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15 Jul 2005

Volume 123, Issue 3, Articles (03xxxx)

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back to top Theoretical Methods and Algorithms

Hybrid approach for ab initio molecular dynamics simulation combining energy density analysis and short-time Fourier transform: Energy transfer spectrogram

Yusuke Yamauchi and Hiromi Nakai

J. Chem. Phys. 123, 034101 (2005); http://dx.doi.org/10.1063/1.1940635 (9 pages) | Cited 16 times

Online Publication Date: 22 July 2005

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We propose a new analysis technique for specifying molecular vibrational modes related with intramolecular and/or intermolecular energy transfer in ab initio molecular dynamics simulation of chemical reaction. The technique combines the short-time Fourier transform method with energy density analysis, which partitions the quantum chemical potential energy in the system into atomic contributions. The image obtained by the combined scheme, termed an energy transfer spectrogram (ETS), enables us to understand the dynamics of energy transfer by time-frequency representation. The time change of the local energy is quite important in chemical reactions. In order to assess the performance of the ETS, its application to the collision reaction between two carbon dioxide molecules is shown.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Rp State to state energy transfer
82.20.Wt Computational modeling; simulation

A refined ring polymer molecular dynamics theory of chemical reaction rates

Ian R. Craig and David E. Manolopoulos

J. Chem. Phys. 123, 034102 (2005); http://dx.doi.org/10.1063/1.1954769 (10 pages) | Cited 30 times

Online Publication Date: 22 July 2005

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We further develop the ring polymer molecular dynamics (RPMD) method for calculating chemical reaction rates [ I. R. Craig and D. E. Manolopoulos, J. Chem. Phys. 122, 084106 (2005) ]. We begin by showing how the rate coefficient we obtained before can be calculated in a more efficient way by considering the side functions of the ring-polymer centroids, rather than averaging over the side functions of the individual ring-polymer beads. This has two distinct advantages. First, the statistics of the phase-space average over the ring-polymer coordinates and momenta are greatly improved. Second, the resulting flux-side correlation function converges to its long-time limit much more rapidly. Indeed the short-time limit of this flux-side correlation function already provides a “quantum transition state theory” approximation to the final rate coefficient. In cases where transition state recrossing effects are negligible, and the transition state dividing surface is put in the right place, the RPMD rate is therefore obtained almost instantly. We then go on to show that the long-time limit of the new flux-side correlation function, and hence the fully converged RPMD reaction rate, is rigorously independent of the choice of the transition state dividing surface. This is especially significant because the optimum dividing surface can often be very difficult to determine for reactions in complex chemical systems.
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82.35.-x Polymers: properties; reactions; polymerization
82.30.-b Specific chemical reactions; reaction mechanisms
82.20.Db Transition state theory and statistical theories of rate constants
82.20.Ej Quantum theory of reaction cross section
82.20.Pm Rate constants, reaction cross sections, and activation energies

Quantitative molecular thermochemistry based on path integrals

Kurt R. Glaesemann and Laurence E. Fried

J. Chem. Phys. 123, 034103 (2005); http://dx.doi.org/10.1063/1.1954771 (7 pages) | Cited 2 times

Online Publication Date: 22 July 2005

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The calculation of thermochemical data requires accurate molecular energies and heat capacities. Traditional methods rely upon the standard harmonic normal-mode analysis to calculate the vibrational and rotational contributions. We utilize path-integral Monte Carlo for going beyond the harmonic analysis and to calculate the vibrational and rotational contributions to ab initio energies. This is an application and an extension of a method previously developed in our group [ J. Chem. Phys. 118, 1596 (2003) ].
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31.15.A- Ab initio calculations
33.15.Mt Rotation, vibration, and vibration-rotation constants
05.10.Ln Monte Carlo methods
02.70.Ss Quantum Monte Carlo methods

Absolute free energy calculations by thermodynamic integration in four spatial dimensions

Tomas Rodinger, P. Lynne Howell, and Régis Pomès

J. Chem. Phys. 123, 034104 (2005); http://dx.doi.org/10.1063/1.1946750 (11 pages) | Cited 6 times

Online Publication Date: 25 July 2005

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An optimized technique for calculating the excess chemical potential of small molecules in dense liquids and the binding affinity of molecular ligands to biomolecules is reported. In this method, a molecular species is coupled to the system of interest via a nonphysical fourth spatial dimension w through which insertion or extraction can be carried out [ R. Pomès, E. Eisenmesser, C. B. Post et al., J. Chem. Phys. 111, 3387 (1999) ]. Molecular simulations are used to compute the potential of mean force (PMF) acting on the solute molecule in the fourth dimension. The excess chemical potential of that molecule is obtained as the difference in the PMF between fully coupled and fully decoupled systems. The simplicity, efficiency, and generality of the method are demonstrated for the calculation of the hydration free energies of water and methanol as well as sodium, cesium, and chloride ions. A significant advantage over other methods is that the 4D-PMF approach provides a single effective and general route for decoupling all nonbonded interactions (i.e., both Lennard-Jones and Coulombic) at once for both neutral and charged solutes. Direct calculation of the mean force from thermodynamic integration is shown to be more computationally efficient than calculating the PMF from umbrella sampling. Statistical error analysis suggests a simple strategy for optimizing sampling. The detailed analysis of systematic errors arising from the truncation of Coulombic interactions in a solvent droplet of finite size leads to straightforward corrections to ionic hydration free energies.
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65.20.-w Thermal properties of liquids
82.60.-s Chemical thermodynamics
82.30.Nr Association, addition, insertion, cluster formation

Simulations of one- and two-electron systems by Bead-Fourier path integral molecular dynamics

Sergei D. Ivanov and Alexander P. Lyubartsev

J. Chem. Phys. 123, 034105 (2005); http://dx.doi.org/10.1063/1.1961312 (10 pages) | Cited 1 time

Online Publication Date: 27 July 2005

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The Bead-Fourier path integral molecular dynamics technique introduced earlier [ S. D. Ivanov, A. P. Lyubartsev, and A. Laaksonen, Phys. Rev. E 67 066710 (2003) ] is applied for simulation of electrons in the simplest molecules: molecular hydrogen, helium atom, and their ions. Special attention is paid to the correct description of electrons in the core region of a nucleus. In an attempt to smooth the Coulomb potential at small distances, a recipe is suggested. The simulation results are in excellent agreement with the analytical solution for the “harmonic helium atom”, as well as with the vibrational potential of the H2 molecule and He ionization energies. It is demonstrated, that the Bead-Fourier path integral molecular dynamics technique is able to provide the accuracy required for the description of electron structure and chemical bonds in cases when electron exchange effects need not be taken into account.
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31.15.xv Molecular dynamics and other numerical methods
31.15.xk Path-integral methods
31.15.ve Electron correlation calculations for atoms and ions: ground state
31.15.vn Electron correlation calculations for diatomic molecules
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions
33.20.Tp Vibrational analysis
32.50.+d Fluorescence, phosphorescence (including quenching)
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
33.15.Fm Bond strengths, dissociation energies
34.70.+e Charge transfer

Atomic spin-orbit pseudopotential definition and its relation to the different relativistic approximations

Emmanuel Fromager, Christian Teichteil, and Laurent Maron

J. Chem. Phys. 123, 034106 (2005); http://dx.doi.org/10.1063/1.1942467 (12 pages) | Cited 3 times

Online Publication Date: 27 July 2005

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A critical analysis of usual shape-consistent spin-orbit pseudopotential extraction procedures is presented, considering the basic requirements of the atomic pseudopotentials. It is based on a perturbative analysis of both reference all-electron Dirac–Coulomb and pseudopotential calculations by means of the formalism developed by Lindgren and Morrisson. In the light of this analysis, we propose a new hybrid extraction of spin-orbit pseudopotentials, taking advantage of both shape-consistent and energy-consistent procedures. These new pseudopotentials are extracted and checked for the math ground state of the halogens.
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31.15.xp Perturbation theory
31.30.J- Relativistic and quantum electrodynamic (QED) effects in atoms, molecules, and ions

Combining the lattice-sum and reaction-field approaches for evaluating long-range electrostatic interactions in molecular simulations

Tim N. Heinz and Philippe H. Hünenberger

J. Chem. Phys. 123, 034107 (2005); http://dx.doi.org/10.1063/1.1955525 (19 pages) | Cited 16 times

Online Publication Date: 28 July 2005

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A new scheme, the lattice-sum-emulated reaction-field (LSERF) method, is presented that combines the lattice-sum (LS) and reaction-field (RF) approaches for evaluating electrostatic interactions in molecular simulations. More precisely, the LSERF scheme emulates a RF calculation (based on an atomic cutoff) via the LS machinery. This is achieved by changing the form of the electrostatic interactions in a standard LS calculation (Coulombic) to the form corresponding to RF electrostatics (Coulombic plus quadratic reaction-field correction term, truncated at the cutoff distance). It is shown (both analytically and numerically) that in the limit of infinite reciprocal-space accuracy, (i) the LSERF scheme with a finite reaction-field cutoff and a given reaction-field permittivity is identical to the RF scheme with the same parameters (and an atomic cutoff), and (ii) the LSERF scheme is identical to the LS scheme in the limit of an infinite reaction-field cutoff, irrespective of the reaction-field permittivity. This new scheme offers two key advantages: (i) from a conceptual point of view, it shows that there is a continuity between the RF and LS schemes and unifies them into a common framework; (ii) from a practical point of view, it allows us to perform RF calculations with arbitrarily large reaction-field cutoff distances for the same computational costs as a corresponding LS calculation. The optimal choice for the cutoff will be the one that achieves the best compromise between artifacts arising from the dielectric heterogeneity of the system (short cutoff) and its artificial periodicity (long cutoff). The implementation of the LSERF method is extremely easy, requiring only very limited modifications of any standard LS code. For practical applications to biomolecular systems, the use of the LSERF scheme with large reaction-field cutoff distances is expected to represent a significant improvement over the current RF simulations involving comparatively much shorter cutoffs.
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87.10.-e General theory and mathematical aspects
87.15.A- Theory, modeling, and computer simulation
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Theoretical and experimental studies of the infrared rovibrational spectrum of He2–N2O

Xiao-Gang Wang, Tucker Carrington, Jian Tang, and A. R. W. McKellar

J. Chem. Phys. 123, 034301 (2005); http://dx.doi.org/10.1063/1.1924408 (15 pages) | Cited 27 times

Online Publication Date: 22 July 2005

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Rovibrational spectra of the He2–N2O complex in the ν1 fundamental band of N2O (2224 cm−1) have been observed using a tunable infrared laser to probe a pulsed supersonic jet expansion, and calculated using five coordinates that specify the positions of the He atoms with respect to the NNO molecule, a product basis, and a Lanczos eigensolver. Vibrational dynamics of the complex are dominated by the torsional motion of the two He atoms on a ring encircling the N2O molecule. The resulting torsional states could be readily identified, and they are relatively uncoupled to other He motions up to at least υt = 7. Good agreement between experiment and theory was obtained with only one adjustable parameter, the band origin. The calculated results were crucial in assigning many weaker observed transitions because the effective rotational constants depend strongly on the torsional state. The observed spectra had effective temperatures around 0.7 K and involved transitions with J ⩽ 3, with υt = 0 and 1, and (with one possible exception) with Δυt = 0. Mixing of the torsion-rotation states is small but significant: some transitions with Δυt ≠ 0 were predicted to have appreciable intensity even assuming that the dipole transition moment coincides perfectly with the NNO axis. One such transition was tentatively assigned in the observed spectra, but confirmation will require further work.
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33.20.Ea Infrared spectra
33.20.Vq Vibration-rotation analysis

1s22p3 and 1s22s23l, l = s,p,d, excited states of boron isoelectronic series from explicitly correlated wave functions

F. J. Gálvez, E. Buendía, and A. Sarsa

J. Chem. Phys. 123, 034302 (2005); http://dx.doi.org/10.1063/1.1961384 (9 pages) | Cited 5 times

Online Publication Date: 26 July 2005

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For some members of the boron isoelectronic series and starting from explicitly correlated wave functions, six low-lying excited states have been studied. Three of them arise from the 1s22p3 configuration, and the other three from the 1s22s23l, l = s,p,d, configurations. This work follows a previous one on both the 1s22s22p-math ground state and the four excited states coming from the 1s22s2p2 configuration. Energies, one- and two-body densities in position space and some other two-body properties in position and momentum spaces have been obtained. A systematic analysis of the energetic ordering of the states as a function of the total orbital angular momentum and spin is performed in terms of the electron-nucleus and electron-electron potential energies and the role of the angular correlation is discussed. All calculations have been carried out by using the Monte Carlo algorithm.
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31.15.vj Electron correlation calculations for atoms and ions: excited states
31.15.-p Calculations and mathematical techniques in atomic and molecular physics

Photodissociation of the water dimer: Three-dimensional quantum dynamics studies on diabatic potential-energy surfaces

Loredana Valenzano, Marc C. van Hemert, and Geert-Jan Kroes

J. Chem. Phys. 123, 034303 (2005); http://dx.doi.org/10.1063/1.1961614 (11 pages) | Cited 10 times

Online Publication Date: 26 July 2005

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The results are presented of three-dimensional model studies of the photodissociation of the water dimer following excitation in the first absorption band. Diabatic potential-energy surfaces are used to investigate the photodissociation following excitation of the hydrogen bond donor molecule and of the hydrogen bond acceptor molecule. In both cases, the degrees of freedom considered are the two OH-stretch modes of the molecule being excited, and the dimer stretch vibration. The diabatic potentials are based on adiabatic potential surfaces computed with the multireference configuration-interaction method, and the dynamics of dissociation was studied using the time-dependent wave-packet method. The dynamics calculations yield a donor spectrum extending over roughly the same range of frequencies as the spectrum of the water monomer computed at the same level of theory. The acceptor spectrum has the same width as the monomer spectrum, but is shifted to the blue by 0.4–0.5 eV. The dimer spectrum obtained by averaging the donor and the acceptor spectrum is broader than the monomer spectrum, with the center of the dimer first absorption band shifted to the blue by about 0.2 eV relative to the monomer band. Our reduced dimensionality calculations do not find the red tail predicted for the dimer first absorption band by Harvey et al. [J. Chem. Phys. 109, 8747 (1998) ]. This conclusion also holds if preexcitation of the dimer stretch vibration with one or two quanta is considered.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
31.50.Df Potential energy surfaces for excited electronic states
31.15.vn Electron correlation calculations for diatomic molecules
33.70.Jg Line and band widths, shapes, and shifts
33.15.Fm Bond strengths, dissociation energies
33.15.Mt Rotation, vibration, and vibration-rotation constants

Aminomethanol water elimination: Theoretical examination

Michael T. Feldmann, Susanna L. Widicus, Geoffrey A. Blake, David R. Kent, and William A. Goddard

J. Chem. Phys. 123, 034304 (2005); http://dx.doi.org/10.1063/1.1935510 (6 pages) | Cited 2 times

Online Publication Date: 27 July 2005

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The mechanism for the formation of hexamethylenetetraamine predicts the formation of aminomethanol from the addition of ammonia to formaldehyde. This molecule subsequently undergoes unimolecular decomposition to form methanimine and water. Aminomethanol is the predicted precursor to interstellar glycine, and is therefore of great interest for laboratory spectroscopic study, which would serve as the basis for observational searches. The height of the water loss barrier is therefore useful in the determination of an appropriate experimental approach for spectroscopic characterization of aminomethanol. We have determined the height of this barrier to be 55 kcal/mol at ambient temperatures. In addition, we have determined the infinite-pressure Rice–Ramsperger–Kassel–Marcus unimolecular decomposition rate to be <10−25 s−1 at 300 K, indicating gas-phase kinetic stability for typical laboratory and hot core temperatures. Therefore, spectroscopic characterization of and observational searches for this molecule should be straightforward provided an efficient formation mechanism can be found.
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82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.20.Pm Rate constants, reaction cross sections, and activation energies

Gas-phase electronic spectra of C18 and C22 rings

A. E. Boguslavskiy, H. Ding, and J. P. Maier

J. Chem. Phys. 123, 034305 (2005); http://dx.doi.org/10.1063/1.1961564 (7 pages) | Cited 12 times

Online Publication Date: 28 July 2005

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The electronic spectra of C18 and C22 in the 15 150–36 900 cm−1 range have been detected in the gas phase by a mass-selective resonant two-color two-photon ionization technique coupled to a laser ablation source. The spectra were assigned to several electronic systems of monocyclic cumulenic isomers with a D9h symmetry for C18 and D11h for C22, based on time-dependent-density-functional calculations and reactivity with respect to H2. The best cooling conditions were achieved with Kr as the buffer gas, and the origin of the math1A2″←math1A1′ transition of C18 at 592.89 nm shows a pair of 1 cm−1 broadbands spaced by 1.5 cm−1. The next electronic transitions exhibited much broader, ∼ 30 (in the visible) to 200 cm−1 (in ultraviolet range), features. The spectrum of C22 exhibits an absorption pattern similar to C18, except that the narrow features to the red are missing; the oscillator strength of the math←math transition is predicted to be low.
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36.40.Mr Spectroscopy and geometrical structure of clusters
33.80.Wz Other multiphoton processes
33.80.Eh Autoionization, photoionization, and photodetachment
33.15.Ta Mass spectra
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
31.15.E- Density-functional theory

Electronic structure and magnetic properties of small manganese oxide clusters

Myung Joon Han, Taisuke Ozaki, and Jaejun Yu

J. Chem. Phys. 123, 034306 (2005); http://dx.doi.org/10.1063/1.1953387 (5 pages) | Cited 7 times

Online Publication Date: 28 July 2005

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To investigate the electronic structure and magnetic properties of manganese oxide clusters, we carried out first-principles electronic structure calculations for small MnO clusters. Among various structural and magnetic configurations of the clusters, the bulklike [111]-antiferromagnetic ordering is found to be favored energetically, while the surface atoms of the clusters exhibit interesting electronic and magnetic characteristics which are different from their bulk ones. The distinct features of the surface atoms are mainly attributed to the reduction of Mn coordination numbers and the bond-length contractions in the clusters, which may serve as a key factor for the understanding of physical and chemical properties of magnetic oxide nanoparticles.
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61.46.-w Structure of nanoscale materials
75.50.Tt Fine-particle systems; nanocrystalline materials
71.15.-m Methods of electronic structure calculations
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)

Photofragment angular momentum distribution beyond the axial recoil approximation: The role of molecular axis rotation

Vladislav V. Kuznetsov and Oleg S. Vasyutinskii

J. Chem. Phys. 123, 034307 (2005); http://dx.doi.org/10.1063/1.1953487 (10 pages) | Cited 22 times

Online Publication Date: 28 July 2005

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We present the quantum-mechanical expressions for the recoil angle dependence of the photofragment multipole moments which explicitly treat the role of molecular axis rotation on the electronic angular momentum polarization of the fragments. The paper generalizes the result of Siebbeles et al. [J. Chem. Phys. 100, 3610 (1994) ] to the case of dissociation of rotating molecules. The electronic wave function of the molecule was used in the adiabatic body-frame representation. The obtained rigorous expressions for the fragment state multipoles have been explicitly derived from the scattering wave-function formalism and then simplified using the quasiclassical approximation in the high-J limit. Possible radial and Coriolis nonadiabatic interactions have been taken into consideration. It is shown that the rotation of the molecular axis is described by a number of rotation factors which depend on the rank of the incident-photon polarization matrix, on the dissociation mechanism, and on the classical angle of rotation of the molecular axis γ.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
33.15.Mt Rotation, vibration, and vibration-rotation constants
03.65.-w Quantum mechanics
33.20.Sn Rotational analysis
back to top Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

Nature of the entropy versus self-diffusivity plot for simple liquids

Charanbir Kaur, Upendra Harbola, and Shankar P. Das

J. Chem. Phys. 123, 034501 (2005); http://dx.doi.org/10.1063/1.1942488 (6 pages) | Cited 6 times

Online Publication Date: 22 July 2005

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The empirical relation (D*)α = a exp[S] between the self-diffusion coefficient D* and the excess entropy S of a liquid is studied here in the context of theoretical model calculation. The coefficient α is dependent on the interaction potential and shows a crossover at an intermediate density, where cooperative dynamics become more important. Around this density a departure from the Stokes–Einstein relation is also observed. The above relation between entropy and diffusion is also tested for the scaled total diffusion coefficient in a binary mixture.
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65.20.-w Thermal properties of liquids
66.10.C- Diffusion and thermal diffusion
61.20.Ne Structure of simple liquids

Relaxation dynamics in (HF)x(H2O)1−x solutions

R. Angelini, P. Giura, G. Monaco, G. Ruocco, and F. Sette

J. Chem. Phys. 123, 034502 (2005); http://dx.doi.org/10.1063/1.1949193 (7 pages)

Online Publication Date: 22 July 2005

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The high-frequency dynamics of (HF)x(H2O)1−x solutions has been investigated by inelastic x-ray scattering. The measurements have been performed as a function of the concentration in the range x = 0.20–0.73 at fixed temperature T = 283 K. The results have been compared with similar data in pure water (x = 0) and pure hydrogen fluoride (x = 1). A viscoelastic analysis of the data highlights the presence of a relaxation process characterized by a relaxation time and a strength directly related to the presence of a hydrogen-bond network in the system. The comparison with the data on water and hydrogen fluoride shows that the structural relaxation time continuously decreases at increasing concentration of hydrogen fluoride passing from the value for water to the one for hydrogen fluoride ταHF, which is three times smaller. This is the consequence of a gradual decreasing number of constraints of the hydrogen-bond networks in passing from one liquid to the other.
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61.25.Em Molecular liquids
78.70.Ck X-ray scattering

NMR relaxation parameters of a Lennard-Jones fluid from molecular-dynamics simulations

Jean-Philippe Grivet

J. Chem. Phys. 123, 034503 (2005); http://dx.doi.org/10.1063/1.1955447 (9 pages) | Cited 9 times

Online Publication Date: 22 July 2005

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Ensembles of soft spheres or of Lennard-Jones atoms were studied by molecular dynamics at reduced temperatures from 0.8 to 3, and radial distribution functions, diffusion coefficients, and magnetic dipole-dipole correlation functions were measured as functions of system size. The expected relation between the values of the correlation functions at zero lag time and the integrals of the radial distribution was verified for each system. The measured correlation functions were compared with theoretical expressions derived by [ Ayant et al., J. Phys. (Paris) 36, 991 (1975) ] and by [ Hwang and Freed, J. Chem. Phys. 63, 4017 (1975) ]. It was shown that, in order to recover the long-time behavior characteristic of diffusion-controlled relaxation processes, the simulation must comprise at least 10 000 particles. By fitting the simulation results to the Hwang-Freed function, independent values of the diffusion coefficient were obtained, similar but not identical to those computed using the Green-Kubo formalism. The spectral densities of the dipole-dipole interaction were computed as Fourier transforms of the correlation functions. These quantities are less sensitive to model imperfections and reproduce quite well the values derived from theory. The dimensionless spin-lattice and spin-spin relaxation rates were derived from the spectral densities. It was shown that the spin-lattice (longitudinal) relaxation rate goes through a maximum as the temperature increases, while the spin-spin (transverse) rate decreases monotonously.
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76.60.Es Relaxation effects
61.20.Ja Computer simulation of liquid structure
66.10.C- Diffusion and thermal diffusion
61.25.Em Molecular liquids

Generalized Langevin theory on the dynamics of simple fluids under external fields

T. Yamaguchi, T. Matsuoka, and S. Koda

J. Chem. Phys. 123, 034504 (2005); http://dx.doi.org/10.1063/1.1955455 (12 pages) | Cited 6 times

Online Publication Date: 22 July 2005

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A theory on the time development of the density and current fields of simple fluids under an external field is formulated through the generalized Langevin formalism. The theory is applied to the linear solvation dynamics of a fixed solute regarding the solute as the external field on the solvent. The solute-solvent-solvent three-body correlation function is taken into account through the hypernetted-chain integral equation theory, and the time correlation function of the random force is approximated by that in the absence of the solute. The theoretical results are compared with those of molecular-dynamics (MD) simulation and the surrogate theory. As for the transient response of the density field, our theory is shown to be free from the artifact of the surrogate theory that the solvent can penetrate into the repulsive core of the solute during the relaxation. We have also found a large quantitative improvement of the solvation correlation function compared with the surrogate theory. In particular, the short-time part of the solvation correlation function is in almost perfect agreement with that from the MD simulation, reflecting that the short-time expansion of the theoretical solvation correlation function is exact up to t2 with the exact three-body correlation function. A quantitative improvement is found in the long-time region, too. Our theory is also applied to the force-force time correlation function of a fixed solute, and similar improvement is obtained, which suggests that our present theory can be a basis to improve the mode-coupling theory on the solute diffusion.
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61.20.Ja Computer simulation of liquid structure

Phase and interface behaviors in type-I and type-V Lennard-Jones mixtures: Theory and simulations

Andrés Mejía, Josep C. Pàmies, Daniel Duque, Hugo Segura, and Lourdes F. Vega

J. Chem. Phys. 123, 034505 (2005); http://dx.doi.org/10.1063/1.1955529 (10 pages) | Cited 15 times

Online Publication Date: 25 July 2005

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Density gradient theory (DGT) and molecular-dynamics (MD) simulations have been used to predict subcritical phase and interface behaviors in type-I and type-V equal-size Lennard-Jones mixtures. Type-I mixtures exhibit a continuum critical line connecting their pure critical components, which implies that their subcritical phase equilibria are gas liquid. Type-V mixtures are characterized by two critical lines and a heteroazeotropic line. One of the two critical lines begins at the more volatile pure component critical point up to an upper critical end point and the other one comes from the less volatile pure component critical point ending at a lower critical end point. The heteroazeotropic line connects both critical end points and is characterized by gas-liquid-liquid equilibria. Therefore, subcritical states of this type exhibit gas-liquid and gas-liquid-liquid equilibria. In order to obtain a correct characterization of the phase and interface behaviors of these types of mixtures and to directly compare DGT and MD results, the global phase diagram of equal-size Lennard-Jones mixtures has been used to define the molecular parameters of these mixtures. According to our results, DGT and MD are two complementary methodologies able to obtain a complete and simultaneous prediction of phase equilibria and their interfacial properties. For the type of mixtures analyzed here, both approaches have shown excellent agreement in their phase equilibrium and interface properties in the full concentration range.
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61.20.Ja Computer simulation of liquid structure
64.60.F- Equilibrium properties near critical points, critical exponents
64.75.-g Phase equilibria
68.03.-g Gas-liquid and vacuum-liquid interfaces

Two-Gaussian excitations model for the glass transition

Dmitry V. Matyushov and C. A. Angell

J. Chem. Phys. 123, 034506 (2005); http://dx.doi.org/10.1063/1.1949211 (12 pages) | Cited 18 times

Online Publication Date: 25 July 2005

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We develop a modified “two-state” model with Gaussian widths for the site energies of both ground and excited states, consistent with expectations for a disordered system. The thermodynamic properties of the system are analyzed in configuration space and found to bridge the gap between simple two-state models (“logarithmic” model in configuration space) and the random energy model (“Gaussian” model in configuration space). The Kauzmann singularity given by the random energy model remains for very fragile liquids but is suppressed or eliminated for stronger liquids. The sharp form of constant-volume heat capacity found by recent simulations for binary mixed Lennard-Jones and soft-sphere systems is reproduced by the model, as is the excess entropy and heat capacity of a variety of laboratory systems, strong and fragile. The ideal glass in all cases has a narrow Gaussian, almost invariant among molecular and atomic glassformers, while the excited-state Gaussian depends on the system and its width plays a role in the thermodynamic fragility. The model predicts the possibility of first-order phase transitions for fragile liquids. The analysis of laboratory data for toluene and o-terphenyl indicates that fragile liquids resolve the Kauzmann paradox by a first-order transition from supercooled liquid to ideal-glass state at a temperature between Tg and Kauzmann temperature extrapolated from experimental data. We stress the importance of the temperature dependence of the energy landscape, predicted by the fluctuation-dissipation theorem, in analyzing the liquid thermodynamics.
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64.70.P- Glass transitions of specific systems
64.70.Q- Theory and modeling of the glass transition
65.20.-w Thermal properties of liquids
61.20.-p Structure of liquids

Diffusion-influenced excited-state reversible transfer reactions, A*+B⇌C*+D, with two different lifetimes: Theories and simulations

Soohyung Park, Kook Joe Shin, Alexander V. Popov, and Noam Agmon

J. Chem. Phys. 123, 034507 (2005); http://dx.doi.org/10.1063/1.1948369 (14 pages) | Cited 7 times

Online Publication Date: 27 July 2005

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We report accurate Brownian simulation results for the kinetics of the pseudo-first-order diffusion-influenced excited-state reversible transfer reaction A*+B⇌C*+D with two different lifetimes using two different propagation algorithms. The results are used to test approximate solutions for this many-particle problem. Available theories fail when one of the two reactions or (decay) rate constants is large. To remedy this situation, we develop two uniform approximations, which are based on introducing a generalized Smoluchowski term into the relaxation-time approximation. The best of these is the extended unified theory of reversible target reactions, which reduces correctly in all limits and exhibits superior agreement with simulations.
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82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
82.20.-w Chemical kinetics and dynamics

Ion association dynamics in aqueous solutions of sulfate salts as studied by Raman band shape analysis

Daisuke Watanabe and Hiro-o Hamaguchi

J. Chem. Phys. 123, 034508 (2005); http://dx.doi.org/10.1063/1.1931660 (7 pages) | Cited 9 times

Online Publication Date: 27 July 2005

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A new perspective is shown on the interaction between the sulfate ion and its counter cation in aqueous solutions. We propose the dynamic exchange model of ion association instead of the conventional static equilibrium model. The concentration dependence of the Raman band shape of the totally symmetric (a1) SO stretch mode of the sulfate ion is investigated systematically for four sulfate ions, MgSO4, (NH4)2SO4, K2SO4, and Li2SO4. The concentration dependence of the a1 Raman band shape in the MgSO4 system is successfully reproduced by the analysis based on the dynamic exchange model. As a result, quantitative information about the extremely dynamic nature of the ion association has been obtained: the mean time between associations is a few picoseconds and the mean lifetime of association is several hundred femtoseconds.
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82.30.Nr Association, addition, insertion, cluster formation
82.20.-w Chemical kinetics and dynamics
82.80.-d Chemical analysis and related physical methods of analysis
78.30.-j Infrared and Raman spectra

On the heat-capacity change of pairwise hydrophobic interactions

Giuseppe Graziano

J. Chem. Phys. 123, 034509 (2005); http://dx.doi.org/10.1063/1.1961476 (4 pages) | Cited 5 times

Online Publication Date: 27 July 2005

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Computer simulations [ S. Shimizu and H. S. Chan, J. Am. Chem. Soc. 123, 2083 (2001); D. Paschek, J. Chem. Phys. 120, 10605 (2004) ] have demonstrated that the heat-capacity change associated with the interaction of two nonpolar spherical particles, at room temperature, shows a complex behavior with a significant maximum at the distance corresponding to the desolvation barrier configuration and a small minimum at the distance corresponding to the contact configuration. Taking advantage of the detailed analysis performed by Paschek, the two-state model of Muller is applied to estimate the energetic strength and the intactness of the H bonds in the hydration shell of a xenon atom and in the concave part of the joint Xe–Xe hydration shell. In both hydration shell regions the H bonds are energetically stronger but more broken than those in bulk water. In addition, those in the concave part of the joint Xe–Xe hydration shell are, in absolute, stronger and more broken. These thermodynamic features coupled to simple geometric arguments allow the calculation of heat-capacity values that are in agreement with those provided by computer simulations for the pairwise Xe–Xe interaction.
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61.20.Ja Computer simulation of liquid structure
65.20.-w Thermal properties of liquids
82.30.Nr Association, addition, insertion, cluster formation
61.50.Lt Crystal binding; cohesive energy

The equation of state of isotropic fluids of hard convex bodies from a high-level virial expansion

X.-M. You, A. Yu. Vlasov, and A. J. Masters

J. Chem. Phys. 123, 034510 (2005); http://dx.doi.org/10.1063/1.1992471 (7 pages) | Cited 5 times

Online Publication Date: 28 July 2005

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We have calculated virial coefficients up to seventh order for the isotropic phases of a variety of fluids composed of hard aspherical particles. The models studied were hard spheroids, hard spherocylinders, and truncated hard spheres, and results are obtained for a variety of length-to-width ratios. We compare the predicted virial equations of state with those determined by simulation. We also use our data to calculate the coefficients of the y expansion [ B. Barboy and W. M. Gelbart, J. Chem. Phys. 71, 3053 (1979) ] and to study its convergence properties. Finally, we use our data to estimate the radius of convergence of the virial series for these aspherical particles. For fairly spherical particles, we estimate the radius of convergence to be similar to that of the density of closest packing. For more anisotropic particles, however, the radius of convergence decreases with increased anisotropy and is considerably less than the close-packed density.
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64.30.-t Equations of state of specific substances
61.20.Gy Theory and models of liquid structure

Molecular simulation of the shear viscosity and the self-diffusion coefficient of mercury along the vapor-liquid coexistence curve

Gabriele Raabe, B. D. Todd, and Richard J. Sadus

J. Chem. Phys. 123, 034511 (2005); http://dx.doi.org/10.1063/1.1955530 (6 pages) | Cited 3 times

Online Publication Date: 28 July 2005

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In earlier work [ G. Raabe and R. J. Sadus, J. Chem. Phys. 119, 6691 (2003) ] we reported that the combination of an accurate two-body ab initio potential with an empirically determined multibody contribution enables the prediction of the phase coexistence properties, the heats of vaporization, and the pair distribution functions of mercury with reasonable accuracy. In this work we present molecular dynamics simulation results for the shear viscosity and self-diffusion coefficient of mercury along the vapor-liquid coexistence curve using our empirical effective potential. The comparison with experiment and calculations based on a modified Enskog theory shows that our multibody contribution yields reliable predictions of the self-diffusion coefficient at all densities. Good results are also obtained for the shear viscosity of mercury at low to moderate densities. Increasing deviations between the simulation and experimental viscosity data at high densities suggest that not only a temperature-dependent but also a density-dependent multibody contribution is necessary to account for the effect of intermolecular interactions in liquid metals. An analysis of our simulation data near the critical point yields a critical exponent of β = 0.39, which is identical to the value obtained from the analysis of the experimental saturation densities.
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61.20.Ja Computer simulation of liquid structure
66.20.-d Viscosity of liquids; diffusive momentum transport
66.10.C- Diffusion and thermal diffusion
64.60.F- Equilibrium properties near critical points, critical exponents
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