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28 Mar 2008

Volume 128, Issue 12, Articles (12xxxx)

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Phase control of molecular fragmentation with a pair of femtosecond-laser pulses

H. G. Breunig, G. Urbasch, and K.-M. Weitzel

J. Chem. Phys. 128, 121101 (2008); http://dx.doi.org/10.1063/1.2898092 (4 pages) | Cited 4 times

Online Publication Date: 25 March 2008

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We demonstrate the control of molecular fragmentation of o-xylene (C8H10) on a femtosecond time scale in two-pulse measurements with a pair of femtosecond-laser pulses. Parent and fragment-ion yields were recorded as a function of interpulse delays, i.e., different relative phases of the excitation pulses. The experiments revealed different fragmentation mechanisms in the temporal region of direct overlapping pulses and for separated pulses. For overlapping pulses all ion yields followed the excitation intensity which oscillated as a function of interpulse delay due to the change of constructive and destructive interference of the light fields. For larger delays, in particular, the oscillations of the C+ and CH3+ fragment-ion yield showed a significant deviation from each other. The results are interpreted as a manifestation of optical phase-dependent electronic excitations mapped onto the nuclear fragmentation dynamics.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
31.50.Df Potential energy surfaces for excited electronic states

Charge patching method for electronic structure of organic systems

Nenad Vukmirović and Lin-Wang Wang

J. Chem. Phys. 128, 121102 (2008); http://dx.doi.org/10.1063/1.2901965 (4 pages) | Cited 10 times

Online Publication Date: 31 March 2008

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The development of the charge patching method for the calculation of the electronic structure of organic systems containing a large number of atoms was presented. The method was tested on a range of systems including alkane and alkene chains, polyacenes, polythiophenes, polypyrroles, polyfuranes, polyphenylene vinylene, and poly(amidoamine) dendrimers. The results obtained by the method are in very good agreement with direct calculations based on density functional theory, since the eigenstate errors are typically of the order of a few tens of meV.
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71.20.Rv Polymers and organic compounds
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
61.41.+e Polymers, elastomers, and plastics
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back to top Theoretical Methods and Algorithms

Born–Oppenheimer approximation and beyond for time-dependent electronic processes

L. S. Cederbaum

J. Chem. Phys. 128, 124101 (2008); http://dx.doi.org/10.1063/1.2895043 (8 pages) | Cited 20 times

Online Publication Date: 24 March 2008

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Explicit computations of electronic motion in time and space are gradually becoming feasible and available. The knowledge of this motion is of relevance by itself but is also important for understanding available and predicting future experiments on the electronic time scale. In electronic processes of interest, usually several and even many stationary electronic states participate and the obvious question arises on how to describe the accompanying quantum nuclear dynamics at least on the time scale of the process. In this work, we attempt to study the nuclear dynamics in the framework of a fully time-dependent Born–Oppenheimer approximation. Additionally, we attempt to go beyond this approximation by introducing the coupling of several electronic wavepackets by the nuclear wavepackets. In this context, we also discuss a time-dependent transformation to diabatic electronic wavepackets. A simple but critical model of charge transfer is analyzed in some detail on various levels of approximation and also solved exactly.
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03.65.Ge Solutions of wave equations: bound states

Simulating prescribed particle densities in the grand canonical ensemble using iterative algorithms

Attila Malasics, Dirk Gillespie, and Dezső Boda

J. Chem. Phys. 128, 124102 (2008); http://dx.doi.org/10.1063/1.2839302 (6 pages) | Cited 18 times

Online Publication Date: 24 March 2008

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We present two efficient iterative Monte Carlo algorithms in the grand canonical ensemble with which the chemical potentials corresponding to prescribed (targeted) partial densities can be determined. The first algorithm works by always using the targeted densities in the kT log(ρi) (ideal gas) terms and updating the excess chemical potentials from the previous iteration. The second algorithm extrapolates the chemical potentials in the next iteration from the results of the previous iteration using a first order series expansion of the densities. The coefficients of the series, the derivatives of the densities with respect to the chemical potentials, are obtained from the simulations by fluctuation formulas. The convergence of this procedure is shown for the examples of a homogeneous Lennard-Jones mixture and a NaCl–CaCl2 electrolyte mixture in the primitive model. The methods are quite robust under the conditions investigated. The first algorithm is less sensitive to initial conditions.
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61.20.Ja Computer simulation of liquid structure
65.20.Jk Studies of thermodynamic properties of specific liquids

Solving the Schrödinger and Dirac equations of hydrogen molecular ion accurately by the free iterative complement interaction method

Atsushi Ishikawa, Hiroyuki Nakashima, and Hiroshi Nakatsuji

J. Chem. Phys. 128, 124103 (2008); http://dx.doi.org/10.1063/1.2842068 (12 pages) | Cited 7 times

Online Publication Date: 24 March 2008

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The nonrelativistic Schrödinger equation and the relativistic four-component Dirac equation of H2+ were solved accurately in an analytical expansion form by the free iterative complement interaction (ICI) method combined with the variational principle. In the nonrelativistic case, we compared the free ICI wave function with the so-called “exact” wave function as two different expansions converging to the unique exact wave function and found that the free ICI method is much more efficient than the exact method. In the relativistic case, we first used the inverse Hamiltonian to guarantee Ritz-type variational principle and obtained accurate result. We also showed that the ordinary variational calculation also gives a nice convergence when the g function is appropriately chosen, since then the free ICI calculation guarantees a correct relationship between the large and small components of each adjacent order, which we call ICI balance. This is the first application of the relativistic free ICI method to molecule. We calculated both ground and excited states in good convergence, and not only the upper bound but also the lower bound of the ground-state energy. The error bound analysis has assured that the present result is highly accurate.
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31.30.J- Relativistic and quantum electrodynamic (QED) effects in atoms, molecules, and ions

Triple excitations in state-specific multireference coupled cluster theory: Application of Mk-MRCCSDT and Mk-MRCCSDT-n methods to model systems

Francesco A. Evangelista, Andrew C. Simmonett, Wesley D. Allen, Henry F. Schaefer, III, and Jürgen Gauss

J. Chem. Phys. 128, 124104 (2008); http://dx.doi.org/10.1063/1.2834927 (13 pages) | Cited 40 times

Online Publication Date: 25 March 2008

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We report the first implementation with correct scaling of the Mukherjee multireference coupled cluster method with singles, doubles, and approximate iterative triples (Mk-MRCCSDT-n, n = 1a,1b,2,3) as well as full triples (Mk-MRCCSDT). These methods were applied to the classic H4, P4, BeH2, and H8 model systems to assess the ability of the Mk-MRCCSDT-n schemes to accurately account for triple excitations. In all model systems the inclusion of triples via the various Mk-MRCCSDT-n approaches greatly reduces the nonparallelism error (NPE) and the mean nonparallelism derivative diagnostics for the potential energy curves, recovering between 59% and 73% of the full triples effect on average. The most complete triples approximation, Mk-MRCCSDT-3, exhibits the best average performance, reducing the mean NPE to below 0.6 mEh, compared to 1.4 mEh for Mk-MRCCSD. Both linear and quadratic truncations of the Mk-MRCC triples coupling terms are viable simplifications producing no significant errors. If the off-diagonal parts of the occupied-occupied and virtual-virtual blocks of the Fock matrices are ignored, the storage of the triples amplitudes is no longer required for the Mk-MRCCSDT-n methods introduced here. This proves to be an effective approximation that gives results almost indistinguishable from those derived from full consideration of the Fock matrices.
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31.15.bw Coupled-cluster theory
31.50.Df Potential energy surfaces for excited electronic states

A unified density-functional treatment of dynamical, nondynamical, and dispersion correlations. II. Thermochemical and kinetic benchmarks

Erin R. Johnson and Axel D. Becke

J. Chem. Phys. 128, 124105 (2008); http://dx.doi.org/10.1063/1.2894878 (3 pages) | Cited 10 times

Online Publication Date: 26 March 2008

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In a previous work [ J. Chem. Phys. 127, 124108 (2007) ] we introduced an exact-exchange-based density-functional methodology incorporating dynamical, nondynamical, and dispersion correlations, called DF07. In this work, the performance of the DF07 method is assessed on a variety of thermochemical and kinetic benchmark data including ionization potentials, electron affinities, proton affinities, isomerization energies, bond dissociation enthalpies, and barrier heights of radical reactions. DF07 gives uniform accuracy over all our benchmark data without any refitting of parameters. The importance of the exact-exchange character of DF07 is highlighted through comparison with a three-parameter hybrid meta-generalized-gradient-approximation functional.
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82.30.Qt Isomerization and rearrangement
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.60.Cx Enthalpies of combustion, reaction, and formation

Interference and quantization in semiclassical response functions

Scott M. Gruenbaum and Roger F. Loring

J. Chem. Phys. 128, 124106 (2008); http://dx.doi.org/10.1063/1.2841943 (12 pages) | Cited 6 times

Online Publication Date: 26 March 2008

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Application of the Herman–Kluk semiclassical propagator to the calculation of spectroscopic response functions for anharmonic oscillators has demonstrated the quantitative accuracy of these approximate dynamics. In this approach, spectroscopic response functions are expressed as multiple phase-space integrals over pairs of classical trajectories and their associated stability matrices. Here we analyze the Herman–Kluk semiclassical approximation to a linear response function and determine the origin of the capacity of this method to reproduce quantum effects in a response function from classical dynamical information. Our analysis identifies those classical trajectories that contribute most significantly to the response function on different time scales. This finding motivates a procedure for computing the linear response function in which the interference between pairs of classical trajectories is treated approximately, resulting in an integral over a single average trajectory, as in a purely classical calculation.
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03.65.Ge Solutions of wave equations: bound states
03.65.Sq Semiclassical theories and applications

Interpreting ultrafast molecular fragmentation dynamics with ab initio electronic structure calculations

Carlos Trallero, Brett J. Pearson, Thomas Weinacht, Kandis Gilliard, and Spiridoula Matsika

J. Chem. Phys. 128, 124107 (2008); http://dx.doi.org/10.1063/1.2850524 (6 pages) | Cited 1 time

Online Publication Date: 26 March 2008

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High-level ab initio electronic structure calculations are used to interpret the fragmentation dynamics of CHBr2COCF3, following excitation with an intense ultrafast laser pulse. The potential energy surfaces of the ground and excited cationic states along the dissociative C–CF3 bond have been calculated using multireference second order perturbation theory methods. The calculations confirm the existence of a charge transfer resonance during the evolution of a dissociative wave packet on the ground state potential energy surface of the molecular cation and yield a detailed picture of the dissociation dynamics observed in earlier work. Comparisons of the ionic spectrum for two similar molecules support a general picture in which molecules are influenced by dynamic resonances in the cation during dissociation.
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31.50.Df Potential energy surfaces for excited electronic states
31.15.A- Ab initio calculations
33.15.Fm Bond strengths, dissociation energies
31.15.xp Perturbation theory
33.80.Gj Diffuse spectra; predissociation, photodissociation
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions

Poisson-transformed density fitting in relativistic four-component Dirac–Kohn–Sham theory

Leonardo Belpassi, Francesco Tarantelli, Antonio Sgamellotti, and Harry M. Quiney

J. Chem. Phys. 128, 124108 (2008); http://dx.doi.org/10.1063/1.2868770 (11 pages) | Cited 4 times

Online Publication Date: 26 March 2008

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We present recent developments in the implementation of the density fitting approach for the Coulomb interaction within the four-component formulation of relativistic density functional theory [ Belpassi et al., J. Chem. Phys. 124, 124104 (2006) ]. In particular, we make use of the Poisson equation to generate suitable auxiliary basis sets and simplify the electron repulsion integrals [ Manby and Knowles, Phys. Rev. Lett. 87, 163001 (2001) ]. We propose a particularly simple and efficient method for the generation of accurate Poisson auxiliary basis sets, based on already available standard Coulomb fitting sets. Just as is found in the nonrelativistic case, we show that the number of standard auxiliary fitting functions that need to be added to the Poisson-generated functions in order to achieve a fitting accuracy equal or, in some cases, better than that of the standard procedure is remarkably small. The efficiency of the present implementation is demonstrated in a detailed study of the spectroscopic properties and energetics of several gold containing systems, including the Au dimer and the CsAu molecule. The extraction reaction of a H2O molecule from a Au(H2O)9+ cluster is also calculated as an example of mixed heavy-light-atom molecular systems. The scaling behavior of the algorithm implemented is illustrated for some closed shell gold clusters up to Au5+. The increased sparsity of the Coulomb matrices involved in the Poisson fitting is identified, as are potential computational applications and the use of the Poisson fitting for the relativistic exchange-correlation problem.
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31.15.E- Density-functional theory
31.30.J- Relativistic and quantum electrodynamic (QED) effects in atoms, molecules, and ions
82.30.-b Specific chemical reactions; reaction mechanisms

A new approach for efficient simulation of Coulomb interactions in ionic fluids

Natalia A. Denesyuk and John D. Weeks

J. Chem. Phys. 128, 124109 (2008); http://dx.doi.org/10.1063/1.2894478 (8 pages) | Cited 10 times

Online Publication Date: 27 March 2008

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We propose a simplified version of local molecular field (LMF) theory to treat Coulomb interactions in simulations of ionic fluids. LMF theory relies on splitting the Coulomb potential into a short-ranged part that combines with other short-ranged core interactions and is simulated explicitly. The averaged effects of the remaining long-ranged part are taken into account through a self-consistently determined effective external field. The theory contains an adjustable length parameter σ that specifies the cutoff distance for the short-ranged interaction. This can be chosen to minimize the errors resulting from the mean-field treatment of the complementary long-ranged part. Here we suggest that in many cases an accurate approximation to the effective field can be obtained directly from the equilibrium charge density given by the Debye theory of screening, thus eliminating the need for a self-consistent treatment. In the limit σ→0, this assumption reduces to the classical Debye approximation. We examine the numerical performance of this approximation for a simple model of a symmetric ionic mixture. Our results for thermodynamic and structural properties of uniform ionic mixtures agree well with similar results of Ewald simulations of the full ionic system. In addition, we have used the simplified theory in a grand-canonical simulation of a nonuniform ionic mixture where an ion has been fixed at the origin. Simulations using short-ranged truncations of the Coulomb interactions alone do not satisfy the exact condition of complete screening of the fixed ion, but this condition is recovered when the effective field is taken into account. We argue that this simplified approach can also be used in the simulations of more complex nonuniform systems.
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61.20.Ja Computer simulation of liquid structure
65.20.De General theory of thermodynamic properties of liquids, including computer simulation

NMR implementation of adiabatic SAT algorithm using strongly modulated pulses

Avik Mitra, T. S. Mahesh, and Anil Kumar

J. Chem. Phys. 128, 124110 (2008); http://dx.doi.org/10.1063/1.2835542 (6 pages) | Cited 2 times

Online Publication Date: 28 March 2008

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NMR implementation of adiabatic algorithms face severe problems in homonuclear spin systems since the qubit selective pulses are long and during this period, evolution under the Hamiltonian and decoherence cause errors. The decoherence destroys the answer as it causes the final state to evolve to mixed state and in homonuclear systems, evolution under the internal Hamiltonian causes phase errors preventing the initial state to converge to the solution state. The resolution of these issues is necessary before one can proceed to implement an adiabatic algorithm in a large system where homonuclear coupled spins will become a necessity. In the present work, we demonstrate that by using “strongly modulated pulses” (SMPs) for the creation of interpolating Hamiltonian, one can circumvent both the problems and successfully implement the adiabatic SAT algorithm in a homonuclear three qubit system. This work also demonstrates that the SMPs tremendously reduce the time taken for the implementation of the algorithm, can overcome problems associated with decoherence, and will be the modality in future implementation of quantum information processing by NMR.
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03.67.Lx Quantum computation architectures and implementations
33.25.+k Nuclear resonance and relaxation

Improvement of the coupled-cluster singles and doubles method via scaling same- and opposite-spin components of the double excitation correlation energy

Tait Takatani, Edward G. Hohenstein, and C. David Sherrill

J. Chem. Phys. 128, 124111 (2008); http://dx.doi.org/10.1063/1.2883974 (7 pages) | Cited 28 times

Online Publication Date: 31 March 2008

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There has been much interest in cost-free improvements to second-order Møller–Plesset perturbation theory (MP2) via scaling the same- and opposite-spin components of the correlation energy (spin-component scaled MP2). By scaling the same- and opposite-spin components of the double excitation correlation energy from the coupled-cluster of single and double excitations (CCSD) method, similar improvements can be achieved. Optimized for a set of 48 reaction energies, scaling factors were determined to be 1.13 and 1.27 for the same- and opposite-spin components, respectively. Preliminary results suggest that the spin-component scaled CCSD (SCS-CCSD) method will outperform all MP2 type methods considered for describing intermolecular interactions. Potential energy curves computed with the SCS-CCSD method for the sandwich benzene dimer and methane dimer reproduce the benchmark CCSD(T) potential curves with errors of only a few hundredths of 1 kcal mol−1 for the minima. The performance of the SCS-CCSD method suggests that it is a reliable, lower cost alternative to the CCSD(T) method.
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31.15.xp Perturbation theory
31.15.bw Coupled-cluster theory
34.20.Gj Intermolecular and atom-molecule potentials and forces
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

On the generalized equipartition theorem in molecular dynamics ensembles and the microcanonical thermodynamics of small systems

Mark J. Uline, Daniel W. Siderius, and David S. Corti

J. Chem. Phys. 128, 124301 (2008); http://dx.doi.org/10.1063/1.2889939 (17 pages) | Cited 5 times

Online Publication Date: 24 March 2008

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We consider various ensemble averages within the molecular dynamics (MD) ensemble, corresponding to those states sampled during a MD simulation in which the application of periodic boundary conditions imposes a constraint on the momentum of the center of mass. As noted by Shirts et al. [J. Chem. Phys. 125, 164102 (2006)] for an isolated system, we find that the principle of equipartition is not satisfied within such simulations, i.e., the total kinetic energy of the system is not shared equally among all the translational degrees of freedom. Nevertheless, we derive two different versions of Tolman’s generalized equipartition theorem, one appropriate for the canonical ensemble and the other relevant to the microcanonical ensemble. In both cases, the breakdown of the principle of equipartition immediately follows from Tolman’s result. The translational degrees of freedom are, however, still equivalent, being coupled to the same bulk property in an identical manner. We also show that the temperature of an isolated system is not directly proportional to the average of the total kinetic energy (in contrast to the direct proportionality that arises between the temperature of the external bath and the kinetic energy within the canonical ensemble). Consequently, the system temperature does not appear within Tolman’s generalized equipartition theorem for the microcanonical ensemble (unlike the immediate appearance of the temperature of the external bath within the canonical ensemble). Both of these results serve to highlight the flaws in the argument put forth by Hertz [Ann. Phys. 33, 225 (1910) ; 33, 537 (1910)] for defining the entropy of an isolated system via the integral of the phase space volume. Only the Boltzmann–Planck entropy definition, which connects entropy to the integral of the phase space density, leads to the correct description of the properties of a finite, isolated system. We demonstrate that the use of the integral of the phase space volume leads to unphysical results, indicating that the property of adiabatic invariance has little to do with the behavior of small systems.
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61.20.Ja Computer simulation of liquid structure
65.20.De General theory of thermodynamic properties of liquids, including computer simulation

The adsorption of CO on charged and neutral Au and Au2: A comparison between wave-function based and density functional theory

Peter Schwerdtfeger, Matthias Lein, Robert P. Krawczyk, and Christoph R. Jacob

J. Chem. Phys. 128, 124302 (2008); http://dx.doi.org/10.1063/1.2834693 (10 pages) | Cited 8 times

Online Publication Date: 25 March 2008

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Quantum theoretical calculations are presented for CO attached to charged and neutral Au and Au2 with the aim to test the performance of currently applied density functional theory (DFT) by comparison with accurate wave-function based results. For this, we developed a compact sized correlation-consistent valence basis set which accompanies a small-core energy-consistent scalar relativistic pseudopotential for gold. The properties analyzed are geometries, dissociation energies, vibrational frequencies, ionization potentials, and electron affinities. The important role of the basis-set superposition error is addressed which can be substantial for the negatively charged systems. The dissociation energies decrease along the series Au+–CO, Au–CO, and Au−–CO and as well as along the series Au2+–CO, Au2–CO, and Au2−–CO. As one expects, a negative charge on gold weakens the carbon oxygen bond considerably, with a consequent redshift in the CO stretching frequency when moving from the positively charged to the neutral and the negatively charged gold atom or dimer. We find that the different density functional approximations applied are not able to correctly describe the rather weak interaction between CO and gold, thus questioning the application of DFT to CO adsorption on larger gold clusters or surfaces.
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68.43.Bc Ab initio calculations of adsorbate structure and reactions
68.43.Fg Adsorbate structure (binding sites, geometry)
68.43.Pq Adsorbate vibrations
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
71.15.Dx Computational methodology (Brillouin zone sampling, iterative diagonalization, pseudopotential construction)

Modeling of HeN+ clusters. II. Calculation of He3+ vibrational spectrum

František Karlický, Bruno Lepetit, René Kalus, Ivana Paidarová, and Florent Xavier Gadéa

J. Chem. Phys. 128, 124303 (2008); http://dx.doi.org/10.1063/1.2841019 (11 pages) | Cited 3 times

Online Publication Date: 25 March 2008

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We have computed the vibrational spectrum of the helium ionized trimer He3+ using three different potential energy surfaces [ D. T. Chang and G. L. Gellene, J. Chem. Phys. 119, 4694 (2003) ; E. Scifoni et al., ibid. 125, 164304 (2006) ; I. Paidarová et al., Chem. Phys. 342, 64 (2007) ]. Differences in the details of these potential energy surfaces induce discrepancies between bound state energies of the order of 0.01 eV. The effects of the geometric phase induced by the conical intersection between the ground electronic potential energy surface and the first excited one are studied by computing vibrational spectra with and without this phase. The six lowest vibrational bound states are negligibly affected by the geometric phase. Indeed, they correspond to wavefunctions localized in the vicinity of the linear symmetric configurations and can be assigned well defined vibrational quantum numbers. On the other hand, higher excited states are delocalized, cannot be assigned definite vibrational quantum numbers, and the geometric phase shifts their energies by approximately 0.005 eV.
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36.40.Mr Spectroscopy and geometrical structure of clusters
36.40.Cg Electronic and magnetic properties of clusters
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Tp Vibrational analysis
31.50.-x Potential energy surfaces

Formation of hydrogenated boron clusters in an external quadrupole static attraction ion trap

Yuji Ohishi, Kaoru Kimura, Masaaki Yamaguchi, Noriyuki Uchida, and Toshihiko Kanayama

J. Chem. Phys. 128, 124304 (2008); http://dx.doi.org/10.1063/1.2894864 (7 pages) | Cited 5 times

Online Publication Date: 26 March 2008

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We report the formation of icosahedral B12H8+ through ion-molecule reactions of the decaborane ion [B10Hx+ (x = 6–14)] with diborane (B2H6) molecules in an external quadrupole static attraction ion trap. The hydrogen content n of B12Hn+ is determined by the analysis of the mass spectrum. The result reveals that B12H8+ is the main product. Ab initio calculations indicate that B12H8+ preferentially forms an icosahedral structure rather than a quasiplanar structure. The energies of the formation reactions of B12H14+ and B12H12+ between B10Hx+ (x = 6,8) ions, which are considered to be involved in the formation of B12Hn+, and a B2H6 molecule are calculated. The calculations of the detachment pathway of H2 molecules and H atoms from the product ions, B12H14+ and B12H12+, indicate that the intermediate state has a relatively low energy, enabling the detachment reaction to proceed owing to the sufficient reaction energy. This autodetachment of H2 accounts for the experimental result that B12H8+ is the most abundant product, even though it does not have the lowest energy among B12Hn+.
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82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
82.80.Ms Mass spectrometry (including SIMS, multiphoton ionization and resonance ionization mass spectrometry, MALDI)
82.20.Fd Collision theories; trajectory models
82.33.Fg Reactions in clusters

Ab initio potential energy surfaces and nonadiabatic interactions in the H++NO collision system

Saieswari Amaran, Sanjay Kumar, and H. Köppel

J. Chem. Phys. 128, 124305 (2008); http://dx.doi.org/10.1063/1.2894308 (11 pages) | Cited 4 times

Online Publication Date: 26 March 2008

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Ab initio calculations on the H++NO system have been carried out in Jacobi coordinates at the multireference configuration interaction level employing Dunning’s correlation-consistent polarized valence triple zeta basis set to analyze the role of low-lying electronic excited states in influencing the collision dynamics relevant to the experimental collision energy range of 9.5–30 eV. The lowest two adiabatic potential energy surfaces, asymptotically correlating to H++NO(X 2Π) and H(2S)+NO+(X 1Σ+), have been obtained. Using ab initio procedures, the (radial) nonadiabatic couplings and the mixing angle between the lowest two electronic states (1 2A′ and 2 2A′) have been obtained to yield the corresponding quasidiabatic potential energy matrix. The strengths of the computed vibrational coupling matrix elements reflect a similar trend, as has been observed experimentally in the magnitudes of the state-to-state transition probability for the inelastic vibrational excitations [ J. Krutein and F. Linder, J. Chem. Phys. 71, 559 (1979); F. A. Gianturco et al., J. Phys. B 14, 667 (1981) ].
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34.50.Ez Rotational and vibrational energy transfer
34.50.Fa Electronic excitation and ionization of atoms (including beam-foil excitation and ionization)
34.50.Gb Electronic excitation and ionization of molecules
31.15.am Relativistic configuration interaction (CI) and many-body perturbation calculations

Quantum dynamics of inelastic excitations and charge transfer processes in the H++NO collisions

Saieswari Amaran and Sanjay Kumar

J. Chem. Phys. 128, 124306 (2008); http://dx.doi.org/10.1063/1.2894311 (7 pages) | Cited 1 time

Online Publication Date: 26 March 2008

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State-resolved differential cross section, integral cross section, average vibrational energy transfer, and the relative transition probability are computed for the H++NO system using our newly obtained ab initio potential energy surfaces (PES) at the multireference configuration interaction level of accuracy employing the correlation consistent polarized valence triple zeta basis set. The quantum dynamics is treated within the vibrational close-coupling rotational infinite-order sudden approximation using the coupled ground state and first excited state ab initio quasidiabatic PES. The computed collision attributes for the inelastic vibrational excitation are compared with the state-to-state scattering data available at Ec.m. = 9.5 eV and Ec.m. = 29.03 eV and are found to be in overall good agreement with those of the experiments. The results for the vibrational charge transfer processes at these collision energies are also presented.
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34.70.+e Charge transfer
34.50.Ez Rotational and vibrational energy transfer
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions
31.15.am Relativistic configuration interaction (CI) and many-body perturbation calculations

Automatic generation of active coordinates for quantum dynamics calculations: Application to the dynamics of benzene photochemistry

Benjamin Lasorne, Fabrizio Sicilia, Michael J. Bearpark, Michael A. Robb, Graham A. Worth, and Lluìs Blancafort

J. Chem. Phys. 128, 124307 (2008); http://dx.doi.org/10.1063/1.2839607 (10 pages) | Cited 5 times

Online Publication Date: 26 March 2008

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A new practical method to generate a subspace of active coordinates for quantum dynamics calculations is presented. These reduced coordinates are obtained as the normal modes of an analytical quadratic representation of the energy difference between excited and ground states within the complete active space self-consistent field method. At the Franck-Condon point, the largest negative eigenvalues of this Hessian correspond to the photoactive modes: those that reduce the energy difference and lead to the conical intersection; eigenvalues close to 0 correspond to bath modes, while modes with large positive eigenvalues are photoinactive vibrations, which increase the energy difference. The efficacy of quantum dynamics run in the subspace of the photoactive modes is illustrated with the photochemistry of benzene, where theoretical simulations are designed to assist optimal control experiments.
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82.50.-m Photochemistry
82.20.Ej Quantum theory of reaction cross section
82.20.Wt Computational modeling; simulation

On the validity of the Arrhenius equation for electron attachment rate coefficients

Ilya I. Fabrikant and Hartmut Hotop

J. Chem. Phys. 128, 124308 (2008); http://dx.doi.org/10.1063/1.2841079 (8 pages) | Cited 18 times

Online Publication Date: 27 March 2008

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The validity of the Arrhenius equation for dissociative electron attachment rate coefficients is investigated. A general analysis allows us to obtain estimates of the upper temperature bound for the range of validity of the Arrhenius equation in the endothermic case and both lower and upper bounds in the exothermic case with a reaction barrier. The results of the general discussion are illustrated by numerical examples whereby the rate coefficient, as a function of temperature for dissociative electron attachment, is calculated using the resonance R-matrix theory. In the endothermic case, the activation energy in the Arrhenius equation is close to the threshold energy, whereas in the case of exothermic reactions with an intermediate barrier, the activation energy is found to be substantially lower than the barrier height.
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82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.20.Pm Rate constants, reaction cross sections, and activation energies

Basis-set limit of the aurophilic attraction using the MP2 method: The examples of [ClAuPH3]2 dimer and [P(AuPH3)4]+ ion

Pekka Pyykkö and Patryk Zaleski-Ejgierd

J. Chem. Phys. 128, 124309 (2008); http://dx.doi.org/10.1063/1.2842081 (6 pages) | Cited 14 times

Online Publication Date: 27 March 2008

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The basis-set limit of the aurophilic attraction is studied at the MP2 level for the free model dimer [ClAuPH3]2 and for a [P(AuPH3)4]+ ion. The latter system is found to prefer a C4v symmetry, instead of Td, in agreement with Li and Pyykkö [Inorg. Chem. 32, 2630 (1993)] but in contradiction to recent results of Fang and Wang [J. Phys. Chem. A. 111, 1562 (2006)] . The Karlsruhe split valence and the Dunning correlation-consistent basis sets converge to the same limit.
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33.15.Bh General molecular conformation and symmetry; stereochemistry
31.15.xp Perturbation theory

An ab initio investigation on (CO2)n and CO2(Ar)m clusters: Geometries and IR spectra

K. V. Jovan Jose and Shridhar R. Gadre

J. Chem. Phys. 128, 124310 (2008); http://dx.doi.org/10.1063/1.2838202 (9 pages) | Cited 11 times

Online Publication Date: 28 March 2008

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An ab initio investigation on CO2 homoclusters is done at MPWB1K/6-31++G(2d) level of theory. Electrostatic guidelines are found to be useful for generating initial structures of (CO2)n clusters. The ab initio minimum energy geometries of (CO2)n with n = 2–8 are T shaped, cyclic, trigonal pyramidal, tetragonal pyramidal, tetragonal bipyramidal, pentagonal bipyramidal, and pentagonal bipyramid with one CO2 molecule attached to it. A test calculation on (CO2)20 cluster is also reported. The geometric parameters of the energetically most favored (CO2)n clusters match quite well their experimental counterparts (wherever available) as well as those derived from molecular dynamics studies. The effect of clustering is quantified through the asymmetric C–O stretching frequency shift relative to the single CO2 molecule. (CO2)n clusters show an increasing blueshift from 1.8 to 9.6 cm−1 on increasing number of CO2 molecules from n = 2 to 8. The energetics and geometries of CO2(Ar)m clusters have also been explored at the same level of theory. The geometries for m = 1–6 show a predominant T type of the argon-CO2 molecule interaction. Higher clusters with m = 7–12 show that the argon atoms cluster around the oxygen atom after the saturation of the central carbon atom. The CO2(Ar)m clusters exhibit an increasing redshift in the C–O asymmetric stretch relative to CO2 molecule of 0.7–5.6 cm−1 with increasing number of argon atoms through m = 1–8.
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36.40.Mr Spectroscopy and geometrical structure of clusters
33.20.Ea Infrared spectra
33.70.Jg Line and band widths, shapes, and shifts
31.15.ae Electronic structure and bonding characteristics
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Near-resonant energy transfer from highly vibrationally excited OH to N2

Kelly D. Burtt and Ramesh D. Sharma

J. Chem. Phys. 128, 124311 (2008); http://dx.doi.org/10.1063/1.2884343 (8 pages) | Cited 3 times

Online Publication Date: 28 March 2008

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The probability per collision P(T) of near-resonant vibration-to-vibration energy transfer (ET) of one quantum of vibrational energy from vibrational levels ν = 8 and ν = 9 of OH to N2(ν = 0), OH(ν)+N2(0)→OH(ν−1)+N2(1), is calculated in the 100–350 K temperature range. These processes represent important steps in a model that explains the enhanced 4.3 μm emission from CO2 in the nocturnal mesosphere. The calculated energy transfer is mediated by weak long-range dipole-quadrupole interaction. The results of this calculation are very sensitive to the strength of the two transition moments. Because of the long range of the intermolecular potential, the resonance function, a measure of energy that can be efficiently exchanged between translation and vibration-rotation degrees of freedom, is rather narrow. A narrow resonance function coupled with the large rotational constant of OH is shown to render the results of the calculation very sensitive to the rotational distribution, or the rotational temperature if one exists, of this molecule. The calculations are carried out in the first and second orders of perturbation theory with the latter shown to give ET probabilities that are an order of magnitude larger than the former. The reasons for the difference in magnitude and temperature dependence of the first- and second-order calculations are discussed. The results of the calculations are compared with room temperature measurements as well as with an earlier calculation. Our calculated results are in good agreement with the room temperature measurements for the transfer of vibrational energy for the exothermic OH(ν = 9) ET process but are about an order lower than the room temperature measurements for the exothermic OH(ν = 8) ET process. The cause of this discrepancy is explored. This calculation does not give the large values of the rate coefficients needed by the model that explains the enhanced 4.3 μm emission from CO2 in the nocturnal mesosphere.
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34.50.Ez Rotational and vibrational energy transfer
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions
31.15.xp Perturbation theory

Fragmentation branching ratios of highly excited hydrocarbon molecules CnH and their cations CnH+ (n ⩽ 4)

T. Tuna, M. Chabot, T. Pino, P. Désesquelles, A. LePadellec, G. Martinet, M. Barat, B. Lucas, F. Mezdari, L. Montagnon, N. T. Van-Oanh, L. Lavergne, A. Lachaize, Y. Carpentier, and K. Béroff

J. Chem. Phys. 128, 124312 (2008); http://dx.doi.org/10.1063/1.2884862 (11 pages) | Cited 1 time

Online Publication Date: 28 March 2008

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We have measured fragmentation branching ratios of neutral CnH and CnH+ cations produced in high velocity (4.5 a.u) collisions between incident CnH+ cations and helium atoms. Electron capture gives rise to excited neutral species CnH and electronic excitation to excited cations CnH+. Thanks to a dedicated setup, based on coincident detection of all fragments, the dissociations of the neutral and cationic parents were recorded separately and in a complete way. For the fragmentation of CnH, the H-loss channel is found to be dominant, as already observed by other authors. By contrast, the H-loss and C-loss channels equally dominate the two-fragment break up of CnH+ species. For these cations, we provide the first fragmentation data (n>2). Results are also discussed in the context of astrochemistry.
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34.50.Gb Electronic excitation and ionization of molecules
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
34.70.+e Charge transfer
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