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21 Jul 2009

Volume 131, Issue 3, Articles (03xxxx)

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J. Chem. Phys. 131, 031101 (2009); http://dx.doi.org/10.1063/1.3173609 (4 pages)

Thomas Gustavsson, Pedro B. Coto, Luis Serrano-Andrés, Takashige Fujiwara, and Edward C. Lim
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Do fluorescence and transient absorption probe the same intramolecular charge transfer state of 4-(dimethylamino)benzonitrile?

Thomas Gustavsson, Pedro B. Coto, Luis Serrano-Andrés, Takashige Fujiwara, and Edward C. Lim

J. Chem. Phys. 131, 031101 (2009); http://dx.doi.org/10.1063/1.3173609 (4 pages) | Cited 6 times

Online Publication Date: 15 July 2009

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We present here the results of time-resolved absorption and emission experiments for 4-(dimethylamino)benzonitrile in solution, which suggest that the fluorescent intramolecular charge transfer (ICT) state may differ from the twisted ICT (TICT) state observed in transient absorption.
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82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
82.80.Dx Analytical methods involving electronic spectroscopy
78.55.Bq Liquids
78.40.Dw Liquids
78.47.D- Time resolved spectroscopy (>1 psec)
78.40.Me Organic compounds and polymers
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Heterogeneous boiling-up of superheated liquid at achievable superheat threshold

G. V. Ermakov, E. V. Lipnyagov, S. A. Perminov, and A. L. Gurashkin

J. Chem. Phys. 131, 031102 (2009); http://dx.doi.org/10.1063/1.3179678 (3 pages) | Cited 2 times

Online Publication Date: 15 July 2009

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The classical theory of homogeneous nucleation describes well the superheat threshold observed in experiments. It may be assumed therefore that homogeneous boiling-up of a liquid takes place in experiments, and the theory has been verified experimentally well. The streak photography used in this study showed that boiling-up of a superheated liquid at the threshold of the achievable superheat occurs at a limited number of surface fluctuation centers in a vessel, rather than in the bulk as one would expect with homogeneous nucleation. Thus, the homogeneous theory, which rather accurately describes the heterogeneous threshold of the achievable superheat, obviously is not confirmed in experiments.
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64.70.fh Boiling and bubble dynamics
64.60.qj Studies of nucleation in specific substances
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Increased fraction of weakened hydrogen bonds of water in aerosol OT reverse micelles

Iradwikanari Waluyo, Dennis Nordlund, Uwe Bergmann, Lars G. M. Pettersson, and Anders Nilsson

J. Chem. Phys. 131, 031103 (2009); http://dx.doi.org/10.1063/1.3184764 (3 pages) | Cited 4 times

Online Publication Date: 17 July 2009

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Water in aerosol OT reverse micelles has been extensively studied as a model system for nanoconfined water. Results from previous vibrational studies suggest that water confined this way has slower dynamics compared to bulk water; however, the effects on the hydrogen bonding network are unclear. From our study of the structure of water hydrogen bonding using x-ray Raman spectroscopy, we found an increased fraction of weakened hydrogen bonds upon confinement, similar to the effect seen in temperature increase and NaCl solvation, as evidenced by the changes in specific spectral features.
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78.30.C- Liquids
82.70.Dd Colloids
61.25.Em Molecular liquids
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Exact two-component Hamiltonians revisited

Wenjian Liu and Daoling Peng

J. Chem. Phys. 131, 031104 (2009); http://dx.doi.org/10.1063/1.3159445 (4 pages) | Cited 17 times

Online Publication Date: 21 July 2009

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Two routes for deriving the exact two-component Hamiltonians are compared. In the first case, as already known, we start directly from the matrix representation of the Dirac operator in a restricted kinetically balanced basis and make a single block diagonalization. In the second case, not considered before, we start instead from the Foldy–Wouthuysen operator and make proper use of resolutions of the identity. The expressions are surprisingly different. It turns out that a mistake was made in the former formulation when going from the Dirac to the Schrödinger picture. The two formulations become equivalent after the mistake is corrected.
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03.65.Pm Relativistic wave equations
03.65.Ge Solutions of wave equations: bound states
03.65.Fd Algebraic methods
02.10.Yn Matrix theory
02.30.Tb Operator theory
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Mechanism of carbon nanotubes unzipping into graphene ribbons

Norma L. Rangel, Juan C. Sotelo, and Jorge M. Seminario

J. Chem. Phys. 131, 031105 (2009); http://dx.doi.org/10.1063/1.3170926 (4 pages) | Cited 3 times

Online Publication Date: 21 July 2009

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The fabrication of graphene nanoribbons from carbon nanotubes (CNTs) treated with potassium permanganate in a concentrated sulfuric acid solution has been reported by Kosynkin et al. [Nature (London) 458, 872 (2009) ]. Here we report ab initio density functional theory calculations of such unzipping process. We find that the unzipping starts with the potassium permanganate attacking one of the internal C–C bonds of the CNT, stretching and breaking it. The created defect weakens neighboring bonds along the length of the CNT, making them energetically prone to be attacked too.
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81.16.-c Methods of micro- and nanofabrication and processing
61.46.Fg Nanotubes
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Thermal lens measurement of the Soret coefficient in acetone/water mixtures

Humberto Cabrera, Luis Martí-López, Eloy Sira, Kareem Rahn, and Máximo García-Sucre

J. Chem. Phys. 131, 031106 (2009); http://dx.doi.org/10.1063/1.3182806 (3 pages) | Cited 3 times

Online Publication Date: 21 July 2009

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The Soret coefficient of acetone/water mixtures has been experimentally determined by a recently developed thermal lens technique [ Appl. Phys. Lett. 94, 051103 (2009)]. The behavior of the Soret coefficient was reproduced, including its sign change with composition. For concentrations around the equimolar ones, we have also confirmed the disagreement between the experimental and simulation data that had previously been reported by Ning and Wiegand [ J. Chem. Phys. 125, 221102 (2006) ] using a transient holographic grating technique of thermal diffusion forced Rayleigh scattering. Additionally, we compare our experimental results with the theoretical values predicted by a recently developed viscous energy model [ J. Chem. Phys. 130, 064506 (2009) ].
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66.10.cd Thermal diffusion and diffusive energy transport
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Realization of the CNOT quantum gate operation in six-dimensional ammonia using the OCT-MCTDH approach

Markus Schröder and Alex Brown

J. Chem. Phys. 131, 034101 (2009); http://dx.doi.org/10.1063/1.3168438 (10 pages) | Cited 12 times

Online Publication Date: 15 July 2009

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Comparative studies of implementations of the controlled NOT quantum gate operation using vibrational states of ammonia as the qubit states are presented. The quantum gate operations are realized using tailored laser pulses, which are calculated using a combined approach of optimal control theory and the multiconfiguration time-dependent Hartree method. We compare results obtained with a reduced model of ammonia with three degrees of freedom (all N–H bond distances fixed) to those obtained with a full six-dimensional model. In our study, the optimal laser pulses of both models induce similar underlying physical mechanisms while the gate quality within the reduced model (>98%) is much higher than within the six-dimensional model ( ≈ 80%).
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03.67.Lx Quantum computation architectures and implementations
33.15.Dj Interatomic distances and angles
31.15.xr Self-consistent-field methods
33.20.Tp Vibrational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants

The multiscale coarse-graining method. III. A test of pairwise additivity of the coarse-grained potential and of new basis functions for the variational calculation

Avisek Das and Hans C. Andersen

J. Chem. Phys. 131, 034102 (2009); http://dx.doi.org/10.1063/1.3173812 (11 pages) | Cited 13 times

Online Publication Date: 15 July 2009

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The multiscale coarse-graining (MS-CG) method, proposed by Izvekov and Voth [J. Phys. Chem. B 109, 2469 (2005) ; Izvekov and VothJ. Chem. Phys. 123, 134105 (2005) ], is a method for determining the effective potential energy function for a coarse-grained model of a fluid using data obtained from molecular dynamics (MD) simulation of the corresponding atomically detailed model. The method has been given a rigorous statistical mechanical basis [ Noid et al. J. Chem. Phys. 128, 244114 (2008) ; Noid et al.,J. Chem. Phys. 128, 244115 (2008) ]. The coarse-grained (CG) potentials obtained using the MS-CG method are an approximate variational solution for the exact many-body potential of mean force for the coarse-grained sites. In this paper we apply this method to study the many-body potential of mean force among solutes in a simple model of a solution of Lennard-Jones particles. We use a new set of basis functions for the variational calculation that is useful when the coarse-grained potential is approximately equal to an arbitrarily complicated pairwise additive, central interaction among the sites of the coarse-grained model. For this model, pairwise additivity of the many-body potential of mean force is a very good approximation when the solute concentration is low, and it becomes less accurate for high concentrations, indicating the importance of many-body contributions to the coarse-grained potential. The best possible pairwise additive CG potential of the solute particles is found to be quite long ranged for all concentrations except those for which the mole fraction of solute is very close to unity. We discuss strategies for construction of short-ranged potentials for efficient but accurate CG MD simulation. We also discuss how the choice of basis functions for the variational calculation can be used to provide smoothing of the calculated CG potential function to overcome statistical sampling error in the atomistic simulation data used for the generation of the potential.
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61.20.Ja Computer simulation of liquid structure

Density functional theory calculation on many-cores hybrid central processing unit-graphic processing unit architectures

Luigi Genovese, Matthieu Ospici, Thierry Deutsch, Jean-François Méhaut, Alexey Neelov, and Stefan Goedecker

J. Chem. Phys. 131, 034103 (2009); http://dx.doi.org/10.1063/1.3166140 (8 pages) | Cited 7 times

Online Publication Date: 16 July 2009

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We present the implementation of a full electronic structure calculation code on a hybrid parallel architecture with graphic processing units (GPUs). This implementation is performed on a free software code based on Daubechies wavelets. Such code shows very good performances, systematic convergence properties, and an excellent efficiency on parallel computers. Our GPU-based acceleration fully preserves all these properties. In particular, the code is able to run on many cores which may or may not have a GPU associated, and thus on parallel and massive parallel hybrid machines. With double precision calculations, we may achieve considerable speedup, between a factor of 20 for some operations and a factor of 6 for the whole density functional theory code.
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31.10.+z Theory of electronic structure, electronic transitions, and chemical binding
31.15.E- Density-functional theory
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Energy and charge-transfer dynamics using projected modes

Andrey Pereverzev, Eric R. Bittner, and Irene Burghardt

J. Chem. Phys. 131, 034104 (2009); http://dx.doi.org/10.1063/1.3174447 (10 pages) | Cited 4 times

Online Publication Date: 16 July 2009

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For electron-phonon Hamiltonians with the couplings linear in the phonon operators, we construct a class of unitary transformations that separate the normal modes into two groups. The modes in the first group interact with the electronic degrees of freedom directly. The modes in the second group interact directly only with the modes in the first group but not with the electronic system. These transformations can be carried out independently for different types of phonon modes, e.g., high- versus low-frequency phonon bands. This construction generalizes recently introduced transformations for systems exhibiting a conical intersection topology. The separation of the normal modes into several groups allows one to develop new approximation schemes. We apply one of such schemes to study electronic relaxation at a semiconducting polymer interface.
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71.38.-k Polarons and electron-phonon interactions
71.35.Gg Exciton-mediated interactions
71.20.Rv Polymers and organic compounds
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions

Electromagnetically induced transparency in a three-level lambda system with permanent dipole moments

Fengxue Zhou, Yueping Niu, and Shangqing Gong

J. Chem. Phys. 131, 034105 (2009); http://dx.doi.org/10.1063/1.3176018 (6 pages) | Cited 2 times

Online Publication Date: 16 July 2009

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Electromagnetically induced transparency in a three-level Λ-type molecular system with nonzero permanent dipole moments is investigated. It is shown that in the (2+2)-transition processes, when the sign of d21, the difference in permanent dipole moments of the probe transition, is positive, perfect electromagnetically induced transparency with steep normal dispersion could be obtained under specific conditions. In contrast, when the sign of d21 is negative, surprisingly gain without inversion with steep anomalous dispersion could be attained.
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42.50.Gy Effects of atomic coherence on propagation, absorption, and amplification of light; electromagnetically induced transparency and absorption
33.80.-b Photon interactions with molecules
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility

Exchange and correlation in molecular wire conductance: Nonlocality is the key

Jeremy S. Evans, Oleg A. Vydrov, and Troy Van Voorhis

J. Chem. Phys. 131, 034106 (2009); http://dx.doi.org/10.1063/1.3179754 (10 pages) | Cited 6 times

Online Publication Date: 16 July 2009

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We study real-time electron dynamics in a molecular junction with a variety of approximations to the electronic structure, toward the ultimate aim of determining what ingredients are crucial for the accurate prediction of charge transport. We begin with real-time, all electron simulations using some common density functionals that differ in how they treat long-range Hartree–Fock exchange. We find that the inclusion or exclusion of nonlocal exchange is the dominant factor determining the transport behavior, with all semilocal contributions having a smaller effect. In order to study nonlocal correlation, we first map our junction onto a simple Pariser–Parr–Pople (PPP) model Hamiltonian. The PPP dynamics are shown to faithfully reproduce the all electron results, and we demonstrate that nonlocal correlation can be readily included in the model space using the generator coordinate method (GCM). Our PPP-GCM simulations suggest that nonlocal correlation has a significant impact on the I-V character that is not captured even qualitatively by any of the common semilocal approximations to exchange and correlation. The implications of our results for transport calculations are discussed.
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31.15.V- Electron correlation calculations for atoms, ions and molecules
31.15.eg Exchange-correlation functionals (in current density functional theory)
31.15.xr Self-consistent-field methods
31.15.bu Semi-empirical and empirical calculations (differential overlap, Hückel, PPP methods, etc.)
34.70.+e Charge transfer

Coarse-grained ions without charges: Reproducing the solvation structure of NaCl in water using short-ranged potentials

Robert C. DeMille and Valeria Molinero

J. Chem. Phys. 131, 034107 (2009); http://dx.doi.org/10.1063/1.3170982 (16 pages) | Cited 7 times

Online Publication Date: 17 July 2009

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A coarse-grained model of NaCl in water is presented where the ions are modeled without charge to avoid computationally challenging electrostatics. A monatomic model of water [ V. Molinero and E. B. Moore, J. Phys. Chem. B 113, 4008 (2009) ] is used as the basis for this coarse-grain approach. The ability of Na+ to disrupt the native tetrahedral arrangement of water molecules, and of Cl− to integrate within this organization, is preserved in this mW-ion model through parametrization focused on water’s solvation of these ions. This model successfully reproduces the structural effect of ions on water, referenced to observations from experiments and atomistic molecular dynamics simulations, while using extremely short-ranged potentials. Without Coulomb interactions the model replicates details of the ion-water structure such as distinguishing contact and solvent-separated ion pairs and the free energy barriers between them. The approach of mimicking ionic effects with short-ranged interactions results in performance gains of two orders of magnitude compared to Ewald methods. Explored over a broad range of salt concentration, the model reproduces the solvation structure and trends of diffusion relative to atomistic simulations and experimental results. The functional form of the mW-ion model can be parametrized to represent other electrolytes. With increased computational efficiency and reliable structural fidelity, this model promises to be an asset for accessing significantly longer simulation time scales with an explicit solvent in a coarse-grained system involving, for example, polyelectrolytes such as proteins, nucleic acids, and fuel-cell membranes.
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82.30.Nr Association, addition, insertion, cluster formation
82.60.Lf Thermodynamics of solutions
61.25.Em Molecular liquids
66.10.C- Diffusion and thermal diffusion
65.20.Jk Studies of thermodynamic properties of specific liquids
61.20.Ja Computer simulation of liquid structure

“Compressing liquid”: An efficient global minima search strategy for clusters

R. L. Zhou, L. Y. Zhao, and B. C. Pan

J. Chem. Phys. 131, 034108 (2009); http://dx.doi.org/10.1063/1.3176511 (6 pages)

Online Publication Date: 17 July 2009

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In this paper we present a new global search strategy named as “compressing liquid” for atomic clusters. In this strategy, a random fragment of liquid structure is adopted as a starting geometry, followed by iterative operations of “compressing” and Monte Carlo adjustment of the atom positions plus structural optimization. It exhibits fair efficiency when it is applied to seeking the global minima of Lennard-Jones clusters. We also employed it to search the low-lying candidates of medium silicon clusters Sin(n = 40–60), where the global search is absent. We found the best candidates for most sizes. More importantly, we obtained non-fullerene-based structures for some sized clusters, which were not found from the endohedral-fullerene strategy. These results indicate that the “compressing-liquid” method is highly efficient for global minima search of clusters.
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61.25.Em Molecular liquids
61.20.Ja Computer simulation of liquid structure

Umbrella integration in two or more reaction coordinates

Johannes Kästner

J. Chem. Phys. 131, 034109 (2009); http://dx.doi.org/10.1063/1.3175798 (8 pages) | Cited 4 times

Online Publication Date: 17 July 2009

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Umbrella integration is a method to analyze umbrella sampling simulations by calculating and integrating the mean force. Here, the method is extended to multidimensional reaction coordinates. Approximation of the probability distribution obtained from sampling by a multivariate normal distribution allows to calculate the mean force from the average and the covariance matrix of the reaction coordinate. Integration schemes of the free-energy gradient field are discussed. Integration on a real-space grid is compared to expansion of the gradient in a series of analytic functions (such as a Fourier analysis), which can be integrated, and the expansion of the gradient only at the window means in a series of analytic functions. The Fourier analysis was found particularly useful for periodic reaction coordinates, such as torsion angles. An expression is provided to calculate the Hessian of the free energy with respect to the reaction coordinates from sampling data. The utility of the method is demonstrated at the example of the free-energy surface of the alanine dipeptide in vacuum calculated with respect to the backbone torsion angles Φ and Ψ. Relevance of the Jacobian term for non-Cartesian reaction coordinates is discussed.
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87.15.R- Reactions and kinetics
87.14.E- Proteins

Long-range-corrected hybrid density functionals including random phase approximation correlation: Application to noncovalent interactions

Benjamin G. Janesko, Thomas M. Henderson, and Gustavo E. Scuseria

J. Chem. Phys. 131, 034110 (2009); http://dx.doi.org/10.1063/1.3176514 (9 pages) | Cited 24 times

Online Publication Date: 17 July 2009

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We recently presented a combination of a short-range density functional approximation with long-range random phase approximation (RPA) correlation [ B. G. Janesko, T. M. Henderson, and G. E. Scuseria, J. Chem. Phys. 130, 081105 (2009) ]. Here we show that this approach provides very accurate interaction energy predictions for a range of noncovalent complexes. Calculations on representative sets of hydrogen bonded, dipole-dipole, charge transfer, and weakly bound (van der Waals) complexes show that long-range RPA provides statistical errors comparable to CCSD(T) in moderate basis sets. This approach shows promise for providing accurate and computationally tractable models of noncovalent interactions in biological systems.
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31.15.eg Exchange-correlation functionals (in current density functional theory)
34.70.+e Charge transfer
33.15.Fm Bond strengths, dissociation energies

Oscillations in meta-generalized-gradient approximation potential energy surfaces for dispersion-bound complexes

Erin R. Johnson, Axel D. Becke, C. David Sherrill, and Gino A. DiLabio

J. Chem. Phys. 131, 034111 (2009); http://dx.doi.org/10.1063/1.3177061 (7 pages) | Cited 30 times

Online Publication Date: 17 July 2009

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Meta-generalized-gradient approximations (meta-GGAs) in density-functional theory are exchange-correlation functionals whose integrands depend on local density, density gradient, and also the kinetic-energy density. It has been pointed out by Johnson et al. [Chem. Phys. Lett. 394, 334 (2004) ] that meta-GGA potential energy curves in dispersion-bound complexes are susceptible to spurious oscillations unless very large integration grids are used. This grid sensitivity originates from the saddle-point region of the density near the intermonomer midpoint. Various dimensionless ratios involving the kinetic-energy density, found in typical meta-GGAs, may be ill-behaved in this region. Grid sensitivity thus arises if the midpoint region is sampled by too sparse a grid. For most meta-GGAs, standard grids do not suffice. Care must be taken to avoid this problem when using, or constructing, meta-GGAs.
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31.15.eg Exchange-correlation functionals (in current density functional theory)
31.50.-x Potential energy surfaces
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility

Accelerating simulation of metastable decay

Isamu Kusaka

J. Chem. Phys. 131, 034112 (2009); http://dx.doi.org/10.1063/1.3179685 (8 pages)

Online Publication Date: 20 July 2009

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We present a new adaptation of umbrella sampling technique in studying nucleation, allowing for a large free energy barrier to be overcome within a time scale accessible to simulation. The method requires a set of order parameters that can distinguish a metastable state and a more stable state toward which the system relaxes, but does not demand that the order parameters be adequate for describing the process around the top of the free energy barrier. To illustrate its effectiveness, the method was implemented using Monte Carlo simulation and was applied to study bubble nucleation in single component truncated and shifted Lennard-Jones fluids. Under the conditions studied, the rate of nucleation, measured in Monte Carlo cycle, was enhanced by a factor ranging from 102 to 107. The simulation also yields various thermodynamic and structural information on the system during nucleation.
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64.60.qe General theory and computer simulations of nucleation
64.60.My Metastable phases
61.20.Ja Computer simulation of liquid structure
65.20.-w Thermal properties of liquids

Systematic truncation of the virtual space in multiconfigurational perturbation theory

Francesco Aquilante, Tanya Kumanova Todorova, Laura Gagliardi, Thomas Bondo Pedersen, and Björn Olof Roos

J. Chem. Phys. 131, 034113 (2009); http://dx.doi.org/10.1063/1.3157463 (7 pages) | Cited 12 times

Online Publication Date: 20 July 2009

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A method is suggested which allows truncation of the virtual space in Cholesky decomposition-based multiconfigurational perturbation theory (CD-CASPT2) calculations with systematic improvability of the results. The method is based on a modified version of the frozen natural orbital (FNO) approach used in coupled cluster theory. The idea is to exploit the near-linear dependence among the eigenvectors of the virtual-virtual block of the second-order Møller–Plesset density matrix. It is shown that FNO-CASPT2 recovers more than 95% of the full CD-CASPT2 correlation energy while requiring only a fraction of the total virtual space, especially when large atomic orbital basis sets are in use. Tests on various properties commonly investigated with CASPT2 demonstrate the reliability of the approach and the associated reduction in computational cost and storage demand of the calculations.
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31.15.xp Perturbation theory
02.10.Yn Matrix theory
02.10.Ud Linear algebra

Probing hot spots on protein-protein interfaces with all-atom free-energy simulation

Irene Meliciani, Konstantin Klenin, Timo Strunk, Katja Schmitz, and Wolfgang Wenzel

J. Chem. Phys. 131, 034114 (2009); http://dx.doi.org/10.1063/1.3177008 (11 pages) | Cited 1 time

Online Publication Date: 20 July 2009

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Modulation of protein-protein interactions by competitive small-molecule binding emerges as a promising avenue for drug discovery. Hot spots, i.e., amino acids with important contributions to the overall interaction energy, provide useful targets within these interfaces. To avoid time-consuming mutagenesis experiments, computational alanine screening has been developed for the prediction of hot spots based on existing structural information. Here we use the all-atom free-energy force field PFF02 to identify important amino acid residues in the complexes of the chemokine interleukin-8 (CXCL8) and an N-terminal peptide of its cognate receptor CXCR1, and of ERBIN, a molecular marker of the basolateral membrane in epithelial cells, in complex with the ERBIN-binding domain of tyrosin kinase ERBB2. The results of our analysis agree with available experimental functional assays, indicating that this approach is suitable for computational alanine screening and may help to identify competitive peptides as starting points for the development of inhibitors of protein-protein interactions for pharmaceutically relevant targets.
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87.15.km Protein-protein interactions
87.15.B- Structure of biomolecules
87.15.Fh Bonding; mechanisms of bond breakage
87.16.D- Membranes, bilayers, and vesicles
87.14.ep Membrane proteins
87.14.ej Enzymes

Vibrational coupled cluster theory with full two-mode and approximate three-mode couplings: The VCC[2pt3] model

Peter Seidler, Eduard Matito, and Ove Christiansen

J. Chem. Phys. 131, 034115 (2009); http://dx.doi.org/10.1063/1.3158946 (12 pages) | Cited 7 times

Online Publication Date: 21 July 2009

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Vibrational coupled cluster (VCC) calculations of molecular vibrational energy levels can be characterized by the number of modes coupled in the Hamiltonian operator and the number of modes simultaneously excited in the parameter space. We propose a VCC model which includes all two-mode couplings in the Hamiltonian and excitation space but only an approximate treatment of three-mode couplings. The approximation is based on a perturbational analysis and the introduced concepts can also be used for even more accurate treatments. The method is iterative and allows the use of VCC response theory to obtain excitation energies. Furthermore, the method is shown to scale with the number of vibrational modes to the third power which is no higher than the corresponding VCC model with only two-mode couplings. Encouraging benchmark calculations are given for a test set of three- and four-atomic molecules. The fundamentals of the larger ethylene oxide molecule have been calculated as well using a grid-based potential energy surface obtained from electronic coupled cluster theory with singles, doubles, and perturbative triples (CCSD(T)).
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33.20.Tp Vibrational analysis
31.15.bw Coupled-cluster theory
31.50.-x Potential energy surfaces

Treatment of nonlinear optical properties due to large amplitude anharmonic vibrational motions: Umbrella motion in NH3

Josep M. Luis, Heribert Reis, Manthos Papadopoulos, and Bernard Kirtman

J. Chem. Phys. 131, 034116 (2009); http://dx.doi.org/10.1063/1.3171615 (9 pages) | Cited 8 times

Online Publication Date: 21 July 2009

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A general reduced dimensionality finite field nuclear relaxation method for calculating vibrational nonlinear optical properties of molecules with large contributions due to anharmonic motions is introduced. In an initial application to the umbrella (inversion) motion of NH3 it is found that difficulties associated with a conventional single well treatment are overcome and that the particular definition of the inversion coordinate is not important. Future applications are described.
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33.20.Tp Vibrational analysis
42.65.-k Nonlinear optics
33.15.Mt Rotation, vibration, and vibration-rotation constants
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Energy and density analyses of the H2 molecule from the united atom to dissociation: The 1∑g+ states

Giorgina Corongiu and Enrico Clementi

J. Chem. Phys. 131, 034301 (2009); http://dx.doi.org/10.1063/1.3168506 (16 pages) | Cited 5 times

Online Publication Date: 15 July 2009

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The first 15 1∑g+ states of the H2 molecule are computed with full configuration interaction (CI) both from Hartree–Fock molecular orbitals and Heitler–London atomic orbitals; the computations are correlated with a comprehensive analysis. The basis sets utilized are extended and optimized Slater-type functions [Slater-type orbital (STO)] and spherical Gaussian functions [Gaussian-type orbital (GTO)]. The full CI computations cover the internuclear distances from 0.01 to 10 000 bohr. The available accurate data by Wolniewicz and co-workers for the first five excited states verify the quality of our computations. We focus on the characterization of the orbitals in the wave functions, on the electronic density evolution from the united atom to dissociation, on quantitative decomposition of the total energy into covalent and ionic components, and on detailed analyses of energy contributions to the total state energy from selected STO and GTO subsets. These analyses lead to study (with full CI) the H− negative ion with a proton and the H+H− ion pair systems. The ground and excited states for the He and H atoms and for the H− ion are computed to discuss the united atom and the dissociation products H(1s)+H(nl) of the n state manifolds. With the exception of n = 1, each manifold has one state, specifically the EF, H, 7, and 11, whose second minimum has strong ionic character; state 11 dissociates as H+H−.
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31.15.vj Electron correlation calculations for atoms and ions: excited states
31.15.xr Self-consistent-field methods
31.15.ve Electron correlation calculations for atoms and ions: ground state

Electric and magnetic properties of the four most stable CHNO isomers from ab initio CCSD(T) studies

Mirjana Mladenović, Mohamed Elhiyani, and Marius Lewerenz

J. Chem. Phys. 131, 034302 (2009); http://dx.doi.org/10.1063/1.3173275 (14 pages) | Cited 3 times

Online Publication Date: 15 July 2009

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Electric and magnetic properties obtained from CCSD(T)/(aug-)cc-pCVXZ (X = T, Q, or 5) electronic structure calculations are reported for isocyanic acid, HNCO, cyanic acid, HOCN, fulminic acid, HCNO, and isofulminic acid, HONC, in their ground electronic states. Comparison of the theoretical results with the available experimentally derived values shows very satisfactory agreement. The new data should be helpful for the identification of these molecules due to characteristic hyperfine structure patterns in their microwave spectra. A brief discussion of the electronic structure properties, based on the electric field gradients, Mulliken population analysis of the total electron density, and molecular orbitals, is provided for the four CHNO isomers and the related HCN/HNC system.
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31.15.bw Coupled-cluster theory
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
33.20.Bx Radio-frequency and microwave spectra
33.15.Pw Fine and hyperfine structure
31.15.aj Relativistic corrections, spin-orbit effects, fine structure; hyperfine structure

Complex multireference configuration interaction calculations employing a coupled diabatic representation for the 2Πg resonance states of N2−

Michael Honigmann, Robert J. Buenker, and Heinz-Peter Liebermann

J. Chem. Phys. 131, 034303 (2009); http://dx.doi.org/10.1063/1.3173277 (8 pages) | Cited 3 times

Online Publication Date: 15 July 2009

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Complex multireference configuration interaction calculations have been carried out for the lowest resonance states of 2Πg symmetry of the N2− molecule. It is shown that there is a forbidden crossing between the two lowest roots of this symmetry and that a satisfactory calculation of vibrational levels and cross sections therefore requires inclusion of both states and the coupling between them. A diabatic representation for the two 2Πg states was determined and vibronic calculations of the cross sections for vibrational excitation were carried out with a two-dimensional complex variational program.
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31.15.am Relativistic configuration interaction (CI) and many-body perturbation calculations
34.50.Ez Rotational and vibrational energy transfer
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Tp Vibrational analysis
33.80.Be Level crossing and optical pumping
33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions
31.15.xt Variational techniques
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