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21 Aug 2007

Volume 127, Issue 7, Articles (07xxxx)

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back to top Theoretical Methods and Algorithms

Gauge-origin independent calculation of magnetizabilities and rotational g tensors at the coupled-cluster level

Jürgen Gauss, Kenneth Ruud, and Mihály Kállay

J. Chem. Phys. 127, 074101 (2007); http://dx.doi.org/10.1063/1.2755664 (13 pages) | Cited 17 times

Online Publication Date: 15 August 2007

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An implementation of the gauge-origin independent calculation of magnetizabilities and rotational g tensors at the coupled-cluster (CC) level is presented. The properties of interest are obtained as second derivatives of the energy with respect to the external magnetic field (in the case of the magnetizability) or with respect to magnetic field and rotational angular momentum (in the case of the rotational g tensor), while gauge-origin independence and fast basis-set convergence are ensured by using gauge-including atomic orbitals (London atomic orbitals) as well as their extension to treat rotational perturbations (rotational London atomic orbitals). The implementation within our existing CC analytic second-derivative code is described, focusing on the required modifications concerning integral evaluation and treatment of the unperturbed and perturbed two-particle density matrices. An extensive set of test calculations for LiH and BH (up to the full configuration-interaction limit), for a series of simple hydrides (HF, H2O, NH3, and CH4) as well as the more challenging molecules CO, N2, and O3 [employing the CC singles and doubles (CCSD) and the CCSD approximation augmented by a perturbative treatment of triple excitations] demonstrates the importance of electron correlation for high-accuracy predictions of magnetizabilities and rotational g tensors.
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33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Sn Rotational analysis
31.15.bw Coupled-cluster theory
31.15.ve Electron correlation calculations for atoms and ions: ground state

Automatically generated Coulomb fitting basis sets: Design and accuracy for systems containing H to Kr

Rui Yang, Alistair P. Rendell, and Michael J. Frisch

J. Chem. Phys. 127, 074102 (2007); http://dx.doi.org/10.1063/1.2752807 (14 pages) | Cited 9 times

Online Publication Date: 15 August 2007

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For intermediate sized chemical systems the use of an auxiliary basis set (ABS) to fit the charge density provides a useful means of accelerating the performance of various quantum chemical methods. As a consequence much effort has been devoted to the design of various ABSs. This paper explores a fundamentally new approach where the ABS is created dynamically based on the specific orbital basis set (OBS) being used. The new approach includes a parameter that is used to coalesce candidate fitting functions together but which can also be used to provide some coarse grain control over the number of functions in the ABS. The accuracy of the new automatically generated ABS (auto-ABS) is systemically studied for a variety of small systems containing the elements H–Kr. Errors in the Coulomb energy computed using auto-ABS and with a variety of OBSs are shown to be small compared to errors in the Hartree-Fock energy due to incompleteness in the OBS. In contrast to fixed size ABSs, the use of auto-ABS is shown to lead to smaller errors as the size (quality) of the OBS is expanded. The performance of auto-ABS is also compared with the use of the recently proposed universal fitting sets [ Weigend, Phys. Chem. Chem. Phys. 8, 1057 (2006) ] for 180 compounds containing atoms from H to Kr.
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31.15.xr Self-consistent-field methods
31.15.E- Density-functional theory

Alternative linear-scaling methodology for the second-order Møller-Plesset perturbation calculation based on the divide-and-conquer method

Masato Kobayashi, Yutaka Imamura, and Hiromi Nakai

J. Chem. Phys. 127, 074103 (2007); http://dx.doi.org/10.1063/1.2761878 (8 pages) | Cited 40 times

Online Publication Date: 17 August 2007

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A new scheme for obtaining the approximate correlation energy in the divide-and-conquer (DC) method of Yang [Phys. Rev. Lett. 66, 1438 (1991) ] is presented. In this method, the correlation energy of the total system is evaluated by summing up subsystem contributions, which are calculated from subsystem orbitals based on a scheme for partitioning the correlation energy. We applied this method to the second-order Møller-Plesset perturbation theory (MP2), which we call DC-MP2. Numerical assessment revealed that this scheme provides a reliable correlation energy with significantly less computational cost than the conventional MP2 calculation.
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31.15.xp Perturbation theory
31.15.V- Electron correlation calculations for atoms, ions and molecules

Static and dynamic second hyperpolarizability calculated by time-dependent density functional cubic response theory with local contribution and natural bond orbital analysis

Aijun Ye, Serguei Patchkovskii, and Jochen Autschbach

J. Chem. Phys. 127, 074104 (2007); http://dx.doi.org/10.1063/1.2749505 (13 pages) | Cited 9 times

Online Publication Date: 17 August 2007

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The static and dynamic second hyperpolarizability γ has been investigated by time-dependent density functional cubic response theory. The third-order coupled perturbed Kohn-Sham equations were solved to obtain the third-order perturbed charge density. Calculations on a number of small molecules (N2, CO2, C2H4, CO, HF, H2O, and CH4), paradisubstituted oligoacetylene chains, benzene, and eight paradisubstituted benzenes were performed to verify the implementation and to assess the accuracy of the nonhybrid and hybrid time-dependent density functional theory computations. Nitroaniline and a derivative were taken as examples to investigate the distribution of the “γ density” and to demonstrate the feasibility of analyzing cubic response functions in terms of contributions from natural bond orbitals (NBOs) and natural localized molecular orbitals (NLMOs). The results highlight the contributions from atoms and bonds on different functional groups to the total value of γ based on the NBO/NLMO analysis, which might be helpful for new nonlinear optical materials design.
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31.15.E- Density-functional theory
42.65.An Optical susceptibility, hyperpolarizability
42.70.Nq Other nonlinear optical materials; photorefractive and semiconductor materials

Analytic high-order Douglas-Kroll-Hess electric field gradients

Remigius Mastalerz, Giampaolo Barone, Roland Lindh, and Markus Reiher

J. Chem. Phys. 127, 074105 (2007); http://dx.doi.org/10.1063/1.2761880 (12 pages) | Cited 12 times

Online Publication Date: 20 August 2007

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In this work we present a comprehensive study of analytical electric field gradients in hydrogen halides calculated within the high-order Douglas-Kroll-Hess (DKH) scalar-relativistic approach taking picture-change effects analytically into account. We demonstrate the technical feasibility and reliability of a high-order DKH unitary transformation for the property integrals. The convergence behavior of the DKH property expansion is discussed close to the basis set limit and conditions ensuring picture-change-corrected results are determined. Numerical results are presented, which show that the DKH property expansion converges rapidly toward the reference values provided by four-component methods. This shows that in closed-shell cases, the scalar-relativistic DKH(2,2) approach which is of second order in the external potential for both orbitals and property operator yields a remarkable accuracy. As a parameter-dependence-free high-order DKH model, we recommend DKH(4,3). Moreover, the effect of a finite-nucleus model, different parametrization schemes for the unitary matrices, and the reliability of standard basis sets are investigated.
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31.30.J- Relativistic and quantum electrodynamic (QED) effects in atoms, molecules, and ions
31.15.-p Calculations and mathematical techniques in atomic and molecular physics

General biorthogonal projected bases as applied to second-order Møller-Plesset perturbation theory

Ville Weijo, Pekka Manninen, Poul Jørgensen, Ove Christiansen, and Jeppe Olsen

J. Chem. Phys. 127, 074106 (2007); http://dx.doi.org/10.1063/1.2752813 (14 pages) | Cited 13 times

Online Publication Date: 20 August 2007

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With low-order scaling correlated wave function theories in mind, we present second quantization formalism as well as biorthonormalization procedures for general—singular or nonsingular—bases. Of particular interest are the so-called projected atomic orbital bases, which are obtained from a set of atom-centered functions and feature a separation of occupied and virtual spaces. We demonstrate the formalism by deriving and implementing second-order Møller-Plesset perturbation theory in it, and discuss the convergence and preconditioning of the iterative amplitude equations in detail.
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31.15.xp Perturbation theory

Quadrupole, octopole, and hexadecapole electric moments of Σ, Π, Δ, and Φ electronic states: Cylindrically asymmetric charge density distributions in linear molecules with nonzero electronic angular momentum

Pablo J. Bruna and Friedrich Grein

J. Chem. Phys. 127, 074107 (2007); http://dx.doi.org/10.1063/1.2755691 (12 pages) | Cited 1 time

Online Publication Date: 21 August 2007

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The number of independent components, n, of traceless electric 2l-multipole moments is determined for C∞v molecules in Σ±, Π, Δ, and Φ electronic states (Λ = 0,1,2,3). Each 2l pole is defined by a rank-l irreducible tensor with (2l+1) components Pm(l) proportional to the solid spherical harmonic rlYml(θ,φ). Here we focus our attention on 2l poles with l = 2,3,4 (quadrupole Θ, octopole Ω, and hexadecapole Φ). An important conclusion of this study is that n can be 1 or 2 depending on both the multipole rank l and state quantum number Λ. For Σ±(Λ = 0) states, all 2l poles have one independent parameter (n = 1). For spatially degenerate states—Π, Δ, and Φ (Λ = 1,2,3)—the general rule reads n = 1 for l<2∣Λ∣ (when the 2l-pole rank lies below 2∣Λ∣) but n = 2 for higher 2l poles with l ≥ 2∣Λ∣. The second nonzero term is the off-diagonal matrix element 〈ψ+Λ∣P∣m∣ = 2Λ(l)∣ψ−Λ〉. Thus, a Π(Λ = 1) state has one dipole (μz) but two independent 2l poles for l ≥ 2—starting with the quadrupole [Θzz,(Θxx−Θyy)]. A Δ(Λ = 2) state has n = 1 for 2(1,2,3) poles (μz,Θzz,Ωzzz) but n = 2 for higher 2(l ≥ 4) poles—from the hexadecapole Φ up. For Φ(Λ = 3) states, it holds that n = 1 for 21 to 25 poles but n = 2 for all 2(l ≥ 6) poles. In short, what is usually stated in the literature—that n = 1 for all possible 2l poles of linear molecules—only applies to Σ± states. For degenerate states with n = 2, all Cartesian 2l-pole components (l ≥ 2∣Λ∣) can be expressed as linear combinations of two irreducible multipoles, Pm = 0(l) and P∣m∣ = 2Λ(l) [parallel (z axis) and anisotropy (xy plane)]. Our predictions are exemplified by the Θ, Ω, and Φ moments calculated for Λ = 0−3 states of selected diatomics (in parentheses): X math(CN), X math(NO), a math(C2), X math(NiH), X math(TiO), X math(CoF), and X math(TiF). States of Π symmetry are most affected by the deviation from axial symmetry.
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31.15.-p Calculations and mathematical techniques in atomic and molecular physics
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
33.15.Bh General molecular conformation and symmetry; stereochemistry
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Submillimeter-wave spectroscopy of DCO+ in the excited vibrational states: Does the Stark effect cause anomalies in the (0220) state?

T. Hirao, Shanshan Yu, and T. Amano

J. Chem. Phys. 127, 074301 (2007); http://dx.doi.org/10.1063/1.2766950 (12 pages) | Cited 4 times

Online Publication Date: 17 August 2007

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The lowest two rotational transitions of (0220) were not detected in previous investigations. This nonobservation was ascribed to the Stark broadening caused by the electric field in a hollow cathode discharge and an extended negative glow discharge. However, rotational lines of symmetric-top ions such as CH3CNH+ and SD3+ were observed in extended negative glow discharges with no such Stark effect. Also, no anomalies were observed for similar lines for HCN and HNC produced in an extended negative glow discharge. In the present investigation, we extended the measurements of DCO+ up to 800 GHz. The DCO+ ions were produced in an extended negative glow discharge in a gas mixture of D2 and CO (a couple of millitorr each) in Ar buffer ( ∼ 12 mTorr). The measurements were made mostly at liquid nitrogen temperature. Our observations confirmed that the lowest rotational lines in (0220) within our frequency coverage, J = 4-3, were too weak to be detected. However, a most notable result obtained in the present investigation is that the J = 5-4 and J = 6-5 lines of (0220) and the J = 5-4 line of (0420) have been detected in induced emission. This observation implies that the previous nonobservation of low-J lines in (0220) may not be due to the Stark effect. The l-type splitting in (0330) has been observed for the J = 9-8 transition and higher. However, the splittings for the J = 7-6 and J = 8-7 lines that are expected to be large enough have not been resolved. The reason for this “narrowing” has been unexplained at the present stage. The population inversion suggests that, initially, DCO+ is formed predominantly in stretching vibrational states, and, subsequently, the energy transfer to bending vibrational states takes place through collisional relaxation processes.

Interactions and dynamics in Li+Li2 ultracold collisions

Marko T. Cvitaš, Pavel Soldán, Jeremy M. Hutson, Pascal Honvault, and Jean-Michel Launay

J. Chem. Phys. 127, 074302 (2007); http://dx.doi.org/10.1063/1.2752162 (19 pages) | Cited 24 times

Online Publication Date: 20 August 2007

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A potential energy surface for the lowest quartet electronic state (4A′) of lithium trimer is developed and used to study spin-polarized Li+Li2 collisions at ultralow kinetic energies. The potential energy surface allows barrierless atom exchange reactions. Elastic and inelastic cross sections are calculated for collisions involving a variety of rovibrational states of Li2. Inelastic collisions are responsible for trap loss in molecule production experiments. Isotope effects and the sensitivity of the results to details of the potential energy surface are investigated. It is found that for vibrationally excited states, the cross sections are only quite weakly dependent on details of the potential energy surface.
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34.50.-s Scattering of atoms and molecules
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Kh Potential energy surfaces for chemical reactions
82.20.Tr Kinetic isotope effects including muonium
82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)

The water-oxygen dimer: First-principles calculation of an extrapolated potential energy surface and second virial coefficients

Richard J. Wheatley and Allan H. Harvey

J. Chem. Phys. 127, 074303 (2007); http://dx.doi.org/10.1063/1.2756524 (8 pages) | Cited 6 times

Online Publication Date: 20 August 2007

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The systematic intermolecular potential extrapolation routine (SIMPER) is applied to the water-oxygen complex to obtain a five-dimensional potential energy surface. This is the first application of SIMPER to open-shell molecules, and it is the first use, in this context, of asymptotic dispersion energy coefficients calculated using the unrestricted time-dependent coupled-cluster method. The potential energy surface is extrapolated to the complete basis set limit, fitted as a function of intermolecular geometry, and used to calculate (mixed) second virial coefficients, which significantly extend the range of the available experimental data.
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31.50.Bc Potential energy surfaces for ground electronic states
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions
82.20.Kh Potential energy surfaces for chemical reactions

1Π←X 1Σ+ band systems of jet-cooled ScCo and YCo

Ramya Nagarajan and Michael D. Morse

J. Chem. Phys. 127, 074304 (2007); http://dx.doi.org/10.1063/1.2756533 (8 pages)

Online Publication Date: 20 August 2007

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Rotationally resolved resonant two-photon ionization (R2PI) spectra of ScCo and YCo are reported. The measured spectra reveal that these molecules possess ground electronic states of 1Σ+ symmetry, as previously found in the isoelectronic Cr2 and CrMo molecules. The ground state rotational constants for ScCo and YCo are B0″ = 0.201 31(22) cm−1 and B0″ = 0.120 96(10) cm−1, corresponding to ground state bond lengths of r0″ = 1.812 1(10) Å and r0″ = 1.983 0(8) Å, respectively. A single electronic band system, assigned as a 1Π←X 1Σ+ transition, has been identified in both molecules. In ScCo, the 1Π state is characterized by T0 = 15 428.8, ωe′ = 246.7, and ωe′xe′ = 0.73 cm−1. In YCo, the 1Π state has T0 = 13 951.3, ωe′ = 231.3, and ωe′xe′ = 2.27 cm−1. For YCo, hot bands originating from levels up to v″ = 3 are observed, allowing the ground state vibrational constants ωe″ = 369.8, ωe″xe″ = 1.47, and ΔG1/2″ = 365.7 cm−1 to be deduced. The bond energy of ScCo has been measured as 2.45 eV from the onset of predissociation in a congested vibronic spectrum. A comparison of the chemical bonding in these molecules to related molecules is presented.
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33.80.Eh Autoionization, photoionization, and photodetachment
33.20.Sn Rotational analysis
33.15.Dj Interatomic distances and angles
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.80.Gj Diffuse spectra; predissociation, photodissociation
33.15.Mt Rotation, vibration, and vibration-rotation constants

Extreme ionization of Xe clusters driven by ultraintense laser fields

Andreas Heidenreich, Isidore Last, and Joshua Jortner

J. Chem. Phys. 127, 074305 (2007); http://dx.doi.org/10.1063/1.2762217 (11 pages) | Cited 13 times

Online Publication Date: 21 August 2007

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We applied theoretical models and molecular dynamics simulations to explore extreme multielectron ionization in Xen clusters (n = 2–2171, initial cluster radius R0 = 2.16–31.0 Å) driven by ultraintense infrared Gaussian laser fields (peak intensity IM = 1015–1020 W cm−2, temporal pulse length τ = 10–100 fs, and frequency ν = 0.35 fs−1). Cluster compound ionization was described by three processes of inner ionization, nanoplasma formation, and outer ionization. Inner ionization gives rise to high ionization levels (with the formation of {Xeq+}n with q = 2–36), which are amenable to experimental observation. The cluster size and laser intensity dependence of the inner ionization levels are induced by a superposition of barrier suppression ionization (BSI) and electron impact ionization (EII). The BSI was induced by a composite field involving the laser field and an inner field of the ions and electrons, which manifests ignition enhancement and screening retardation effects. EII was treated using experimental cross sections, with a proper account of sequential impact ionization. At the highest intensities (IM = 1018–1020 W cm−2) inner ionization is dominated by BSI. At lower intensities (IM = 1015–1016 W cm−2), where the nanoplasma is persistent, the EII contribution to the inner ionization yield is substantial. It increases with increasing the cluster size, exerts a marked effect on the increase of the {Xeq+}n ionization level, is most pronounced in the cluster center, and manifests a marked increase with increasing the pulse length (i.e., becoming the dominant ionization channel (56%) for Xe2171 at τ = 100 fs). The EII yield and the ionization level enhancement decrease with increasing the laser intensity. The pulse length dependence of the EII yield at IM = 1015–1016 W cm−2 establishes an ultraintense laser pulse length control mechanism of extreme ionization products.
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36.40.Qv Stability and fragmentation of clusters
33.80.Eh Autoionization, photoionization, and photodetachment
34.80.Gs Molecular excitation and ionization
52.50.-b Plasma production and heating

Ground states of the Mo2, W2, and CrMo molecules: A second and third order multireference perturbation theory study

Celestino Angeli, Alex Cavallini, and Renzo Cimiraglia

J. Chem. Phys. 127, 074306 (2007); http://dx.doi.org/10.1063/1.2768529 (7 pages) | Cited 8 times

Online Publication Date: 21 August 2007

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The potential energy curves of the molecules Mo2, W2, and CrMo have been studied ab initio using large basis sets and the “n-electron valence state perturbation theory” up to the third order in the energy. The third order results for Mo2 and W2 reproduce the equilibrium distances re and the harmonic frequencies ωe in fairly good accordance with the experimental values but tend to underestimate the dissociation energy. The CrMo molecule, for which experimental dissociation energy data do not exist yet, is predicted to have a value for De of ≈ 2.5 eV.
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31.50.Bc Potential energy surfaces for ground electronic states
31.15.A- Ab initio calculations
33.15.Fm Bond strengths, dissociation energies

Electron correlation in the GK state of the hydrogen molecule

Jian Wang, Yu Wang, Shengliang Lv, and Jesus M. Ugalde

J. Chem. Phys. 127, 074307 (2007); http://dx.doi.org/10.1063/1.2768531 (7 pages) | Cited 3 times

Online Publication Date: 21 August 2007

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The second excited 1Σg+ state of the hydrogen molecule, the so-called GK state, has a potential energy curve with double minima. At the united atom limit it converges to the 1s3d configuration of He. At large internuclear distances R, it dissociates to two separated atoms, one in the ground state and another in the 2p excited state. Radial pair density calculations and natural orbital analyses reveal unusual effect of electron correlation around the K minimum of the potential energy curve. As R>2.0 a.u., a natural orbital of σu symmetry joins the two natural orbitals of σg symmetry at smaller R. The average interelectronic distance decreases as the internuclear distance increases from R = 2.0 to 3.0 a.u. Around R = 3.0 a.u. the singly peaked pair density curve splits into two peaks. The inner peak can be attributed to the formation of the ionic electron configuration (1s)2, where both 1s electrons are on the same nucleus. As the two 1s electrons run into different nuclei, one of the two 1s electrons is promoted to the 2p state, which results in the outer peak in the pair density curve. The Rydberg 1s2p configuration persists as the nuclei stretch, and becomes dominant at large R where four natural orbitals, two of σg and two of σu symmetry, become responsible.
back to top Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

Structure and thermodynamics of a two-dimensional Coulomb fluid in the strong association regime

E. Lomba, J. J. Weis, and F. Lado

J. Chem. Phys. 127, 074501 (2007); http://dx.doi.org/10.1063/1.2764478 (7 pages) | Cited 2 times

Online Publication Date: 16 August 2007

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The behavior of a two-dimensional neutral Coulomb fluid in the strong association regime (low density, high ionic charge) is explored by means of computer simulation and the hypernetted chain integral equation. The theory reproduces reasonably well the structure and thermodynamics of the system but presents a no-solution region at temperatures well above the computer simulation estimates of the metal-insulator transition. In contrast with hypernetted chain predictions for the three-dimensional Coulomb fluid, here the breakdown of the solution is not accompanied by divergences in any physical quantity.
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61.20.Qg Structure of associated liquids: electrolytes, molten salts, etc.
71.30.+h Metal-insulator transitions and other electronic transitions
65.20.-w Thermal properties of liquids
61.20.Ja Computer simulation of liquid structure

Solution of the master equation for Wigner’s quasiprobability distribution in phase space for the Brownian motion of a particle in a double well potential

William T. Coffey, Yuri P. Kalmykov, and Serguey V. Titov

J. Chem. Phys. 127, 074502 (2007); http://dx.doi.org/10.1063/1.2759486 (10 pages) | Cited 11 times

Online Publication Date: 16 August 2007

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Quantum effects in the Brownian motion of a particle in the symmetric double well potential V(x) = ax2/2+bx4/4 are treated using the semiclassical master equation for the time evolution of the Wigner distribution function W(x,p,t) in phase space (x,p). The equilibrium position autocorrelation function, dynamic susceptibility, and escape rate are evaluated via matrix continued fractions in the manner customarily used for the classical Fokker-Planck equation. The escape rate so yielded has a quantum correction depending strongly on the barrier height and is compared with that given analytically by the quantum mechanical reaction rate solution of the Kramers turnover problem. The matrix continued fraction solution substantially agrees with the analytic solution. Moreover, the low-frequency part of the spectrum associated with noise assisted Kramers transitions across the potential barrier may be accurately described by a single Lorentzian with characteristic frequency given by the quantum mechanical reaction rate.
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05.40.Jc Brownian motion
02.30.Sa Functional analysis
02.50.Cw Probability theory
03.65.Db Functional analytical methods
03.65.Sq Semiclassical theories and applications
02.10.Yn Matrix theory

Hydration properties of magnesium and calcium ions from constrained first principles molecular dynamics

Takashi Ikeda, Mauro Boero, and Kiyoyuki Terakura

J. Chem. Phys. 127, 074503 (2007); http://dx.doi.org/10.1063/1.2768063 (8 pages) | Cited 10 times

Online Publication Date: 17 August 2007

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We studied the solvation structures of the divalent metal cations Mg2+ and Ca2+ in ambient water by applying a Car-Parrinello-based constrained molecular dynamics method. By employing the metal-water oxygen coordination number as a reaction coordinate, we could identify distinct aqua complexes characterized by structural variations of the first coordination shell. In particular, our estimated free-energy profile clearly shows that the global minimum for Mg2+ is represented by a rather stable sixfold coordination in the octahedral arrangement, in agreement with experiments. Conversely, for Ca2+ the free-energy curve shows several shallow local minima, suggesting that the hydration structure of Ca2+ is highly variable. Implications for water exchange reactions are also discussed.
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82.30.Nr Association, addition, insertion, cluster formation
82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
61.20.Ja Computer simulation of liquid structure
82.60.Lf Thermodynamics of solutions

Toward effective and reliable fluorescence energies in solution by a new state specific polarizable continuum model time dependent density functional theory approach

Roberto Improta, Giovanni Scalmani, Michael J. Frisch, and Vincenzo Barone

J. Chem. Phys. 127, 074504 (2007); http://dx.doi.org/10.1063/1.2757168 (9 pages) | Cited 32 times

Online Publication Date: 17 August 2007

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A state specific (SS) model for the inclusion of solvent effects in time dependent density functional theory (TD-DFT) computations of emission energies has been developed and coded in the framework of the so called polarizable continuum model (PCM). The new model allows for a rigorous and effective treatment of dynamical solvent effects in the computation of fluorescence and phosphorescence spectra in solution, and it can be used for studying different relaxation time regimes. SS and conventional linear response (LR) models have been compared by computing the emission energies for different benchmark systems (formaldehyde in water and three coumarin derivatives in ethanol). Special attention is given to the influence of dynamical solvation effects on LR geometry optimizations in solution. The results on formaldehyde point out the complementarity of LR and SS approaches and the advantages of the latter model especially for polar solvents and/or weak transitions. The computed emission energies for coumarin derivatives are very close to their experimental counterparts, pointing out the importance of a proper treatment of nonequilibrium solvent effects on both the excited and the ground state energies. The availability of SS-PCM/TD-DFT models for the study of absorption and emission processes allows for a consistent treatment of a number of different spectroscopic properties in solution.
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78.55.Bq Liquids

Continuum limit semiclassical initial value representation for dissipative systems

Eli Pollak

J. Chem. Phys. 127, 074505 (2007); http://dx.doi.org/10.1063/1.2753151 (12 pages) | Cited 11 times

Online Publication Date: 17 August 2007

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In this paper, we consider a dissipative system in which the system is coupled linearly to a harmonic bath. In the continuum limit, the bath is defined via a spectral density and the classical system dynamics is given in terms of a generalized Langevin equation. Using the path integral formulation and factorized initial conditions, it is well known that one can integrate out the harmonic bath, leaving only a path integral over the system degrees of freedom. However, the semiclassical initial value representation treatment of dissipative systems has usually been limited to a discretized treatment of the bath in terms of a finite number of bath oscillators. In this paper, the continuum limit of the semiclassical initial value representation is derived for dissipative systems. As in the path integral, the action is modified with an added nonlocal term, which expresses the influence of the bath on the dynamics. The first order correction term to the semiclassical initial value approximation is also derived in the continuum limit.
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03.65.Sq Semiclassical theories and applications
03.65.Ge Solutions of wave equations: bound states
02.30.Hq Ordinary differential equations

Quantum effects in liquid water from an ab initio-based polarizable force field

Francesco Paesani, Satoru Iuchi, and Gregory A. Voth

J. Chem. Phys. 127, 074506 (2007); http://dx.doi.org/10.1063/1.2759484 (15 pages) | Cited 33 times

Online Publication Date: 21 August 2007

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The importance of quantum effects as well as the accuracy of the ab initio-based polarizable TTM2.1-F force field in describing liquid water are quantitatively assessed by a detailed analysis of the temperature dependence of several thermodynamic and dynamical properties computed using the path-integral molecular dynamics and centroid molecular dynamics methods. The results show that quantum effects are not negligible even at relatively high temperatures, and their inclusion in simulations with the TTM2.1-F water model is necessary to achieve a more accurate description of the liquid properties. Comparison with the results reported in the literature for empirical, nonpolarizable force fields demonstrates that the effects of the nuclear quantization on the dielectric constant are dependent in part on how the electronic polarization is described in the underlying water model, while comparison with other ab initio-based force fields shows that the TTM2.1-F model provides an overall accurate description of liquid water. Analysis of the isotope effect on the dynamical properties does not display significant temperature dependence. This suggests that the contribution of quantum tunneling, which has been proposed as a possible cause for the different orientational dynamics observed for the HDO:H2O and HDO:D2O systems, appears to be small.

Low density solid ozone

B. D. Teolis, M. Famá, and R. A. Baragiola

J. Chem. Phys. 127, 074507 (2007); http://dx.doi.org/10.1063/1.2762215 (9 pages) | Cited 14 times

Online Publication Date: 21 August 2007

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We report a very low density ( ∼ 0.5 g/cm3) structure of solid ozone. It is produced by irradiation of solid oxygen with 100 keV protons at 20 K followed by heating to sublime unconverted oxygen. Upon heating to 47 K the porous ozone compacts to a density of ∼ 1.6 g/cm3 and crystallizes. We use a detailed analysis of the main infrared absorption band of the porous ozone to interpret previous research, where solid oxygen was irradiated by UV light and keV electrons.
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61.43.Gt Powders, porous materials
61.80.Jh Ion radiation effects
64.70.Hz Solid-vapor transitions
78.30.Hv Other nonmetallic inorganics
back to top Surfaces, Interfaces, and Materials

A study of the ionic conduction of mica surface by admittance spectroscopy

D. Dolci, G. Aloisi, L. Lanzi, and M. Carlà

J. Chem. Phys. 127, 074701 (2007); http://dx.doi.org/10.1063/1.2754679 (7 pages) | Cited 2 times

Online Publication Date: 15 August 2007

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The ionic conduction on the surface of humid mica has been analyzed by admittance spectroscopy as a function of relative humidity for different surface treatments. Measurements at low frequency indicate that water adsorption proceeds first in the form of a strongly adsorbed uniform thin layer, then with the formation of highly inhomogeneous thick aggregates.
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66.30.H- Self-diffusion and ionic conduction in nonmetals
68.43.Mn Adsorption kinetics

Thermodynamic pressure of simple fluids confined in cylindrical nanopores by isothermal-isobaric Monte Carlo: Influence of fluid/substrate interactions

Joël Puibasset

J. Chem. Phys. 127, 074702 (2007); http://dx.doi.org/10.1063/1.2764477 (8 pages) | Cited 1 time

Online Publication Date: 15 August 2007

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The thermodynamic pressure or grand potential density is calculated by isobaric-isothermal Monte Carlo algorithm for simple Lennard-Jones fluid confined in cylindrical pores presenting chemical heterogeneities along their axis. Heuristic arguments and simulation results show that the thermodynamic pressure of the confined fluid contains two contributions. The first term is the usual pressure of the bulk fluid for a density equal to the confined fluid density defined as the total number of confined particles divided by the accessible volume due to thermal agitation. A second term has to be added, which is empirically shown to be proportional to the fluid/wall interface area and almost constant along the adsorption and desorption branches. This interfacial contribution, calculated for various pore models, has small variations reminiscent of the fluid adsorption/desorption properties calculated in the various pores. In particular, it is shown that this interfacial quantity is maximum for a fluid/substrate interaction intensity of the same order as the fluid/fluid one, while the thermodynamic pressure at which rapid desorption occurs presents a minimum. Stronger or weaker fluid/wall affinity favors gas state nucleation on the desorption of confined fluids.
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65.20.-w Thermal properties of liquids
61.20.Ja Computer simulation of liquid structure
68.43.Mn Adsorption kinetics

Kinetic coefficient of steps at the Si(111) crystal-melt interface from molecular dynamics simulations

Dorel Buta, Mark Asta, and Jeffrey J. Hoyt

J. Chem. Phys. 127, 074703 (2007); http://dx.doi.org/10.1063/1.2754682 (10 pages) | Cited 12 times

Online Publication Date: 15 August 2007

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Nonequilibrium molecular dynamics simulations are applied to the investigation of step-flow kinetics at crystal-melt interfaces of silicon, modeled with the Stillinger-Weber potential [ Phys. Rev. B 31, 5262 (1985) ]. Step kinetic coefficients are calculated from crystallization rates of interfaces that are vicinals of the faceted (111) orientation. These vicinal interfaces contain periodic arrays of bilayer steps, and they are observed to crystallize in a step-flow growth mode at undercoolings lower than 40 K. Kinetic coefficients for both [110] and [121] oriented steps are determined for several values of the average step separation, in the range of 7.7–62.4 Å. The values of the step kinetic coefficients are shown to be highly isotropic, and are found to increase with increasing step separation until they saturate at step separations larger than ∼ 50 Å. The largest step kinetic coefficients are found to be in the range of 0.7–0.8 m/(sK), values that are more than five times larger than the kinetic coefficient for the rough (100) crystal-melt interface in the same system. The dependence of step mobility on step separation and the relatively large value of the step kinetic coefficient are discussed in terms of available theoretical models for crystal growth kinetics from the melt.
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81.10.Aj Theory and models of crystal growth; physics and chemistry of crystal growth, crystal morphology, and orientation
81.10.Fq Growth from melts; zone melting and refining
64.70.D- Solid-liquid transitions

Oxygen vacancy formation energy in Pd-doped ceria: A DFT+U study

Zongxian Yang, Gaixia Luo, Zhansheng Lu, and Kersti Hermansson

J. Chem. Phys. 127, 074704 (2007); http://dx.doi.org/10.1063/1.2752504 (5 pages) | Cited 25 times

Online Publication Date: 16 August 2007

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Using the DFT+U method, i.e., first principles density functional theory calculations with the inclusion of on-site Coulomb interaction, the effects of Pd doping on the O vacancy formation energy (Evac) in CeO2 has been studied. We find that Evac is lowered from 3.0 eV in undoped ceria to 0.6 eV in the Pd-doped compound. Much of this decrease can be attributed to emerging Pd-induced gap states above the valence band and below the empty Ce 4f states. These localized defect states involve the Pd ion and its nearest neighbors, which are also the main acceptors of the extra electrons left on reduction. The effect of the Pd dopant on the geometric structure is very modest for CeO2 but considerable for CeO2−x.
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61.72.J- Point defects and defect clusters
61.72.up Other materials
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
71.55.Ht Other nonmetals
71.20.Ps Other inorganic compounds
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