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14 Jan 2012

Volume 136, Issue 2, Articles (02xxxx)

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J. Chem. Phys. 136, 024101 (2012); http://dx.doi.org/10.1063/1.3670417 (14 pages)

Mark A. Watson, Dmitrij Rappoport, Elizabeth M. Y. Lee, Roberto Olivares-Amaya, and Alán Aspuru-Guzik
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Communication: Beyond Boltzmann's H-theorem: Demonstration of the relaxation theorem for a non-monotonic approach to equilibrium

James C. Reid, Denis J. Evans, and Debra J. Searles

J. Chem. Phys. 136, 021101 (2012); http://dx.doi.org/10.1063/1.3675847 (4 pages) | Cited 1 time

Online Publication Date: 11 January 2012

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Relaxation of a system to equilibrium is as ubiquitous, essential, and as poorly quantified as any phenomena in physics. For over a century, the most precise description of relaxation has been Boltzmann's H-theorem, predicting that a uniform ideal gas will relax monotonically. Recently, the relaxation theorem has shown that the approach to equilibrium can be quantified in terms of the dissipation function first defined in the proof of the Evans-Searles fluctuation theorem. Here, we provide the first demonstration of the relaxation theorem through simulation of a simple fluid system that generates a non-monotonic relaxation to equilibrium.
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05.60.-k Transport processes
05.70.Ce Thermodynamic functions and equations of state
02.60.-x Numerical approximation and analysis
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Communication: High speed optical investigations of a character of boiling-up onset

A. L. Gurashkin, A. A. Starostin, G. V. Ermakov, and P. V. Skripov

J. Chem. Phys. 136, 021102 (2012); http://dx.doi.org/10.1063/1.3678831 (4 pages) | Cited 1 time

Online Publication Date: 13 January 2012

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In this communication, we discuss the phenomenon of attainable superheat of liquid and the peculiarities of its release by spontaneous boiling-up. We have combined the apparatus for superheating, namely, bubble chamber, with a high speed micro-optical method for detailed monitoring of the initial stage of boiling-up. In experiments on the isothermal pressure drop, it was found that the boiling-up onset of n-hexane is accompanied by characteristic step signal. The signal has proved to be typical of the heterogeneous character of boiling-up onset in a whole range of superheating degrees. The performance of the method for investigation of the refractive index and density for superheated liquids as functions of temperature and pressure has been revealed. The experimental error is estimated to be 0.1%.
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78.47.J- Ultrafast spectroscopy (<1 psec)
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
81.40.Gh Other heat and thermomechanical treatments
64.70.fh Boiling and bubble dynamics
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Electronic structure calculations in arbitrary electrostatic environments

Mark A. Watson, Dmitrij Rappoport, Elizabeth M. Y. Lee, Roberto Olivares-Amaya, and Alán Aspuru-Guzik

J. Chem. Phys. 136, 024101 (2012); http://dx.doi.org/10.1063/1.3670417 (14 pages) | Cited 4 times

Online Publication Date: 9 January 2012

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Modeling of electronic structure of molecules in electrostatic environments is of considerable relevance for surface-enhanced spectroscopy and molecular electronics. We have developed and implemented a novel approach to the molecular electronic structure in arbitrary electrostatic environments that is compatible with standard quantum chemical methods and can be applied to medium-sized and large molecules. The scheme denoted CheESE (chemistry in electrostatic environments) is based on the description of molecular electronic structure subject to a boundary condition on the system/environment interface. Thus, it is particularly suited to study molecules on metallic surfaces. The proposed model is capable of describing both electrostatic effects near nanostructured metallic surfaces and image-charge effects. We present an implementation of the CheESE model as a library module and show example applications to neutral and negatively charged molecules.
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31.15.E- Density-functional theory

Coarse-grained potential models for structural prediction of carbon dioxide (CO2) in confined environments

T. Sanghi and N. R. Aluru

J. Chem. Phys. 136, 024102 (2012); http://dx.doi.org/10.1063/1.3674979 (13 pages) | Cited 3 times

Online Publication Date: 9 January 2012

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In this paper, we propose coarse-grained single-site (CGSS), wall-CO2, and CO2-CO2 interaction potential models to study the structure of carbon dioxide under confinement. The CGSS potentials are used in an empirical potential based quasi-continuum theory, EQT, to compute the center-of-mass density and potential profiles of CO2 confined inside different size graphite slit pores. Results obtained from EQT are compared with those obtained from all-atom molecular dynamics (AA-MD) simulations, and are found to be in good agreement with each other. Though these CGSS interaction potentials are primarily developed and parameterized for EQT, they are also used to perform coarse-grained molecular dynamics (CG-MD) simulations. The results obtained from CG-MD simulations are also found to be in reasonable agreement with AA-MD simulation results.
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34.20.Gj Intermolecular and atom-molecule potentials and forces
33.15.Bh General molecular conformation and symmetry; stereochemistry

An energy decomposition analysis for intermolecular interactions from an absolutely localized molecular orbital reference at the coupled-cluster singles and doubles level

R. Julian Azar and Martin Head-Gordon

J. Chem. Phys. 136, 024103 (2012); http://dx.doi.org/10.1063/1.3674992 (8 pages) | Cited 5 times

Online Publication Date: 9 January 2012

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We propose a wave function-based method for the decomposition of intermolecular interaction energies into chemically-intuitive components, isolating both mean-field- and explicit correlation-level contributions. We begin by solving the locally-projected self-consistent field for molecular interactions equations for a molecular complex, obtaining an intramolecularly polarized reference of self-consistently optimized, absolutely-localized molecular orbitals (ALMOs), determined with the constraint that each fragment MO be composed only of atomic basis functions belonging to its own fragment. As explicit inter-electronic correlation is integral to an accurate description of weak forces underlying intermolecular interaction potentials, namely, coordinated fluctuations in weakly interacting electronic densities, we add dynamical correlation to the ALMO polarized reference at the coupled-cluster singles and doubles level, accounting for explicit dispersion and charge-transfer effects, which map naturally onto the cluster operator. We demonstrate the stability of energy components with basis set extension, follow the hydrogen bond-breaking coordinate in the Cs-symmetry water dimer, decompose the interaction energies of dispersion-bound rare gas dimers and other van der Waals complexes, and examine charge transfer-dominated donor-acceptor interactions in borane adducts. We compare our results with high-level calculations and experiment when possible.
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31.15.bw Coupled-cluster theory
31.15.xr Self-consistent-field methods
34.20.Cf Interatomic potentials and forces
82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
34.70.+e Charge transfer
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)

Calculation of the surface tension and pressure components from a non-exponential perturbation method of the thermodynamic route

A. Ghoufi and P. Malfreyt

J. Chem. Phys. 136, 024104 (2012); http://dx.doi.org/10.1063/1.3676056 (6 pages) | Cited 6 times

Online Publication Date: 10 January 2012

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Surface tension is probably the most important interfacial property and a large number of techniques have been devoted to its calculation. Usually, this calculation is carried out using mechanical or thermodynamic definitions. The mechanical route uses an arbitrary choice to affect the contribution of the pairwise force. To overcome this arbitrariness, a thermodynamic route based on the area perturbation (test-area (TA) method) has been developed for the calculation of surface tension. The volume perturbation (VP) method provides an original route to compute the components of the pressure tensor. These two routes are developed from the perturbation theory leading to working expressions using exponential averages of energy. The use of exponential averages makes the calculation strongly dependent on the occurrence of low values of ΔU. Additionally, the decomposition of the energy to obtain local surface tension is nontrivial. From the explicit derivation of the partition function the exponential average is avoided providing an interesting alternative to TA, VP, and mechanical methods. To make a consistent comparison, we study the profiles of the surface tension along the direction normal to the surface for the different definitions and techniques in the cases of liquid-vapor interfaces of acids gases, binary, and apolar systems.
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68.03.Cd Surface tension and related phenomena

A spin-adapted size-extensive state-specific multi-reference perturbation theory. I. Formal developments

Shuneng Mao, Lan Cheng, Wenjian Liu, and Debashis Mukherjee

J. Chem. Phys. 136, 024105 (2012); http://dx.doi.org/10.1063/1.3672083 (14 pages) | Cited 8 times

Online Publication Date: 11 January 2012

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We present in this paper a comprehensive formulation of a spin-adapted size-extensive state-specific multi-reference second-order perturbation theory (SA-SSMRPT2) as a tool for applications to molecular states of arbitrary complexity and generality. The perturbative theory emerges in the development as a result of a physically appealing quasi-linearization of a rigorously size-extensive state-specific multi-reference coupled cluster (SSMRCC) formalism [U. S. Mahapatra, B. Datta, and D. Mukherjee, J. Chem. Phys. 110, 6171 (1999)10.1063/1.478523]. The formulation is intruder-free as long as the state-energy is energetically well-separated from the virtual functions. SA-SSMRPT2 works with a complete active space (CAS), and treats each of the model space functions on the same footing. This thus has the twin advantages of being capable of handling varying degrees of quasi-degeneracy and of ensuring size-extensivity. This strategy is attractive in terms of the applicability to bigger systems. A very desirable property of the parent SSMRCC theory is the explicit maintenance of size-extensivity under a variety of approximations of the working equations. We show how to generate both the Rayleigh-Schrödinger (RS) and the Brillouin-Wigner (BW) versions of SA-SSMRPT2. Unlike the traditional naive formulations, both the RS and the BW variants are manifestly size-extensive and both share the avoidance of intruders in the same manner as the parent SSMRCC. We discuss the various features of the RS as well as the BW version using several partitioning strategies of the hamiltonian. Unlike the other CAS based MRPTs, the SA-SSMRPT2 is intrinsically flexible in the sense that it is constructed in a manner that it can relax the coefficients of the reference function, or keep the coefficients frozen if we so desire. We delineate the issues pertaining to the spin-adaptation of the working equations of the SA-SSMRPT2, starting from SSMRCC, which would allow us to incorporate essentially any type open-shell configuration-state functions (CSF) within the CAS. The formalisms presented here will be applied extensively in a companion paper to assess their efficacy.
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31.15.bw Coupled-cluster theory
31.15.xp Perturbation theory

A spin-adapted size-extensive state-specific multi-reference perturbation theory with various partitioning schemes. II. Molecular applications

Shuneng Mao, Lan Cheng, Wenjian Liu, and Debashis Mukherjee

J. Chem. Phys. 136, 024106 (2012); http://dx.doi.org/10.1063/1.3672085 (15 pages) | Cited 11 times

Online Publication Date: 11 January 2012

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Following the theoretical development of a spin-adapted state-specific multi-reference second-order perturbation theory (SA-SSMRPT2) as expounded in the preceding publication, we discuss here its implementation and the results of its applications to potential energy curves (PECs) of various electronic states of small molecules. In particular, we illustrate its efficacy in states of various spin multiplicities and varying multi-reference character. Both Møller–Plesset (MP) and Epstein–Nesbet (EN) type of partitions have been explored. Also, a straightforward Rayleigh–Schrödinger (RS) and Brillouin–Wigner (BW) version of the SA-SSMRPT2 have been studied. Ground state PECs were computed for singlet states of HF, BH, and H2O molecules as well as the doublet state of NH2 and BeH radicals and compared to corresponding full configuration interaction numbers, which serve as benchmark results. As an extensive application on a production level, the ground state PECs of N2, a classic example of multiple-bond breaking, were calculated using cc-pVXZ (X = 3,4,5) basis and then extrapolated to obtain estimates of the complete basis set limit. Vibrational energy levels were extracted from these N2 PECs, which compare favorably to the experimental values. In addition, extensive studies were also carried out on PECs of the seven low-lying excited states of the N2 molecule. Finally, it is shown that the flexibility to relax configuration coefficients in SA-SSMRPT2 helps to provide good descriptions for the avoided crossing between the two lowest 1Σ states of the LiF molecule. Our results indicate (1) that more studies are needed to draw firm conclusions about the relative efficacies of the MP and EN results and (2) that the RS version works so well as compared to the BW version that the extra computational expenses needed in the later formalism is not warranted.
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31.15.xp Perturbation theory
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.50.Gh Surface crossings, non-adiabatic couplings
31.50.-x Potential energy surfaces
31.15.V- Electron correlation calculations for atoms, ions and molecules

Theoretical and numerical assessments of spin-flip time-dependent density functional theory

Zhendong Li and Wenjian Liu

J. Chem. Phys. 136, 024107 (2012); http://dx.doi.org/10.1063/1.3676736 (14 pages) | Cited 6 times

Online Publication Date: 12 January 2012

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Spin-flip time-dependent density functional theory (SF-TD-DFT) with the full noncollinear hybrid exchange-correlation kernel and its approximate variants are critically assessed, both formally and numerically. As demonstrated by the ethylene torsion and the C2v ring-opening of oxirane, SF-TD-DFT is very useful for describing nearly degenerate situations. However, it may occasionally yield unphysical results. This stems from the noncollinear form of the generalized gradient approximation, which becomes numerically instable in the presence of spin-flip excitations from the closed- to vacant-shell orbitals of an open-shell reference. To cure this defect, a simple modification, dubbed as ALDA0, is proposed in the spirit of adiabatic local density approximation (ALDA). It is applicable to all kinds of density functionals and yields stable results without too much loss of accuracy. In particular, the combination of ALDA0 with the Tamm-Dancoff approximation is a promising tool for studying global potential energy surfaces. In addition to the kernel problem, SF-TD-DFT is also rather sensitive to the choice of reference states, as demonstrated by the spin multiplet states of closed-shell molecules of H2O, CH2O, and C2H4. Surprisingly, SF-TD-DFT with pure density functionals may also fail for valance excitations with large orbital overlaps, at variance with the spin-conserving counterpart (SC-TD-DFT). In this case, the inclusion of a large amount of Hartree-Fock exchange is mandatory for quantitative results. Nonetheless, for spatially degenerate cases such as CF, CH, and NH+, SF-TD-DFT is more advantageous than SC-TD-DFT, unless the latter is also space adapted. These findings are very instructive for future development and applications of TD-DFT.
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31.15.ee Time-dependent density functional theory
31.15.eg Exchange-correlation functionals (in current density functional theory)
31.15.vq Electron correlation calculations for polyatomic molecules
31.15.xr Self-consistent-field methods
31.50.-x Potential energy surfaces

A new general Renner-Teller (including ɛ ≳ 1) spectroscopic formalism for triatomic molecules

Alexander O. Mitrushchenkov

J. Chem. Phys. 136, 024108 (2012); http://dx.doi.org/10.1063/1.3672162 (12 pages) | Cited 2 times

Online Publication Date: 12 January 2012

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We present a general approach and its implementation for calculating the rovibronic levels and spectra of rigid or floppy Renner-Teller triatomic molecules, which is not limited in the number and quantum numbers (Λ, S) of electronic states. The case of strong Renner-Teller coupling (ɛ ≳ 1) is specially studied and it is shown that the proposed implementation provides very accurate and stable results for this complicated problem. Few examples (BF2, HBF, and CCO) illustrate the performance of the code.
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33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions
33.20.Vq Vibration-rotation analysis
33.15.Bh General molecular conformation and symmetry; stereochemistry
31.15.X- Alternative approaches

Intermolecular torsional motion of a π-aggregated dimer probed by two-dimensional electronic spectroscopy

Joachim Seibt and Alexander Eisfeld

J. Chem. Phys. 136, 024109 (2012); http://dx.doi.org/10.1063/1.3674993 (15 pages) | Cited 3 times

Online Publication Date: 13 January 2012

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The energetic splitting of the two exciton states of a molecular dimer depends strongly on the relative orientation of the monomers with respect to each other. The curvature of the corresponding adiabatic potential energy surfaces can lead to torsional motion of the monomers. It has been suggested recently that this torsional motion could provide a possible relaxation mechanism for the upper state which proceeds via a crossing of the two singly excited state potentials. Another, competing, relaxation mechanism is provided by coupling to the environment, leading to direct exciton relaxation. Here we examine theoretically the combined dynamics of torsional motion and excitonic relaxation for a π-aggregated dimer. Using two-dimensional (2D) spectroscopy, it is shown how torsional motion through a crossing of the adiabatic excitonic potential surfaces could be distinguished from direct relaxation. For the calculations a mixed quantum/classical approach is used, where the torsional motion is treated by an Ehrenfest type of equation, while the excitonic dynamics including dephasing and direct relaxation is described by a quantum master equation.
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34.20.Gj Intermolecular and atom-molecule potentials and forces
32.10.Fn Fine and hyperfine structure
31.50.Df Potential energy surfaces for excited electronic states
82.56.Fk Multidimensional NMR

Correlation effects of π electrons on the band structures of conjugated polymers using the self-consistent GW approximation with vertex corrections

Yao-Wen Chang and Bih-Yaw Jin

J. Chem. Phys. 136, 024110 (2012); http://dx.doi.org/10.1063/1.3675640 (9 pages) | Cited 2 times

Online Publication Date: 13 January 2012

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Many-body perturbation theory is used to investigate the effect of π-electron correlations on the quasi-particle band structures of conjugated polymers at the level of the Pariser-Parr-Pople model. The self-consistent GW approximation with vertex corrections to both the self-energy and the polarization in Hedin's equations is employed in order to eliminate self-interaction errors and include the effects of electron-hole attraction in screening processes. The dynamic inverse dielectric function is constructed from the generalized plasmon-pole approximation with the static dressed polarization given by the coupled-perturbed Hartree-Fock equation. The bandgaps of trans-polyacetylene, trans-polyphenylenevinylene and poly(para)phenylene are calculated by both the Hartree-Fock and GW approximation, and a lowering of bandgaps due to electron correlations is found. We conclude that both dielectric screening and vertex corrections are important for calculating the quasi-particle bandgaps of conjugated polymers.
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71.20.Rv Polymers and organic compounds
71.45.Gm Exchange, correlation, dielectric and magnetic response functions, plasmons
71.15.Ap Basis sets (LCAO, plane-wave, APW, etc.) and related methodology (scattering methods, ASA, linearized methods, etc.)

Nonspherical model density matrices for Rung 3.5 density functionals

Benjamin G. Janesko and Austin Aguero

J. Chem. Phys. 136, 024111 (2012); http://dx.doi.org/10.1063/1.3675681 (8 pages) | Cited 5 times

Online Publication Date: 13 January 2012

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“Rung 3.5” exchange-correlation functionals for Kohn-Sham density functional theory depend linearly on the nonlocal one-particle density matrix of the noninteracting Kohn-Sham reference system. Rung 3.5 functionals also require a semilocal model for the one-particle density matrix. This work presents new model density matrices for Rung 3.5 functionals. The resulting functionals give reasonable predictions for total energies, molecular thermochemistry and kinetics, odd-electron bonds, and conjugated polymer bandgaps. Global-hybrid-like combinations of semilocal and Rung 3.5 exchange, and empirical density matrix models, also show promise.
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31.15.E- Density-functional theory
33.15.Fm Bond strengths, dissociation energies

Phonon-mediated path-interference in electronic energy transfer

Hoda Hossein-Nejad, Alexandra Olaya-Castro, and Gregory D. Scholes

J. Chem. Phys. 136, 024112 (2012); http://dx.doi.org/10.1063/1.3675844 (9 pages) | Cited 3 times

Online Publication Date: 13 January 2012

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We present a formalism to quantify the contribution of path-interference in phonon-mediated electronic energy transfer. The transfer rate between two molecules is computed by considering the quantum mechanical amplitudes associated with pathways connecting the initial and final sites. This includes contributions from classical pathways, but also terms arising from interference of different pathways. We treat the vibrational modes coupled to the molecules as a non-Markovian harmonic oscillator bath, and investigate the correction to transfer rates due to the lowest-order interference contribution. We show that depending on the structure of the harmonic bath, the correction due to path-interference may have a dominant vibrational or electronic character, and can make a notable contribution to the transfer rate in the steady state.
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33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Tp Vibrational analysis
36.20.Ng Vibrational and rotational structure, infrared and Raman spectra

A new fragment-based approach for calculating electronic excitation energies of large systems

Yingjin Ma, Yang Liu, and Haibo Ma

J. Chem. Phys. 136, 024113 (2012); http://dx.doi.org/10.1063/1.3675915 (11 pages) | Cited 2 times

Online Publication Date: 13 January 2012

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We present a new fragment-based scheme to calculate the excited states of large systems without necessity of a Hartree-Fock (HF) solution of the whole system. This method is based on the implementation of the renormalized excitonic method [M. A. Hajj et al., Phys. Rev. B 72, 224412 (2005)10.1103/PhysRevB.72.224412] at ab initio level, which assumes that the excitation of the whole system can be expressed by a linear combination of various local excitations. We decomposed the whole system into several blocks and then constructed the effective Hamiltonians for the intra- and inter-block interactions with block canonical molecular orbitals instead of widely used localized molecular orbitals. Accordingly, we avoided the prerequisite HF solution and the localization procedure of the molecular orbitals in the popular local correlation methods. Test calculations were implemented for hydrogen molecule chains at the full configuration interaction, symmetry adapted cluster/symmetry adapted cluster configuration interaction, HF/configuration interaction singles (CIS) levels and more realistic polyene systems at the HF/CIS level. The calculated vertical excitation energies for lowest excited states are in reasonable accordance with those determined by the calculations of the whole systems with traditional methods, showing that our new fragment-based method can give good estimates for low-lying energy spectra of both weak and moderate interaction systems with economic computational costs.
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31.15.xr Self-consistent-field methods
31.15.am Relativistic configuration interaction (CI) and many-body perturbation calculations
31.15.vn Electron correlation calculations for diatomic molecules

Visualizing the zero order basis of the spectroscopic Hamiltonian

George L. Barnes and Michael E. Kellman

J. Chem. Phys. 136, 024114 (2012); http://dx.doi.org/10.1063/1.3674994 (8 pages) | Cited 1 time

Online Publication Date: 13 January 2012

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Recent works have shown that a generalization of the spectroscopic effective Hamiltonian can describe spectra in surprising regions, such as isomerization barriers. In this work, we seek to explain why the effective Hamiltonian is successful where there was reason to doubt that it would work at all. All spectroscopic Hamiltonians have an underlying abstract zero-order basis (ZOB) which is the “ideal” basis for a given form and parameterization of the Hamiltonian. Without a physical model there is no way to transform this abstract basis into a coordinate representation. To this end, we present a method of obtaining the coordinate space representation of the abstract ZOB of a spectroscopic effective Hamiltonian. This method works equally well for generalized effective Hamiltonians that encompass above-barrier multiwell behavior, and standard effective Hamiltonians for the vicinity of a single potential minimum. Our approach relies on a set of converged eigenfunctions obtained from a variational calculation on a potential surface. By making a one-to-one correspondence between the energy eigenstates of the effective Hamiltonian and those of the coordinate space Hamiltonian, a physical representation of the abstract ZOB is calculated. We find that the ZOB basis naturally adjusts its complexity depending on the underlying nature of phase space, which allows spectroscopic Hamiltonians to succeed for systems sampling multiple stationary points.
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82.30.Qt Isomerization and rearrangement
82.20.Kh Potential energy surfaces for chemical reactions
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Cavity ring-down spectroscopy of the oxygen B-band with absolute frequency reference to the optical frequency comb

J. Domysławska, S. Wójtewicz, D. Lisak, A. Cygan, F. Ozimek, K. Stec, Cz. Radzewicz, R. S. Trawiński, and R. Ciuryło

J. Chem. Phys. 136, 024201 (2012); http://dx.doi.org/10.1063/1.3675903 (6 pages) | Cited 15 times

Online Publication Date: 10 January 2012

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Absolute positions of several oxygen B-band lines were measured with the Pound-Drever-Hall-locked frequency-stabilized cavity ring-down spectrometer. The frequency axis of spectra was linked to the optical frequency comb. Achieved uncertainties of line positions are between 0.9 and 2.9 MHz. Self-pressure shifts coefficients are also reported.
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32.30.Jc Visible and ultraviolet spectra
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Singly and doubly lithium doped silicon clusters: Geometrical and electronic structures and ionization energies

Nguyen Minh Tam, Vu Thi Ngan, Jorg de Haeck, Soumen Bhattacharyya, Hai Thuy Le, Ewald Janssens, Peter Lievens, and Minh Tho Nguyen

J. Chem. Phys. 136, 024301 (2012); http://dx.doi.org/10.1063/1.3672164 (11 pages) | Cited 3 times

Online Publication Date: 9 January 2012

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The geometric structures of neutral and cationic SinLim0/+ clusters with n = 2–11 and m = 1, 2 are investigated using combined experimental and computational methods. The adiabatic ionization energy and vertical ionization energy (VIE) of SinLim clusters are determined using quantum chemical methods (B3LYP/6-311+G(d), G3B3, and CCSD(T)/aug-cc-pVxZ with x = D,T), whereas experimental values are derived from threshold photoionization experiments in the 4.68–6.24 eV range. Among the investigated cluster sizes, only Si6Li2, Si7Li, Si10Li, and Si11Li have ionization thresholds below 6.24 eV and could be measured accurately. The ionization threshold and VIE obtained from the experimental photoionization efficiency curves agree well with the computed values. The growth mechanism of the lithium doped silicon clusters follows some simple rules: (1) neutral singly doped SinLi clusters favor the Li atom addition on an edge or a face of the structure of the corresponding Sin− anion, while the cationic SinLi+ binds with one Si atom of the bare Sin cluster or adds on one of its edges, and (2) for doubly doped SinLi20/+ clusters, the neutrals have the shape of the Sin+1 counterparts with an additional Li atom added on an edge or a face of it, while the cations have both Li atoms added on edges or faces of the Sin− clusters.
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36.40.-c Atomic and molecular clusters
31.15.am Relativistic configuration interaction (CI) and many-body perturbation calculations
31.15.bw Coupled-cluster theory
31.15.xp Perturbation theory
33.80.Eh Autoionization, photoionization, and photodetachment
34.50.Gb Electronic excitation and ionization of molecules

Accurate calculation of the intensity dependence of the refractive index using polarized basis sets

Angelika Baranowska-Łączkowska, Krzysztof Z. Łączkowski, and Berta Fernández

J. Chem. Phys. 136, 024302 (2012); http://dx.doi.org/10.1063/1.3674286 (12 pages) | Cited 4 times

Online Publication Date: 9 January 2012

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Using the single and double excitation coupled cluster level of theory (CCSD) and the density functional theory/Becke 3-parameter Lee-Yang and Parr (DFT/B3LYP) methods, we test the performance of the Pol, ZPol, and LPol-n (n = ds, dl, fs, fl) basis sets in the accurate description of the intensity dependence of the refractive index in the Ne atom, and the N2 and the CO molecules. Additionally, we test the aug-pc-n (n = 1, 2) basis sets of Jensen, and the SVPD, TZVPD, and QZVPD bases by Rappoport and Furche. Tests involve calculations of dynamic polarizabilities and frequency dependent second hyperpolarizabilities. The results are interpreted in terms of the medium constants entering the expressions for optically induced birefringences. In all achiral systems, the performance of the LPol-n sets is very good. Also the aug-pc-2 set yields promising results. Accurate CCSD results available in the literature allow us to select the best basis sets in order to carry out DFT/B3LYP calculations of medium constants in larger molecules. As applications, we show results for (R)-fluoro-oxirane and (R)-methyloxirane.
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33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
31.15.bw Coupled-cluster theory
31.15.E- Density-functional theory

Energy landscapes of ion clusters in isotropic quadrupolar and octupolar traps

F. Calvo, E. Yurtsever, and D. J. Wales

J. Chem. Phys. 136, 024303 (2012); http://dx.doi.org/10.1063/1.3673318 (8 pages) | Cited 1 time

Online Publication Date: 9 January 2012

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The energy landscapes of ion clouds confined in isotropic quadrupolar and octupolar traps are characterized for several representative cluster sizes. All clusters exhibit stable multishell structures that belong to separate funnels. Quadrupolar confinement leads to more homogeneous clusters and denser distributions of isomers than octupolar confinement. Statistical analysis of the transition states indicates that the barriers associated with intrashell motion are lower but more asymmetric and more cooperative compared to intershell motion. The relaxation between low-energy funnels with different arrangements of shells mostly exhibits Arrhenius kinetics, with a weak variation of the activation energy at higher temperatures.
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82.20.Db Transition state theory and statistical theories of rate constants
82.20.Pm Rate constants, reaction cross sections, and activation energies
37.10.Ty Ion trapping
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Experimental and theoretical investigation of the triple differential cross section for electron impact ionization of pyrimidine molecules

J. D. Builth-Williams, S. M. Bellm, D. B. Jones, Hari Chaluvadi, D. H. Madison, C. G. Ning, B. Lohmann, and M. J. Brunger

J. Chem. Phys. 136, 024304 (2012); http://dx.doi.org/10.1063/1.3675167 (6 pages) | Cited 14 times

Online Publication Date: 9 January 2012

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Cross-section data for electron impact induced ionization of bio-molecules are important for modelling the deposition of energy within a biological medium and for gaining knowledge of electron driven processes at the molecular level. Triply differential cross sections have been measured for the electron impact ionization of the outer valence 7b2 and 10a1 orbitals of pyrimidine, using the (e, 2e) technique. The measurements have been performed with coplanar asymmetric kinematics, at an incident electron energy of 250 eV and ejected electron energy of 20 eV, for scattered electron angles of −5°, −10°, and −15°. The ejected electron angular range encompasses both the binary and recoil peaks in the triple differential cross section. Corresponding theoretical calculations have been performed using the molecular 3-body distorted wave model and are in reasonably good agreement with the present experiment.
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34.80.Gs Molecular excitation and ionization
87.15.-v Biomolecules: structure and physical properties
36.20.-r Macromolecules and polymer molecules

Electronic structure and bonding of HBeLi, HmgLi, and HCaLi in their bent equilibrium geometries

Fabio E. Penotti

J. Chem. Phys. 136, 024305 (2012); http://dx.doi.org/10.1063/1.3673539 (14 pages)

Online Publication Date: 9 January 2012

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Compact, yet accurate, non-orthogonal multi-configuration wavefunctions have been computed for HBeLi, HMgLi, and HCaLi in their respective nonlinear equilibrium geometries. They appear to be dominated by just two configurations, “orbitally relaxed” versions of the single-configuration spin-coupled and generalized valence bond (GVB) wavefunctions, respectively, with a smaller contribution from a self-consistent field (SCF)-like configuration. Double excitations out of the main configurations, while required for quantitative accuracy, enter the wavefunction with such small weights that they do not alter the qualitative picture that emerges from the orbital structure of the two main configurations. For comparison, calculations have also been carried out with two orthogonality-free configurations as reference, and no GVB-like or SCF-like configuration. Atoms-in-molecules (AIM) topological analyses of the overall electron densities, and considerations of local energetics in the differential neighbourhood of the equilibrium geometries, have been used to provide independent assessments of the nature of bonding in these molecules. Orbital structure and AIM results together suggest the existence of three-centre two-electron M–H–M′ bonds through hydrogen in all three molecules. Orbital pictures suggest these bonds are at least partially covalent, while a strict interpretation of values of the electron density Laplacian at AIM bond critical points would imply closed-shell interactions. Also for all three molecules, the orbital structures of the two main configurations suggest the presence of a one-electron two-centre bond between Li and the alkaline-earth atom. This bond may provide at least a partial explanation for the relative shortness of the inter-metallic distances, but is apparently too spread out to show up in AIM analyses of the total electron density. Considerations of local energetics support the more nuanced description of bonding that emerges, for these three molecules, from their orbital structure.
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31.15.xr Self-consistent-field methods
31.50.Df Potential energy surfaces for excited electronic states
33.15.Bh General molecular conformation and symmetry; stereochemistry

Nuclear motion captured by the slow electron velocity imaging technique in the tunnelling predissociation of the S1 methylamine

Doo-Sik Ahn, Jeongmook Lee, Young Choon Park, Yoon Sup Lee, and Sang Kyu Kim

J. Chem. Phys. 136, 024306 (2012); http://dx.doi.org/10.1063/1.3675566 (6 pages) | Cited 1 time

Online Publication Date: 9 January 2012

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Predissociation dynamics of methylamines (CH3NH2 and CH3ND2) on the first electronically excited states are studied using the slow-electron velocity imaging method to unravel the multi-dimensional nature of the N–H(D) chemical bond dissociation reaction which occurs via tunnelling. The nearly free internal rotation around the C–N bond axis is found to be strongly coupled to the reaction pathway, revealing nuclear motions actively involved in the tunnelling process on the S1 potential energy surfaces. The vibrational state-resolved energy and angular distributions of photoelectron, ejected from the ionization mediated by the metastable intermediate S1 state provide a unique way for mapping the predissociative potential energy surfaces.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
33.60.+q Photoelectron spectra
31.30.Gs Hyperfine interactions and isotope effects
31.50.Df Potential energy surfaces for excited electronic states
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
33.15.Mt Rotation, vibration, and vibration-rotation constants

Competitive partitioning of rotational energy in gas ensemble equilibration

Anthony J. McCaffery and Richard J. Marsh

J. Chem. Phys. 136, 024307 (2012); http://dx.doi.org/10.1063/1.3675638 (9 pages) | Cited 1 time

Online Publication Date: 9 January 2012

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A wide-ranging computational study of equilibration in binary mixtures of diatomic gases reveals the existence of competition between the constituent species for the orbital angular momentum and energy available on collision with the bath gas. The ensembles consist of a bath gas AB(v;j), and a highly excited minor component CD(v′;j′), present in the ratio AB:CD = 10:1. Each ensemble contains 8000 molecules. Rotational temperatures (Tr) are found to differ widely at equilibration with TrAB/TrCD varying from 2.74 to 0.92, indicating unequal partitioning of rotational energy and angular momentum between the two species. Unusually, low values of Tr are found generally to be associated with diatomics of low reduced mass. To test effects of the equi-partition theorem on low Tr we undertook calculations on HF(6;4) in N2(0;10) over the range 100–2000 K. No significant change in TrN2/TrHF was found. Two potential sources of rotational inequality are examined in detail. The first is possible asymmetry of −Δj and +Δj probabilities for molecules in mid- to high j states resulting from the quadratic dependence of rotational energy on j. The second is the efficiency of conversion of orbital angular momentum, generated on collision with bath gas molecules, into molecular rotation. Comparison of these two possible effects with computed TrAB/TrCD shows the efficiency factor to be an excellent predictor of partitioning between the two species. Our finding that Tr values for molecules such as HF and OH are considerably lower than other modal temperatures suggests that the determination of gas ensemble temperatures from Boltzmann fits to rotational distributions of diatomics of low reduced mass may require a degree of caution.
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33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Sn Rotational analysis

Comparison of two-body and three-body decomposition of ethanedial, propanal, propenal, n-butane, 1-butene, and 1,3-butadiene

Chih-Hao Chin and Shih-Huang Lee

J. Chem. Phys. 136, 024308 (2012); http://dx.doi.org/10.1063/1.3675682 (11 pages) | Cited 1 time

Online Publication Date: 10 January 2012

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We investigated two-body (binary) and three-body (triple) dissociations of ethanedial, propanal, propenal, n-butane, 1-butene, and 1,3-butadiene on the ground potential-energy surfaces using quantum-chemical and Rice-Ramsperger-Kassel-Marcus calculations; most attention is paid on the triple dissociation mechanisms. The triple dissociation includes elimination of a hydrogen molecule from a combination of two separate terminal hydrogen atoms; meanwhile, the rest part simultaneously decomposes to two stable fragments, e.g., C2H4, C2H2, or CO. Transition structures corresponding to the concerted triple dissociation were identified using the B3LYP/6-311G(d,p) level of theory and total energies were computed using the method CCSD(T)/6-311+G(3df, 2p). The forward barrier height of triple dissociation has a trend of ethanedial < propanal < propenal < n-butane < 1-butene < 1,3-butadiene, pertaining to the reaction enthalpy. Ratios of translational energies of three separate fragments could be estimated from the transition structure of triple dissociation. The synchronous concerted dissociation of propanal, propenal, and 1-butene leading to three different types of molecular fragments by breaking nonequivalent chemical bonds is rare. The triple dissociation of propanal, n-butane, 1-butene, and 1,3-butadiene were investigated for the first time. To outline a whole picture of dissociation mechanisms, some significant two-body dissociation channels were investigated for the calculations of product branching ratios. The triple dissociation plays an important role in the three carbonyl compounds, but plays a minor or negligible role in the three hydrocarbons.
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82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
31.15.bw Coupled-cluster theory
33.15.Bh General molecular conformation and symmetry; stereochemistry
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