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28 Sep 2009

Volume 131, Issue 12, Articles (12xxxx)

Issue Cover Spotlight Figure

J. Chem. Phys. 131, 124101 (2009); http://dx.doi.org/10.1063/1.3216567 (11 pages)

Gregory R. Bowman, Kyle A. Beauchamp, George Boxer, and Vijay S. Pande
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Cavity ring-down ellipsometry

Anna Karaiskou, Vassilis Papadakis, Benoit Loppinet, and T. Peter Rakitzis

J. Chem. Phys. 131, 121101 (2009); http://dx.doi.org/10.1063/1.3236819 (3 pages) | Cited 3 times

Online Publication Date: 23 September 2009

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We demonstrate the enhancement of ellipsometric measurements by multiple reflections of a polarized light pulse on a highly reflective target surface, using an optical cavity. The principle is demonstrated by measuring the adsorbed amount of a molecular vapor (fenchone) onto the ring-cavity mirrors. A phase shift sensitivity of about 10−2° in a single laser pulse is achieved in 1 μs. Further improvements are discussed that should allow sensitivities of at least 10−4°, surpassing current commercial ellipsometers, but also surpassing their time resolution by several orders of magnitude, allowing the uses of sensitive ellipsometry to be expanded to include the study of fast surface phenomena with submicrosecond resolution.
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07.60.Fs Polarimeters and ellipsometers
42.79.Bh Lenses, prisms and mirrors
42.25.Gy Edge and boundary effects; reflection and refraction
42.25.Ja Polarization
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Strong correlations via constrained-pairing mean-field theory

Takashi Tsuchimochi and Gustavo E. Scuseria

J. Chem. Phys. 131, 121102 (2009); http://dx.doi.org/10.1063/1.3237029 (4 pages) | Cited 20 times

Online Publication Date: 25 September 2009

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We present a mean-field approach for accurately describing strong correlations via electron number fluctuations and pairings constrained to an active space. Electron number conservation is broken and correct only on average, but both spin and spatial symmetries are preserved. Optimized natural orbitals and occupations are determined by diagonalization of a mean-field Hamiltonian. This constrained-pairing mean-field theory (CPMFT) yields a two-particle density matrix ansatz that exclusively describes strong correlations. We demonstrate CPMFT accuracy with applications to the metal-insulator transition of large hydrogen clusters and molecular dissociation curves.
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82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.20.-w Chemical kinetics and dynamics
36.40.Jn Reactivity of clusters
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Al13H−: Hydrogen atom site selectivity and the shell model

A. Grubisic, X. Li, S. T. Stokes, K. Vetter, G. F. Ganteför, K. H. Bowen, P. Jena, B. Kiran, R. Burgert, and H. Schnöckel

J. Chem. Phys. 131, 121103 (2009); http://dx.doi.org/10.1063/1.3234363 (4 pages) | Cited 9 times

Online Publication Date: 25 September 2009

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Using a combination of anion photoelectron spectroscopy and density functional theory calculations, we explored the influence of the shell model on H atom site selectivity in Al13H−. Photoelectron spectra revealed that Al13H− has two anionic isomers and for both of them provided vertical detachment energies (VDEs). Theoretical calculations found that the structures of these anionic isomers differ by the position of the hydrogen atom. In one, the hydrogen atom is radially bonded, while in the other, hydrogen caps a triangular face. VDEs for both anionic isomers as well as other energetic relationships were also calculated. Comparison of the measured versus calculated VDE values permitted the structure of each isomer to be confirmed and correlated with its observed photoelectron spectrum. Shell model, electron-counting considerations correctly predicted the relative stabilities of the anionic isomers and identified the stable structure of neutral Al13H.
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36.40.Mr Spectroscopy and geometrical structure of clusters
36.40.Wa Charged clusters
33.60.+q Photoelectron spectra
31.15.E- Density-functional theory
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Combined temperature-programmed reaction and in situ x-ray scattering studies of size-selected silver clusters under realistic reaction conditions in the epoxidation of propene

Stefan Vajda, Sungsik Lee, Kristian Sell, Ingo Barke, Armin Kleibert, Viola von Oeynhausen, Karl-Heinz Meiwes-Broer, Arantxa Fraile Rodríguez, Jeffrey W. Elam, Michael M. Pellin, Byeongdu Lee, Sönke Seifert, and Randall E. Winans

J. Chem. Phys. 131, 121104 (2009); http://dx.doi.org/10.1063/1.3237158 (4 pages) | Cited 8 times

Online Publication Date: 25 September 2009

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The catalytic activity and dynamical shape changes in size-selected nanoclusters at work are studied under realistic reaction conditions by using a combination of simultaneous temperature-programmed reaction with in situ grazing-incidence small angle x-ray scattering. This approach allows drawing a direct correlation between nanocatalyst size, composition, shape, and its function under realistic reaction conditions for the first time. The approach is illustrated in a chemical industry highly relevant selective partial oxidation of propene on a monodisperse silver nanocatalyst. The shape of the catalyst undergoes rapid change already at room temperature upon the exposure to the reactants, followed by a complex evolution of shape with increasing temperature. Acrolein formation is observed around 50 °C while the formation of the propylene oxide exhibits a sharp onset at 80 °C and is leveling off at 150 °C. At lower temperatures acrolein is produced preferentially to propylene oxide; at temperatures above 100 °C propylene oxide is favored.
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82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
78.70.Ck X-ray scattering
82.30.-b Specific chemical reactions; reaction mechanisms
61.46.Bc Structure of clusters (e.g., metcars; not fragments of crystals; free or loosely aggregated or loosely attached to a substrate)
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Progress and challenges in the automated construction of Markov state models for full protein systems

Gregory R. Bowman, Kyle A. Beauchamp, George Boxer, and Vijay S. Pande

J. Chem. Phys. 131, 124101 (2009); http://dx.doi.org/10.1063/1.3216567 (11 pages) | Cited 33 times

Online Publication Date: 22 September 2009

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Markov state models (MSMs) are a powerful tool for modeling both the thermodynamics and kinetics of molecular systems. In addition, they provide a rigorous means to combine information from multiple sources into a single model and to direct future simulations/experiments to minimize uncertainties in the model. However, constructing MSMs is challenging because doing so requires decomposing the extremely high dimensional and rugged free energy landscape of a molecular system into long-lived states, also called metastable states. Thus, their application has generally required significant chemical intuition and hand-tuning. To address this limitation we have developed a toolkit for automating the construction of MSMs called MSMBUILDER (available at https://simtk.org/home/msmbuilder). In this work we demonstrate the application of MSMBUILDER to the villin headpiece (HP-35 NleNle), one of the smallest and fastest folding proteins. We show that the resulting MSM captures both the thermodynamics and kinetics of the original molecular dynamics of the system. As a first step toward experimental validation of our methodology we show that our model provides accurate structure prediction and that the longest timescale events correspond to folding.
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87.14.E- Proteins
36.20.-r Macromolecules and polymer molecules

Robust and efficient density fitting

Víctor D. Domínguez-Soria, Gerald Geudtner, José Luis Morales, Patrizia Calaminici, and Andreas M. Köster

J. Chem. Phys. 131, 124102 (2009); http://dx.doi.org/10.1063/1.3216476 (9 pages) | Cited 2 times

Online Publication Date: 22 September 2009

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In this paper we propose an iterative method for solving the inhomogeneous systems of linear equations associated with density fitting. The proposed method is based on a version of the conjugate gradient method that makes use of automatically built quasi-Newton preconditioners. The paper gives a detailed description of a parallel implementation of the new method. The computational performance of the new algorithms is analyzed by benchmark calculations on systems with up to about 35 000 auxiliary functions. Comparisons with the standard, direct approach show no significant differences in the computed solutions.
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31.15.E- Density-functional theory
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions

Excited states of methylene from quantum Monte Carlo

Paul M. Zimmerman, Julien Toulouse, Zhiyong Zhang, Charles B. Musgrave, and C. J. Umrigar

J. Chem. Phys. 131, 124103 (2009); http://dx.doi.org/10.1063/1.3220671 (7 pages) | Cited 11 times

Online Publication Date: 22 September 2009

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The ground and lowest three adiabatic excited states of methylene are computed using the variational Monte Carlo and diffusion Monte Carlo (DMC) methods using progressively larger Jastrow–Slater multideterminant complete active space (CAS) wave functions. The highest of these states has the same symmetry, 1A1, as the first excited state. The DMC excitation energies obtained using any of the CAS wave functions are in excellent agreement with experiment, but single-determinant wave functions do not yield accurate DMC energies of the states of 1A1 symmetry, indicating that it is important to include in the wave function Slater determinants that describe static (strong) correlation. Excitation energies obtained using recently proposed pseudopotentials [ Burkatzki et al., J. Chem. Phys. 126, 234105 (2007) ] differ from the all-electron excitation energies by at most 0.04 eV.
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31.15.vq Electron correlation calculations for polyatomic molecules
33.15.Bh General molecular conformation and symmetry; stereochemistry
31.15.xr Self-consistent-field methods

Analytic evaluation of the nonadiabatic coupling vector between excited states using equation-of-motion coupled-cluster theory

Attila Tajti and Péter G. Szalay

J. Chem. Phys. 131, 124104 (2009); http://dx.doi.org/10.1063/1.3232011 (9 pages) | Cited 8 times

Online Publication Date: 23 September 2009

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Theory and implementation for evaluation of the nonadiabatic coupling vector between excited electronic states described by equation-of-motion excitation energy coupled-cluster singles and doubles (EOMEE-CCSD) method is presented. Problems arising from the non-Hermitian nature of the theory are discussed in detail. The performance of the new approach is demonstrated by the nice agreement of the nonadiabatic coupling curves for LiH obtained at the EOMEE-CCSD and MR-CISD levels. Using the tools developed we also present a computational procedure to evaluate the interstate coupling constants used in vibronic coupling theories. As an application of this part of the implementation we present simulation of the electronic absorption spectrum of the pyrazine molecule within the linear vibronic coupling model.
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33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions
31.15.bw Coupled-cluster theory
31.50.Df Potential energy surfaces for excited electronic states
31.50.Gh Surface crossings, non-adiabatic couplings
31.15.vn Electron correlation calculations for diatomic molecules

A systematic analysis of the structure and (hyper)polarizability of donor-acceptor substituted polyacetylenes using a Coulomb-attenuating density functional

Stefano Borini, Peter A. Limacher, and Hans Peter Lüthi

J. Chem. Phys. 131, 124105 (2009); http://dx.doi.org/10.1063/1.3216825 (10 pages) | Cited 10 times

Online Publication Date: 23 September 2009

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In this paper we perform a systematic investigation on all-trans polyacetylene chains of different lengths, end-capped with moieties of different donor or acceptor natures and different strengths, to infer useful structure/property relationship rules and behavioral patterns. The values for bond length alternation (BLA), longitudinal polarizability, and first and second hyperpolarizabilities have been computed with the Coulomb-attenuating density function (CAM-B3LYP), using response theory. A comparison of the relative effect that each end-capping combination contributes to BLA, linear, and nonlinear optical coefficients has been performed. This results in useful insights and general rules to ad hoc tailoring the molecular response for a specific characteristic.
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31.15.E- Density-functional theory
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
33.15.Dj Interatomic distances and angles

Penetrable-square-well fluids: Analytical study and Monte Carlo simulations

Riccardo Fantoni, Achille Giacometti, Alexandr Malijevský, and Andrés Santos

J. Chem. Phys. 131, 124106 (2009); http://dx.doi.org/10.1063/1.3236515 (14 pages) | Cited 6 times

Online Publication Date: 23 September 2009

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We study structural and thermophysical properties of a one-dimensional classical fluid made of penetrable spheres interacting via an attractive square-well potential. Penetrability of the spheres is enforced by reducing from infinite to finite the repulsive energy barrier in the pair potentials As a consequence, an exact analytical solution is lacking even in one dimension. Building upon previous exact analytical work in the low-density limit [ A. Santos, R. Fantoni, and A. Giacometti, Phys. Rev. E 77, 051206 (2008) ], we propose an approximate theory valid at any density and in the low-penetrable regime. By comparison with specialized Monte Carlo simulations and integral equation theories, we assess the regime of validity of the theory. We investigate the degree of inconsistency among the various routes to thermodynamics and explore the possibility of a fluid-fluid transition. Finally we locate the dependence of the Fisher–Widom line on the degree of penetrability. Our results constitute the first systematic study of penetrable spheres with attractions as a prototype model for soft systems.
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61.20.Ja Computer simulation of liquid structure
82.70.Dd Colloids
64.70.Ja Liquid-liquid transitions
65.20.De General theory of thermodynamic properties of liquids, including computer simulation

Time correlation functions via forward-backward quantum dynamics using Hamilton’s law of varying action

Jonathan Chen and Nancy Makri

J. Chem. Phys. 131, 124107 (2009); http://dx.doi.org/10.1063/1.3224494 (8 pages) | Cited 1 time

Online Publication Date: 23 September 2009

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We introduce a stable numerical procedure for solving Bohm’s equations of motion to compute quantum trajectories in the forward-backward quantum dynamics (FBQD) formulation of zero-temperature time correlation functions. Rather than integrating the differential equations forward in time, our method is based on a series expansion of the quantum trajectory, exploiting Hamilton’s law of varying action to determine the expansion coefficients. Because in FBQD the quantum trajectories generally are smooth and the quantum potential is well behaved, our method allows accurate determination of time correlation functions in strongly anharmonic bound systems over several oscillation periods.
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03.65.Ge Solutions of wave equations: bound states
02.60.Jh Numerical differentiation and integration
02.60.Lj Ordinary and partial differential equations; boundary value problems
03.65.Fd Algebraic methods
02.10.Ud Linear algebra

Effective-mode representation of non-Markovian dynamics: A hierarchical approximation of the spectral density. II. Application to environment-induced nonadiabatic dynamics

Keith H. Hughes, Clara D. Christ, and Irene Burghardt

J. Chem. Phys. 131, 124108 (2009); http://dx.doi.org/10.1063/1.3226343 (15 pages) | Cited 11 times

Online Publication Date: 24 September 2009

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The non-Markovian approach developed in the companion paper [ Hughes et al., J. Chem. Phys. 131, 024109 (2009) ], which employs a hierarchical series of approximate spectral densities, is extended to the treatment of nonadiabatic dynamics of coupled electronic states. We focus on a spin-boson-type Hamiltonian including a subset of system vibrational modes which are treated without any approximation, while a set of bath modes is transformed to a chain of effective modes and treated in a reduced-dimensional space. Only the first member of the chain is coupled to the electronic subsystem. The chain construction can be truncated at successive orders and is terminated by a Markovian closure acting on the end of the chain. From this Mori-type construction, a hierarchy of approximate spectral densities is obtained which approach the true bath spectral density with increasing accuracy. Applications are presented for the dynamics of a vibronic subsystem comprising a high-frequency mode and interacting with a low-frequency bath. The bath is shown to have a striking effect on the nonadiabatic dynamics, which can be rationalized in the effective-mode picture. A reduced two-dimensional subspace is constructed which accounts for the essential features of the nonadiabatic process induced by the effective environmental mode. Electronic coherence is found to be preserved on the shortest time scale determined by the effective mode, while decoherence sets in on a longer time scale. Numerical simulations are carried out using either an explicit wave function representation of the system and overall bath or else an explicit representation of the system and effective-mode part in conjunction with a Caldeira–Leggett master equation.
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63.50.-x Vibrational states in disordered systems
71.20.Rv Polymers and organic compounds

Model core potentials for studies of scalar-relativistic effects and spin-orbit coupling at Douglas–Kroll level. I. Theory and applications to Pb and Bi

Tao Zeng, Dmitri G. Fedorov, and Mariusz Klobukowski

J. Chem. Phys. 131, 124109 (2009); http://dx.doi.org/10.1063/1.3211955 (17 pages) | Cited 8 times

Online Publication Date: 24 September 2009

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A theory of model core potentials that can treat spin-orbit-coupling (SOC) effects at the level of Douglas–Kroll formalism has been developed. By storing the damping effect of kinematic operator in the Douglas–Kroll spin-orbit operator into an additional set of basis set contraction coefficients, the Breit–Pauli spin-orbit code in the GAMESS-US program was successfully used to perform Douglas–Kroll spin-orbit calculations. It was found that minute errors in the radial functions of valence orbitals lead to large errors in the spin-orbit energy levels and thus fitting the radial part of the spin-orbit matrix elements is necessary in model core potential parametrization. The first model core potentials that include the new formalism were developed for two 6p-block elements, Pb and Bi. The valence space of the 5p, 5d, 6s, and 6p orbitals was used because of the large SOC between the 5p and 6p orbitals. The model core potentials were validated in the calculations of atomic properties as well as spectroscopic constants of diatomic metal hydrides. The agreement between results of the model core potential and all-electron calculations was excellent, with energy errors of hundreds of cm−1 and hundredths of eV, re errors of thousandths of Å, and ωe errors under 20 cm−1. Two kinds of interplay between SOC effect and bonding process (antibonding and bonding SOC) were demonstrated using spin-free term potential curves of PbH and BiH. The present study is the first extension of the model core potential method beyond Breit–Pauli to Douglas–Kroll SOC calculations.
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31.50.Bc Potential energy surfaces for ground electronic states

Integral approximations in ab initio, electron propagator calculations

Roberto Flores-Moreno and J. V. Ortiz

J. Chem. Phys. 131, 124110 (2009); http://dx.doi.org/10.1063/1.3238243 (6 pages) | Cited 4 times

Online Publication Date: 24 September 2009

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Treatments of interelectronic repulsion that avoid four-center integrals have been incorporated in ab initio, electron-propagator calculations with diagonal self-energy matrices. Whereas the formal scaling of arithmetic operations in the propagator calculations is unaffected, the reduction of storage requirements is substantial. Moreover, the scaling of integral transformations to the molecular orbital base is lowered by one order. Four-index, electron-repulsion integrals are regenerated from three-index intermediates. Test calculations with widely applied self-energy approximations demonstrate the accuracy of this approach. Only small errors are introduced when this technique is used with quasiparticle virtual orbitals, provided that conventional techniques of integral evaluation are used in the construction of density-difference matrices.
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31.15.A- Ab initio calculations

The effect of spin polarization on zero field splitting parameters in paramagnetic π-electron molecules

Maurice van Gastel

J. Chem. Phys. 131, 124111 (2009); http://dx.doi.org/10.1063/1.3233859 (7 pages) | Cited 3 times

Online Publication Date: 25 September 2009

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Spin polarization effects play an important role in the theory of isotropic hyperfine interactions for aromatic protons. The spin polarization gives rise to significant isotropic proton hyperfine interactions—spin-dependent one-electron properties—smaller than 0 MHz and the effect has been theoretically described [ H. M. McConnell and D. B. J. Chesnut, Chem. Phys. 28, 107 (1958) ]. The influence of spin polarization on the zero field splitting parameters, which are spin-dependent two-electron properties, has not been clearly identified yet. A phenomenological equation is proposed here for the contribution of spin polarization to the zero field splitting parameter D in analogy to McConnell’s equation for hyperfine interactions. The presence of the effect is demonstrated in a series of calculations on polyacenes in the triplet state and turns out to be responsible for up to 50% of the D parameter in the case of naphthalene! It is found that spin-unrestricted single-determinant methods, including the widely used density functional theory methods, do not accurately reproduce the two-electron reduced electron density required for the evaluation of two-electron spin-dependent properties. For the accurate calculation of zero field splitting parameters by quantum chemical methods, it thus seems necessary to resort to correlated ab initio methods which do not give rise to spin contamination and which do provide an accurate description of the two-electron reduced electron density.
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71.70.Jp Nuclear states and interactions
72.25.-b Spin polarized transport

Maximum locality in occupied and virtual orbital spaces using a least-change strategy

Marcin Ziółkowski, Branislav Jansík, Poul Jørgensen, and Jeppe Olsen

J. Chem. Phys. 131, 124112 (2009); http://dx.doi.org/10.1063/1.3230604 (15 pages) | Cited 7 times

Online Publication Date: 25 September 2009

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A new strategy is introduced for obtaining localized orthonormal Hartree–Fock (HF) orbitals where the underlying principle is to minimize the size of the transformation matrix from the atomic orbital basis to the HF optimized orbital basis. The new strategy gives both localized occupied and localized virtual orbital spaces. The locality of the occupied orbital space is similar to one obtained using standard localization schemes. For the virtual space, standard localization schemes fail to give local orbitals while the new strategy gives a virtual space which has a locality similar to the one of a Löwdin orthonormalization of the atomic orbital basis. Since Löwdin orthonormalization gives the most local orthonormal basis functions in the sense that they have the largest similarity with the local atomic basis functions, the new strategy thus allows the orthonormal basis to become optimized without introducing significant delocalization.
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31.15.xr Self-consistent-field methods

Hartree–Fock solutions as a quasidiabatic basis for nonorthogonal configuration interaction

Alex J. W. Thom and Martin Head-Gordon

J. Chem. Phys. 131, 124113 (2009); http://dx.doi.org/10.1063/1.3236841 (5 pages) | Cited 1 time

Online Publication Date: 25 September 2009

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Using the method of self-consistent field metadynamics, we locate some of the low-energy solutions to the Hartree–Fock (HF) equations on LiF and O3. The located solutions qualitatively resemble the adiabatic electronic states in these systems. We formulate the method of nonorthogonal Configuration Interaction (CI) to interact these solutions with cubic scaling with system size and quadratic scaling with the number of solutions. The resultant solutions display the avoided crossings and, in O3, a conical intersection expected of the adiabatic states. In LiF the relevant solutions coalesce and disappear from Unrestricted HF space indicating a more general HF theory is required.
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31.15.xr Self-consistent-field methods

Cross sections and photoelectron angular distributions in photodetachment from negative ions using equation-of-motion coupled-cluster Dyson orbitals

C. Melania Oana and Anna I. Krylov

J. Chem. Phys. 131, 124114 (2009); http://dx.doi.org/10.1063/1.3231143 (15 pages) | Cited 12 times

Online Publication Date: 25 September 2009

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We report total and differential cross sections for photodetachment from negative ions using Dyson orbitals calculated from equation-of-motion coupled-cluster wave functions and free wave description of the detached electron. The energy dependence of the cross sections is reproduced well, however, the accuracy of absolute values varies. For F−, C−, NH2−, and H−, the calculated cross sections are within the error bars from the experimental values, whereas the errors for Li− and OH− are about 20%. The largest errors are observed for O− and O2− for which the calculated cross sections differ from the experimental ones by factors of 3 and 2, respectively. Calculated anisotropy parameters for atomic anions exhibit too slow decrease, which suggests that the diffuseness of the computed Dyson orbitals is underestimated. Moreover, in the asymptotic region, the orbitals exhibit artifactual oscillations probably due to the limitations of Gaussian basis sets. The analysis of the trends in the experimental anisotropy parameters suggests that the interaction of the detached electron with the core, which is neglected in the present model, is important.
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32.80.Gc Photodetachment of atomic negative ions
33.60.+q Photoelectron spectra
31.15.bw Coupled-cluster theory
33.80.Eh Autoionization, photoionization, and photodetachment

Properties of the exact universal functional in multicomponent density functional theory

Arindam Chakraborty, Michael V. Pak, and Sharon Hammes-Schiffer

J. Chem. Phys. 131, 124115 (2009); http://dx.doi.org/10.1063/1.3236844 (8 pages) | Cited 9 times

Online Publication Date: 25 September 2009

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Multicomponent density functional theory has been developed to treat systems with more than one type of quantum particle, such as electrons and nuclei, in an external potential. The existence of the exact universal multicomponent density functional in terms of the one-particle densities for each type of quantum particle has been proven. In the present paper, a number of important mathematical properties of the exact universal multicomponent density functional are derived. The expression relating the electron-proton pair density to the one-particle densities leads to an inequality for the potential energy component of the electron-proton correlation functional under well-defined conditions. General inequalities for the kinetic energy correlation functionals and the total electron-proton correlation functional are also derived. The coordinate scaling analysis leads to mathematical inequalities describing the effect of scaled densities on the kinetic, potential, and total energy functionals. The adiabatic connection formula defines the exact electron-proton functional in terms of an adiabatic scaling parameter that smoothly connects the noninteracting system with the fully interacting system. The virial expression provides the relation between the exact kinetic and potential energy functionals for the ground state densities of multicomponent systems. These mathematical relationships provide insight into the fundamental properties of the exact universal multicomponent density functional and serve as a guide for the development of approximate electron-proton density functionals.
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31.15.ve Electron correlation calculations for atoms and ions: ground state
31.50.Bc Potential energy surfaces for ground electronic states
34.20.Gj Intermolecular and atom-molecule potentials and forces

The molecular mean-field approach for correlated relativistic calculations

Jetze Sikkema, Lucas Visscher, Trond Saue, and Miroslav Iliaš

J. Chem. Phys. 131, 124116 (2009); http://dx.doi.org/10.1063/1.3239505 (9 pages) | Cited 21 times

Online Publication Date: 28 September 2009

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A new approach for relativistic correlated electron structure calculations is proposed by which a transformation to a two-spinor basis is carried out after solving the four-component relativistic Hartree–Fock equations. The method is shown to be more accurate than approaches that apply an a priori transformation to a two-spinor basis. We also demonstrate how the two-component relativistic calculations with properly transformed two-electron interaction can be simulated at the four-component level by projection techniques, thus allowing an assessment of errors introduced by more approximate schemes.
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31.30.J- Relativistic and quantum electrodynamic (QED) effects in atoms, molecules, and ions
33.15.Pw Fine and hyperfine structure
31.15.xr Self-consistent-field methods

A multistate local coupled cluster CC2 response method based on the Laplace transform

Danylo Kats and Martin Schütz

J. Chem. Phys. 131, 124117 (2009); http://dx.doi.org/10.1063/1.3237134 (11 pages) | Cited 14 times

Online Publication Date: 29 September 2009

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A new Laplace transform based multistate local CC2 response method for calculating excitation energies of extended molecular systems is presented. By virtue of the Laplace transform trick, the eigenvalue problem involving the local CC2 Jacobian is partitioned along the doubles-doubles block (which is diagonal in the parental canonical method) without losing the sparsity in the integral, amplitude, and amplitude response supermatrices. Hence, only an effective eigenvalue problem involving singles vectors has to be solved, while the doubles part can be computed on-the-fly. Within this framework, a multistate treatment of excited states with state specific and adaptive local approximations imposed on the doubles part is straightforwardly possible. Furthermore, in the context of the density fitting approximation of the two-electron integrals, a procedure to specify the local approximation, i.e., the restricted pair lists and domains, on the basis of an analysis of the object to be approximated itself is proposed. Performance and accuracy of the new Laplace transformed density fitted local CC2 (LT-DF-LCC2) response method are tested for set of different test molecules and states. It turns out that LT-DF-LCC2 response is much more robust than the earlier local CC2 response method proposed before, which failed to find some excited states in difficult cases.
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31.15.bw Coupled-cluster theory

Response properties with explicitly correlated coupled-cluster methods using a Slater-type correlation factor and cusp conditions

Matthias Hanauer and Andreas Köhn

J. Chem. Phys. 131, 124118 (2009); http://dx.doi.org/10.1063/1.3238237 (11 pages) | Cited 12 times

Online Publication Date: 29 September 2009

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The recently proposed extension of the explicitly correlated coupled-cluster ansatz using cusp conditions [ A. Köhn, J. Chem. Phys. 130, 104104 (2009) ] is tested for suitability in the calculation of response properties. For this purpose, static and dynamic electrical properties up to ESHG hyperpolarizabilities as well as optical rotations have been computed within the CCSD(F12) model. It is shown that effectively converged correlation contributions can reliably be obtained using augmented quadruple zeta basis sets already. The ansatz is optionally equipped with an extension capable of reducing the one-electron basis set error. A further simplification of the method specific Lagrangian aimed at reducing the computational effort has been tested and is shown to be uncritical. Furthermore, we examined the impact of conventional triple and quadruple excitations in explicitly correlated property calculations.
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31.15.bw Coupled-cluster theory
33.55.+b Optical activity and dichroism
32.30.-r Atomic spectra
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility

Gauge origin independent calculations of nuclear magnetic shieldings in relativistic four-component theory

Miroslav Iliaš, Trond Saue, Thomas Enevoldsen, and Hans Jørgen Aa. Jensen

J. Chem. Phys. 131, 124119 (2009); http://dx.doi.org/10.1063/1.3240198 (13 pages) | Cited 17 times

Online Publication Date: 29 September 2009

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The use of perturbation-dependent London atomic orbitals, also called gauge including atomic orbitals, has proven efficient for calculations of NMR shielding constants and other magnetic properties in the nonrelativistic framework. In this paper, the theory of London atomic orbitals for NMR shieldings is extended to the four-component relativistic framework and our implementation is described. The relevance of London atomic orbitals in four-component calculations as well as computational aspects are illustrated with test calculations on hydrogen iodide. We find that the use of London atomic orbitals is an efficient method for reliable calculations of NMR shielding constants with standard basis sets, also for four-component calculations with spin-orbit coupling effects included in the wave function optimization. Furthermore, we find that it is important that the small component basis functions fulfill the magnetic balance for accurate description of the diamagnetic shielding and that the role of London atomic orbitals in the relativistic domain is to provide atomic magnetic balance even in the molecular case, thus greatly improving basis set convergence. The Sternheim approximation, which calculates the diamagnetic contribution as an expectation value, leads to significant errors and is not recommended.
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31.30.J- Relativistic and quantum electrodynamic (QED) effects in atoms, molecules, and ions
33.25.+k Nuclear resonance and relaxation
31.15.xp Perturbation theory
31.30.jc Relativistic corrections to atomic structure and properties

Polarization justified Fukui functions

Ludwik Komorowski, Józef Lipiński, and Paweł Szarek

J. Chem. Phys. 131, 124120 (2009); http://dx.doi.org/10.1063/1.3239503 (9 pages) | Cited 3 times

Online Publication Date: 29 September 2009

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New Fukui functions have been derived within the conceptual density functional theory by the analysis of the polarization effect of a system in static electric field. Resulting Fukui functions accurately reproduce the global softness and electronic dipolar polarizability; they meet the condition ∫[f(r)/r]dr = −(∂μ/∂Z)N and lead to very reasonable values of the global hardness for atoms for the group of 29 main group elements. Computational clarity makes the new Fukui functions a promising tool in studies of molecular reactivity.
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31.15.E- Density-functional theory
82.20.Wt Computational modeling; simulation
32.30.-r Atomic spectra
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility

Phase-space geometry of the generalized Langevin equation

Thomas Bartsch

J. Chem. Phys. 131, 124121 (2009); http://dx.doi.org/10.1063/1.3239473 (10 pages) | Cited 4 times

Online Publication Date: 29 September 2009

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The generalized Langevin equation is widely used to model the influence of a heat bath upon a reactive system. This equation will here be studied from a geometric point of view. A dynamical phase space that represents all possible states of the system will be constructed, the generalized Langevin equation will be formally rewritten as a pair of coupled ordinary differential equations, and the fundamental geometric structures in phase space will be described. It will be shown that the phase space itself and its geometric structure depend critically on the preparation of the system: A system that is assumed to have been in existence forever has a larger phase space with a simpler structure than a system that is prepared at a finite time. These differences persist even in the long-time limit, where one might expect the details of preparation to become irrelevant.
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82.20.-w Chemical kinetics and dynamics
02.40.-k Geometry, differential geometry, and topology
02.30.Hq Ordinary differential equations
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