with clear thresholds and the rate constants vary with temperature in the Arrhenius form. The calculated canonical rate constant is in good agreement with the experimental measurements. Our results also indicate that the quasiclassical trajectory method underestimates the rate due to the neglect of tunneling, while the quantum statistical approach overestimates because of the short lifetime of the reaction intermediate.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Xr Quantum effects in rate constants (tunneling, resonances, etc.)
82.20.Kh Potential energy surfaces for chemical reactions
82.40.Bj Oscillations, chaos, and bifurcations
82.20.Hf Product distribution
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back to top Theoretical Methods and Algorithms

Dissipative dynamics of laser induced nonadiabatic molecular alignment

S. Ramakrishna and Tamar Seideman

J. Chem. Phys. 124, 034101 (2006); http://dx.doi.org/10.1063/1.2130708 (11 pages) | Cited 17 times

Online Publication Date: 17 January 2006

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Nonadiabatic alignment induced by short, moderately intense laser pulses in molecules coupled to dissipative environments is studied within a nonperturbative density matrix theory. We focus primarily on exploring and extending a recently proposed approach [ Phys. Rev. Lett. 95, 113001 (2005) ], wherein nonadiabatic laser alignment is used as a coherence spectroscopy that probes the dissipative properties of the solvent. To that end we apply the method to several molecular collision systems that exhibit sufficiently varied behavior to represent a broad variety of chemical environments. These include molecules in low temperature gas jets, in room temperature gas cells, and in dense liquids. We examine also the possibility of prolonging the duration of the field free (post-pulse) alignment in dissipative media by a proper choice of the system parameters.
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34.50.Ez Rotational and vibrational energy transfer
34.50.Rk Laser-modified scattering and reactions
33.80.-b Photon interactions with molecules

On the unphysical impact of complex absorbing potentials on the Hamiltonian and its remedy

S. Scheit, H.-D. Meyer, N. Moiseyev, and L. S. Cederbaum

J. Chem. Phys. 124, 034102 (2006); http://dx.doi.org/10.1063/1.2158991 (8 pages) | Cited 2 times

Online Publication Date: 17 January 2006

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The introduction of complex absorbing potentials as numerical tools to stabilize or increase the efficiency of calculations based on wave-packet propagation or on eigenvalue problems has the drawback of causing a modification of the Hamilton operator of the problem. In this work the consequences of such a modification are analyzed and the corrections required in order to properly describe the original physical process are derived. As an example, the decay of excited molecular states is considered: it is shown that the standard time-independent expression for the decay spectrum loses its validity when a complex absorbing potential is introduced in the nuclear Hamilton operator of the problem. To remedy the situation, a new, very stable formula is derived and tested on relevant model studies. Numerical examples are discussed.
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31.15.vj Electron correlation calculations for atoms and ions: excited states
31.15.E- Density-functional theory

Construction of environment states in quantum-chemical density-matrix renormalization group calculations

Gerrit Moritz and Markus Reiher

J. Chem. Phys. 124, 034103 (2006); http://dx.doi.org/10.1063/1.2139998 (9 pages) | Cited 18 times

Online Publication Date: 17 January 2006

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The application of the quantum-chemical density-matrix renormalization group (DMRG) algorithm is cumbersome for complex electronic structures with many active orbitals. The high computational cost is mainly due to the poor convergence of standard DMRG calculations. A factor which affects the convergence behavior of the calculations is the choice of the start-up procedure. In this start-up step matrix representations of operators have to be calculated in a guessed many-electron basis of the DMRG environment block. Different possibilities for the construction of these basis states exist, and we first compare four procedures to approximate the environment states using Slater determinants explicitly. These start-up procedures are applied to DMRG calculations on a sophisticated test system: the chromium dimer. It is found that the converged energies and the rate of convergence depend significantly on the choice of the start-up procedure. However, since already the most simple start-up procedure, which uses only the Hartree-Fock determinant, is comparatively good, Slater determinants, in general, appear not to be a good choice as approximate environment basis states for convergence acceleration. Based on extensive test calculations it is demonstrated that the computational cost can be significantly reduced if the number of total states m is successively increased. This is done in such a way that the environment states are built up stepwise from system states of previous truncated DMRG sweeps for slowly increasing m values.
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31.15.xr Self-consistent-field methods

Coherent control of pump-probe signals of helical structures by adaptive pulse polarizations

Dmitri Voronine, Darius Abramavicius, and Shaul Mukamel

J. Chem. Phys. 124, 034104 (2006); http://dx.doi.org/10.1063/1.2107667 (12 pages) | Cited 11 times

Online Publication Date: 17 January 2006

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The simplification of the pump-probe spectrum of excitons by pure-phase-polarization pulse shaping is investigated by a simulation study. The state of light is manipulated by varying the phases of two perpendicular polarization components of the pump, holding its total spectral and temporal intensity profiles fixed. Genetic and iterative Fourier transform algorithms are used to search for pulse phase functions that optimize the ratio of the signal at two frequencies. New features are extracted from the congested pump-probe spectrum of a helical pentamer by selecting a combination of Liouville space pathways. Tensor components which dominate the optimized spectra are identified.
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71.35.-y Excitons and related phenomena
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
42.25.Ja Polarization
02.60.Pn Numerical optimization
02.30.Nw Fourier analysis
02.30.Uu Integral transforms

Density-functional theory with effective potential expressed as a mapping of the external potential: Applications to open-shell molecules

Andreas K. Theophilou and Vitaly N. Glushkov

J. Chem. Phys. 124, 034105 (2006); http://dx.doi.org/10.1063/1.2161184 (8 pages) | Cited 15 times

Online Publication Date: 18 January 2006

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In this paper we apply the direct-mapping density-functional theory (DFT) to open-shell systems, in order to get many-electron wave functions having the same transformation properties as the eigenstates of the exact Hamiltonians. Such a case is that of spin, where in order to get the magnetic properties, the many-particle states must be eigenstates not only of Sz but also of S2. In this theory the Kohn and Sham [Phys. Rev. A 140, 1133 (1965)] potential is expressed directly as a mapping of the external potential. The total energies of the molecules calculated were satisfactory as their relative deviations (ΔE/E) from the exact Hartree-Fock ones were of the order of 10−4. This accuracy is much higher than that of the standard DFT in its local exchange potential approximation. This method does not need an approximate density as input, as the effective potential is derived directly from the external potential.
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31.15.E- Density-functional theory
31.15.xr Self-consistent-field methods

Density fitting of two-electron integrals in extended systems with translational periodicity: The Coulomb problem

Štefan Varga, Matúš Milko, and Jozef Noga

J. Chem. Phys. 124, 034106 (2006); http://dx.doi.org/10.1063/1.2145877 (7 pages) | Cited 13 times

Online Publication Date: 18 January 2006

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Density fitting approach to Coulomb integrals for infinite systems with translational periodicity is reformulated in direct space. Despite of the Coulomb infinite decay of some integrals, direct-space calculation is shown to be feasible. Moreover, we show that the direct-space ansatz is completely equivalent to our previous formulation in reciprocal space. Computational demands scale linearly with the number of unit cells. In addition, direct-space treatment has some practical advantages over the reciprocal-space formulation. The efficiency of our scheme is demonstrated on systems with translational periodicity in one dimension. Computation time takes only a small fraction of the conventional calculation with exact integrals. We show that for infinite systems auxiliary basis sets of equally good quality as for molecules can be constructed in a systematic way.
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31.15.-p Calculations and mathematical techniques in atomic and molecular physics
02.30.Rz Integral equations
02.60.Nm Integral and integrodifferential equations

Relativistic small-core energy-consistent pseudopotentials for the alkaline-earth elements from Ca to Ra

Ivan S. Lim, Hermann Stoll, and Peter Schwerdtfeger

J. Chem. Phys. 124, 034107 (2006); http://dx.doi.org/10.1063/1.2148945 (9 pages) | Cited 21 times

Online Publication Date: 18 January 2006

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Small-core ten-valence electron energy-consistent scalar- and two-component relativistic pseudopotentials for the alkaline-earth elements from Ca to Ra are presented. The accuracy and reliability of these pseudopotentials are discussed in terms of their ability to reproduce all-electron calculated and experimental dipole polarizabilities and ionization potentials.
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31.30.J- Relativistic and quantum electrodynamic (QED) effects in atoms, molecules, and ions
32.50.+d Fluorescence, phosphorescence (including quenching)
32.30.-r Atomic spectra

Semiempirical hybrid density functional with perturbative second-order correlation

Stefan Grimme

J. Chem. Phys. 124, 034108 (2006); http://dx.doi.org/10.1063/1.2148954 (16 pages) | Cited 275 times

Online Publication Date: 18 January 2006

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A new hybrid density functional for general chemistry applications is proposed. It is based on a mixing of standard generalized gradient approximations (GGAs) for exchange by Becke (B) and for correlation by Lee, Yang, and Parr (LYP) 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 contains only two global parameters that describe the mixture of HF and GGA exchange (ax) and of the PT2 and GGA correlation (c), respectively. The parameters are obtained in a least-squares-fit procedure to the G2/97 set of heat of formations. Opposed to conventional hybrid functionals, the optimum ax is found to be quite large (53% with c = 27%) which at least in part explains the success for many problematic molecular systems compared to conventional approaches. The performance of the new functional termed B2-PLYP is assessed by the G2/97 standard benchmark set, a second test suite of atoms, molecules, and reactions that are considered as electronically very difficult (including transition-metal compounds, weakly bonded complexes, and reaction barriers) and comparisons with other hybrid functionals of GGA and meta-GGA types. According to many realistic tests, B2-PLYP can be regarded as the best general purpose density functional for molecules (e.g., a mean absolute deviation for the two test sets of only 1.8 and 3.2 kcal/mol compared to about 3 and 5 kcal/mol, respectively, for the best other density functionals). Very importantly, also the maximum and minium errors (outliers) are strongly reduced (by about 10–20 kcal/mol). Furthermore, very good results are obtained for transition state barriers but unlike previous attempts at such a good description, this definitely comes not at the expense of equilibrium properties. Preliminary calculations of the equilibrium bond lengths and harmonic vibrational frequencies for diatomic molecules and transition-metal complexes also show very promising results. The uniformity with which B2-PLYP improves for a wide range of chemical systems emphasizes the need of (virtual) orbital-dependent terms that describe nonlocal electron correlation in accurate exchange-correlation functionals. From a practical point of view, the new functional seems to be very robust and it is thus suggested as an efficient quantum chemical method of general purpose.
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31.15.E- Density-functional theory
31.15.xp Perturbation theory
31.15.xr Self-consistent-field methods
33.15.Dj Interatomic distances and angles
33.15.Mt Rotation, vibration, and vibration-rotation constants

A reversible minimum-to-minimum mapping method for the calculation of free-energy differences

Doros N. Theodorou

J. Chem. Phys. 124, 034109 (2006); http://dx.doi.org/10.1063/1.2138701 (19 pages) | Cited 7 times

Online Publication Date: 19 January 2006

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A general method is introduced for the calculation of the free-energy difference between two systems, 0 and 1, with configuration spaces Ω(0), Ω(1) of the same dimensionality. The method relies upon establishing a bijective mapping between disjoint subsets Γi(0) of Ω(0) and corresponding disjoint subsets Γi(1) of Ω(1), and averaging a function of the ratio of configurational integrals over Γi(0) and Γi(1) with respect to the probability densities of the two systems. The mapped subsets Γi(0) and Γi(1) need not span the entire configuration spaces Ω(0) and Ω(1). The method is applied for the calculation of the excess chemical potential μex in a Lennard-Jones (LJ) fluid. In this case, Ω(0) is the configuration space of a (N−1) real molecule plus one ideal-gas molecule system, while Ω(1) is the configuration space of a N real molecule system occupying the same volume. Γi(0) and Γi(1) are constructed from hyperspheres of the same radius centered at minimum-energy configurations of a set of “active” molecules lying within distance a from the ideal-gas molecule and the last real molecule, respectively. An algorithm is described for sampling Γi(0) and Γi(1) given a point in Ω(0) or in Ω(1). The algorithm encompasses three steps: “quenching” (minimization with respect to the active-molecule degrees of freedom), “mutation” (gradual conversion of the ideal-gas molecule into a real molecule, with simultaneous minimization of the energy with respect to the active-molecule degrees of freedom), and “excitation” (generation of points on a hypersphere centered at the active-molecule energy minimum). These steps are also carried out in reverse, as required by the bijective nature of the mapping. The mutation step, which establishes a reversible mapping between energy minima with respect to the active degrees of freedom of systems 0 and 1, ensures that excluded volume interactions emerging in the process of converting the ideal-gas molecule into a real molecule are relieved through appropriate rearrangement of the surrounding active molecules. Thus, the insertion problem plaguing traditional methods for the calculation of chemical potential at high densities is alleviated. Results are presented at two state points of the LJ system for a variety of radii a of the active domain. It is shown that the estimated values of μex are correct in all cases and subject to an order of magnitude lower statistical uncertainty than values based on the same number of Widom [J. Chem. Phys. 39, 2808 (1963) ] insertions at high fluid densities. Optimal settings for the new algorithm are identified and distributions of the quantities involved in it [number of active molecules, energy at the sampled minima of systems 0 and 1, and free-energy differences between subsets Γi(0) and Γi(1) that are mapped onto each other] are explored.
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65.20.-w Thermal properties of liquids
61.20.Gy Theory and models of liquid structure

On the calculation of time correlation functions by potential scaling

Chenyue Xing and Ioan Andricioaei

J. Chem. Phys. 124, 034110 (2006); http://dx.doi.org/10.1063/1.2159476 (10 pages) | Cited 10 times

Online Publication Date: 19 January 2006

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We present and analyze a general method to calculate time correlation functions from molecular dynamics on scaled potentials for complex systems for which simulation is affected by broken ergodicity. Depending on the value of the scaling factor, correlations can be calculated for times that can be orders of magnitude longer than those accessible to direct simulations. We show that the exact value of the time correlation functions of the original system (i.e., with unscaled potential) can be obtained, in principle, using an action-reweighting scheme based on a stochastic path-integral formalism. Two tests (involving a bistable potential model and a dipeptide bond-vector orientational relaxation) are exemplified to showcase the strengths, as well as the limitations of the approach, and a procedure for the estimation of the time-dependent standard deviation error is outlined.
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87.15.A- Theory, modeling, and computer simulation
87.15.H- Dynamics of biomolecules
87.15.B- Structure of biomolecules
87.14.E- Proteins

Quantum computing based on vibrational eigenstates: Pulse area theorem analysis

Taiwang Cheng and Alex Brown

J. Chem. Phys. 124, 034111 (2006); http://dx.doi.org/10.1063/1.2164457 (8 pages) | Cited 40 times

Online Publication Date: 19 January 2006

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In a recent paper [ D. Babikov, J. Chem. Phys. 121, 7577 (2004) ], quantum optimal control theory was applied to analyze the accuracy of quantum gates in a quantum computer based on molecular vibrational eigenstates. The effects of the anharmonicity parameter of the molecule, the target time of the pulse, and the penalty function on the accuracy of the qubit transformations were investigated. We demonstrate that the effects of all the molecular and laser-pulse parameters can be explained utilizing the analytical pulse area theorem, which originates from the standard two-level model. Moreover, by analyzing the difference between the optimal control theory results and those obtained using the pulse area theorem, it is shown that extremely high quantum gate fidelity can be achieved for a qubit system based on vibrational eigenstates.
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03.67.Lx Quantum computation architectures and implementations
03.65.Ta Foundations of quantum mechanics; measurement theory
33.20.Tp Vibrational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
02.30.Yy Control theory

General-model-space state-universal coupled-cluster methods for excited states: Diagonal noniterative triple corrections

Xiangzhu Li and Josef Paldus

J. Chem. Phys. 124, 034112 (2006); http://dx.doi.org/10.1063/1.2151893 (14 pages) | Cited 32 times

Online Publication Date: 20 January 2006

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The recently developed multireference, general-model-space, state-universal coupled-cluster approach considering singles and doubles (GMS SU CCSD) has been extended to account perturbatively for triples, similar to the ubiquitous single-reference CCSD(T) method. The effectiveness of this extension in handling of excited states and its ability to account for the static and nondynamic correlation effects when considering spin- and/or space-symmetry degenerate levels within the spin-orbital formalism is examined on the example of low-lying excitation energies of the C2, N2, and CO molecules and a comparison is made with the (N,N)-CCSD method used for the same puropose. It is shown that while the triple corrections are very effective in improving the absolute energies, they have only a modest effect on the corresponding excitation energies, which may be even detrimental if both the ground- and excited-state levels cannot be given a balanced treatment. While the triple corrections help to avoid the symmetry-breaking effects arising due to the use of the spin-orbital formalism, they are much less effective in this regard than the (N,N)-CCSD approach.
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31.15.bw Coupled-cluster theory
31.15.vj Electron correlation calculations for atoms and ions: excited states

Chirality-induced signals in coherent multidimensional spectroscopy of excitons

Darius Abramavicius and Shaul Mukamel

J. Chem. Phys. 124, 034113 (2006); http://dx.doi.org/10.1063/1.2104527 (17 pages) | Cited 12 times

Online Publication Date: 20 January 2006

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The nonlocal second- and third-order susceptibilities of an isotropic ensemble of aggregates are calculated by solving the nonlinear exciton equations which map the system into coupled anharmonic oscillators. Both electric and magnetic contributions are included using the minimal-coupling Hamiltonian. The various tensor components are evaluated to first order in the optical wave vector k. Additional structural information about the interchromophore distances, which is not accessible through zeroth-order contributions (the dipole approximation), is contained to the first order in k. New resonant second- and third-order signals predicted for chiral molecules provide multidimensional extensions of circular dichroism spectroscopy. Numerical simulations demonstrate the sensitivity of third-order signals to the secondary structural motiffs of peptides.
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33.55.+b Optical activity and dichroism
33.15.Bh General molecular conformation and symmetry; stereochemistry
31.15.-p Calculations and mathematical techniques in atomic and molecular physics

Quantum-mechanical evaluation of the Boltzmann operator in correlation functions for large molecular systems: A multilayer multiconfiguration time-dependent Hartree approach

Haobin Wang and Michael Thoss

J. Chem. Phys. 124, 034114 (2006); http://dx.doi.org/10.1063/1.2161178 (11 pages) | Cited 17 times

Online Publication Date: 20 January 2006

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It is shown that the Boltzmann operator in time correlation functions for complex molecular systems can be evaluated in a numerically exact way employing the multilayer formulation of the multiconfiguration time-dependent Hartree theory in combination with Monte Carlo importance sampling techniques. The performance of the method is illustrated by selected applications to photoinduced intervalence electron transfer reactions in the condensed phase. Furthermore, the validity of approximate schemes to evaluate the Boltzmann is discussed.
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82.53.-k Femtochemistry
82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
82.50.-m Photochemistry
02.70.Uu Applications of Monte Carlo methods

Reconciling semiclassical and Bohmian mechanics. II. Scattering states for discontinuous potentials

Corey Trahan and Bill Poirier

J. Chem. Phys. 124, 034115 (2006); http://dx.doi.org/10.1063/1.2145883 (18 pages) | Cited 11 times

Online Publication Date: 20 January 2006

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In a previous paper [B. Poirier, J. Chem. Phys. 121, 4501 (2004)] a unique bipolar decomposition, Ψ = Ψ1+Ψ2, was presented for stationary bound states Ψ of the one-dimensional Schrödinger equation, such that the components Ψ1 and Ψ2 approach their semiclassical WKB analogs in the large action limit. Moreover, by applying the Madelung-Bohm ansatz to the components rather than to Ψ itself, the resultant bipolar Bohmian mechanical formulation satisfies the correspondence principle. As a result, the bipolar quantum trajectories are classical-like and well behaved, even when Ψ has many nodes or is wildly oscillatory. In this paper, the previous decomposition scheme is modified in order to achieve the same desirable properties for stationary scattering states. Discontinuous potential systems are considered (hard wall, step potential, and square barrier/well), for which the bipolar quantum potential is found to be zero everywhere, except at the discontinuities. This approach leads to an exact numerical method for computing stationary scattering states of any desired boundary conditions, and reflection and transmission probabilities. The continuous potential case will be considered in a companion paper [C. Trahan and B. Poirier, J. Chem. Phys. 124, 034116 (2006), following paper].
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03.65.Sq Semiclassical theories and applications
03.65.Ge Solutions of wave equations: bound states

Reconciling semiclassical and Bohmian mechanics. III. Scattering states for continuous potentials

Corey Trahan and Bill Poirier

J. Chem. Phys. 124, 034116 (2006); http://dx.doi.org/10.1063/1.2145923 (14 pages) | Cited 11 times

Online Publication Date: 20 January 2006

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In a previous paper [ B. Poirier, J. Chem. Phys. 121, 4501 (2004) ] a unique bipolar decomposition Ψ = Ψ1+Ψ2 was presented for stationary bound states Ψ of the one-dimensional Schrödinger equation, such that the components Ψ1 and Ψ2 approach their semiclassical WKB analogs in the large-action limit. The corresponding bipolar quantum trajectories, as defined in the usual Bohmian mechanical formulation, are classical-like and well behaved, even when Ψ has many nodes or is wildly oscillatory. A modification for discontinuous potential stationary scattering states was presented in a second, companion paper [ C. Trahan and B. Poirier, J. Chem. Phys.124, 034115 (2006), previous paper ], whose generalization for continuous potentials is given here. The result is an exact quantum scattering methodology using classical trajectories. For additional convenience in handling the tunneling case, a constant-velocity-trajectory version is also developed.
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03.65.Sq Semiclassical theories and applications
03.65.Ge Solutions of wave equations: bound states
03.65.Xp Tunneling, traversal time, quantum Zeno dynamics
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Doppler effect in fluorine K-Auger line produced in electron-induced core ionization of SF6

S. Mondal, R. K. Singh, and R. Shanker

J. Chem. Phys. 124, 034301 (2006); http://dx.doi.org/10.1063/1.2158995 (4 pages)

Online Publication Date: 17 January 2006

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An experimental evidence is reported on the observation of the Doppler effect in fluorine K-Auger line emitted from a core-ionized SF6 molecule under an impact of 16 keV electrons. The emitting source of the Auger line is found to acquire a kinetic energy of 4.7±0.3 keV. We propose that such large energy is released from the Coulomb repulsion taking place between F+ and SF5+ fragment ions under influence of an intense focusing field of the incident electrons. In the presence of the Coulomb field of these ions, the Auger line obtains a polarization P = 76%±7%.
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34.80.Gs Molecular excitation and ionization
34.80.Ht Dissociation and dissociative attachment
33.80.Eh Autoionization, photoionization, and photodetachment

Spectroscopic analysis of small organic molecules: A comprehensive near-edge x-ray-absorption fine-structure study of C6-ring-containing molecules

C. Kolczewski, R. Püttner, M. Martins, A. S. Schlachter, G. Snell, M. M. Sant’Anna, K. Hermann, and G. Kaindl

J. Chem. Phys. 124, 034302 (2006); http://dx.doi.org/10.1063/1.2139674 (13 pages) | Cited 15 times

Online Publication Date: 18 January 2006

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We report high-resolution C 1s near-edge x-ray-absorption fine-structure (NEXAFS) spectra of the C6-ring-containing molecules benzene (C6H6), 1,3- and 1,4-cyclohexadiene (C6H8), cyclohexene (C6H10), cyclohexane (C6H12), styrene (C8H8), and ethylbenzene (C8H10) which allow us to examine the gradual development of delocalization of the corresponding π electron systems. Due to the high experimental resolution, vibrational progressions can be partly resolved in the spectra. The experimental spectra are compared with theoretical NEXAFS spectra obtained from density-functional theory calculations where electronic final-state relaxation is accounted for. The comparison yields very good agreement between theoretical spectra and experimental results. In all cases, the spectra can be described by excitations to π*- and σ*-type final-state orbitals with valence character, while final-state orbitals of Rydberg character make only minor contributions. The lowest C 1s→1π* excitation energy is found to agree in the (experimental and theoretical) spectra of all molecules except for 1,3-cyclohexadiene (C6H8) where an energy smaller by about 0.6 eV is obtained. The theoretical analysis can explain this result by different binding properties of this molecule compared to the others.
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33.20.Rm X-ray spectra
31.15.bu Semi-empirical and empirical calculations (differential overlap, Hückel, PPP methods, etc.)
33.15.Bh General molecular conformation and symmetry; stereochemistry
82.50.-m Photochemistry

A systematic multireference perturbation-theory study of the low-lying states of SiC3

Jamie M. Rintelman, Mark S. Gordon, Graham D. Fletcher, and Joseph Ivanic

J. Chem. Phys. 124, 034303 (2006); http://dx.doi.org/10.1063/1.2140687 (5 pages) | Cited 6 times

Online Publication Date: 18 January 2006

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The three known lowest-energy isomers of SiC3, two cyclic singlets (2s and 3s) and a linear triplet (1t), have been reinvestigated using multireference second-order perturbation theory (MRPT2). The dependence of the relative energies of the isomers upon the quality of the basis sets and the sizes of the reference active spaces is explored. When using a complete-active-space self-consistent-field reference wave function with 12 electrons in 11 orbitals [CASSCF (12, 11)] together with basis sets that increase in size up to the correlation-consistent polarized core-valence quadruple zeta basis set (cc-pCVQZ), the MRPT2 method consistently predicts the linear triplet to be the most stable isomer. A new parallel direct determinant MRPT2 code has been used to systematically explore reference spaces that vary in size from CASSCF (8,8) to full optimized reaction space [FORS or CASSCF (16,16)] with the cc-pCVQZ basis. It is found that the relative energies of the isomers change substantially as the active space is increased. At the best level of theory, MRPT2 with a full valence FORS reference, the 2s isomer is predicted to be more stable than 3s and 1t by 4.7 and 2.2 kcal/mol, respectively.
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31.15.xp Perturbation theory
31.15.xr Self-consistent-field methods
33.15.Bh General molecular conformation and symmetry; stereochemistry

Coherent phase control of the product branching ratio in the photodissociation of dimethylsulfide

Hidekazu Nagai, Hideki Ohmura, Fumiyuki Ito, Taisuke Nakanaga, and Masanori Tachiya

J. Chem. Phys. 124, 034304 (2006); http://dx.doi.org/10.1063/1.2165200 (4 pages) | Cited 1 time

Online Publication Date: 19 January 2006

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Coherent phase control of the photodissociation reaction of the dimethylsulfide has been achieved by means of quantum-mechanical interference between one- and three-photon transitions. Dimethylsulfide was irradiated by fundamental and frequency-tripled outputs of a visible laser (600.5–602.5 nm), simultaneously to yield CH3S+ and CH3SCH2+ fragment ions. The branching ratio of the two product channels could be modulated with variation of the phase difference between the light fields. This accounted for the difference between the molecular phases of the two product channels. The phase lag was observed to have a maximum value of 8° at 601.5 nm. This is the first result of a selective bond breaking in a polyatomic molecule by the coherent phase control.
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82.50.Hp Processes caused by visible and UV light
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)

Quantum dynamics of Ne–Br2 vibrational predissociation: The role of continuum resonances as doorway states

A. García-Vela and K. C. Janda

J. Chem. Phys. 124, 034305 (2006); http://dx.doi.org/10.1063/1.2162167 (13 pages) | Cited 16 times

Online Publication Date: 19 January 2006

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Wave-packet simulations of the Ne–Br2(B,υ′) vibrational predissociation dynamics in the range υ′ = 16–29 are reported. The aim is to interpret recent time-dependent pump-probe experiments [ Cabrera et al., J. Chem. Phys. 123, 054311 (2005) ]. Good agreement is found between the calculated and the experimental lifetimes corresponding to decay of the Ne–Br2(B,υ′) initial state and to appearance of Br2(B,υ<υ′) products. The simulations show that up to υ′ ∼ 22 the dynamics is dominated by direct predissociation, while for higher υ′ levels an indirect intramolecular vibrational relaxation mechanism of dissociation becomes increasingly important. Such a mechanism occurs via coupling of the initial state in the υ′ vibrational manifold to nearly degenerate resonances embedded in the continuum of the lower υ<υ′ manifolds, which act as intermediate doorway states to dissociation. The role of the intermediate resonances manifests itself in multiexponential behavior and oscillations in the time-dependent population curves associated with the initial complex state, the final product states, and the Ne–Br2(B,υ<υ′) intermediate complexes. Analysis of the Ne–Br2(B,υ<υ′) intermediate population shows that the continuum resonances are supported by centrifugal barriers involving excitation of the internal rotation of the complex. We find that the coupling between the intermediate state resonances and the continuum product state wave functions extend to Ne–Br2 distances greater than 15 Å. In the light of the results, a structure of the spectrum of continuum resonances is suggested and discussed.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
33.20.Tp Vibrational analysis
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
31.50.-x Potential energy surfaces

On the photophysics of all-trans polyenes: Hexatriene versus octatetraene

J. Catalán and J. L. G. de Paz

J. Chem. Phys. 124, 034306 (2006); http://dx.doi.org/10.1063/1.2158992 (11 pages) | Cited 12 times

Online Publication Date: 19 January 2006

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The disparate photophysical behavior of trans-1,3,5-hexatriene (nonfluorescent) and trans-1,3,5,7-octatetraene (with two fluorescence emissions) in the gas phase is explained in terms of the tendency of their 1Bu excited states to rotate about their terminal carbon-carbon single bonds in order to adopt a quasiplanar molecular form of lower energy than the 1Bu state in the parent all-trans structure. The origin of their disparate photophysical behavior is that such a transformation is subject to a small energy barrier in octatetraene; the barrier produces two minima (two fluorescence emissions) in the corresponding potential-energy curve. Instead of an energy barrier, hexatriene gives a 1,3-diene species which falls to the ground state so rapidly that no emission is produced.
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33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.50.Dq Fluorescence and phosphorescence spectra
33.15.Fm Bond strengths, dissociation energies
33.15.Bh General molecular conformation and symmetry; stereochemistry
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions

The electronic spectrum of AgCl2: Ab initio benchmark versus density-functional theory calculations on the lowest ligand-field states including spin-orbit effects

A. Ramírez-Solís, R. Poteau, and J. P. Daudey

J. Chem. Phys. 124, 034307 (2006); http://dx.doi.org/10.1063/1.2145879 (10 pages) | Cited 5 times

Online Publication Date: 20 January 2006

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The X math, math, and math states of AgCl2 have been studied through benchmark ab initio complete active space self-consistent field plus second-order complete active space multireference Möller-Plesset algorithm (CASSCF+CASPT2) and complete active space self-consistent field plus averaged coupled pair functional (CASSCF+ACPF) and density-functional theory (DFT) calculations using especially developed basis sets to study the transition energies, geometries, vibrational frequencies, Mulliken charges, and spin densities. The spin-orbit (SO) effects were included through the effective Hamiltonian formalism using the ΛSΣ ACPF energies as diagonal elements. At the ACPF level, the ground state is math in contradiction with ligand-field theory, SCF, and large CASSCF; the adiabatic excitation energies for the math and math states are 1640 and 18 230 cm−1, respectively. The inclusion of the SO effects leads to a pure Ω = 3/2(math) ground state, a Ω = 1/2 (66%math and 34%math) A state, a Ω = 1/2 (34%math and 66%math) B state, a Ω = 5/2(math)C state, and a Ω = 3/2(99%math)D state. The X-A, X-B, X-C, and X-D transition energies are 485, 3715, 17 246, and 20 110 cm−1, respectively. The B97-2, B3LYP, and PBE0 functionals overestimate by ≈ 100% the X math−mathTe but provide a qualitative energetic ordering in good agreement with ACPF results. B3LYP with variable exchange leads to a 42% optimal Hartree-Fock exchange for transition energies but all equilibrium geometries get worsened. Asymptotic corrections to B3LYP do not provide improved values. The nature of the bonding in the X math state is very different from that of CuCl2 since the Mulliken charge on the metal is 1.1 while the spin density is only 0.35. DFT strongly delocalizes the spin density providing even smaller values of around 0.18 on Ag not only for the ground state, but also for the math state.
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31.15.A- Ab initio calculations
31.15.E- Density-functional theory
31.15.xr Self-consistent-field methods
31.15.xp Perturbation theory
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.20.Tp Vibrational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.15.Fm Bond strengths, dissociation energies

New exchange-Coulomb N2–Ar potential-energy surface and its comparison with other recent N2–Ar potential-energy surfaces

Ashok K. Dham, William J. Meath, Jason W. Jechow, and Frederick R. W. McCourt

J. Chem. Phys. 124, 034308 (2006); http://dx.doi.org/10.1063/1.2159001 (23 pages) | Cited 8 times

Online Publication Date: 20 January 2006

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The reliability of five N2–Ar potential-energy surfaces in representing the N2–Ar interaction has been investigated by comparing their abilities to reproduce a variety of experimental results, including interaction second virial coefficients, bulk transport properties, relaxation phenomena, differential scattering cross sections, and the microwave and infrared spectra of the van der Waals complexes. Four of the surfaces are the result of high-level ab initio quantal calculations; one of them utilized fine tuning by fitting to microwave data. To date, these four potential-energy surfaces have only been tested against experimental microwave data. The fifth potential-energy surface, based upon the exchange-Coulomb potential-energy model for the interaction of closed-shell species, is developed herein: it is a combination of a damped dispersion energy series and ab initio calculations of the Heitler-London interaction energy, and has adjustable parameters determined by requiring essentially simultaneous agreement with selected quality interaction second virial coefficient and microwave data. Comparisons are also made with the predictions of three other very good literature potential-energy surfaces, including the precursor of the new exchange-Coulomb potential-energy surface developed here. Based upon an analysis of a large body of information, the new exchange-Coulomb and microwave-tuned ab initio potential-energy surfaces provide the best representations of the N2–Ar interaction; nevertheless, the other potential-energy surfaces examined still have considerable merit with respect to the prediction of specific properties of the N2–Ar van der Waals complex. Of the two recommended surfaces, the new exchange-Coulomb surface is preferred on balance due to its superior predictions of the effective cross sections related to various relaxation phenomena, and to its reliable, and relatively simple, representation of the long-range part of the potential-energy surface. Moreover, the flexibility still inherent in the exchange-Coulomb potential form can be further exploited, if required, in future studies of the N2–Ar interaction.
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31.50.-x Potential energy surfaces
31.15.A- Ab initio calculations
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