bluuebunny/crossref_metadata_2025 huggingface dataset:
On the contact values of the derivatives of the hard-sphere radial distribution function
Authors: Robles M. , López de Haro M.
Published: July 2002

Approximate analytical expressions for the values of the derivatives of the radial distribution function at contact in a hard-sphere fluid are derived. These expressions are obtained within a rational function approximation (RFA) method recently developed. The results are compared with those based on the Percus-Yevick approximation, as well as with those obtained from expressions for the cavity function inside the core or through computer simulations. While the qualitative dependence on the packing fraction is similar for the values of the first two derivatives at contact in all cases, the one corresponding to the third derivative is definitely different in our approach from that previously reported. The integral moments of the total correlation function as given by the RFA method are also computed.
Phase separations induced by partial stiffening of semiflexible polymers
Authors: Matsuyama Akihiko , Sumikawa Yasuo , Kato Tadaya
Published: July 2002

A simple model is introduced to describe phase separations and nematic ordering in solutions and melts of semiflexible polymer chains, taking into account the partial orientational ordering of flexible bonds on the chain. We obtain the free energy by combining the Flory-Huggins theory for isotropic solutions and the Maier-Saupe model for describing the coupling between the nematic ordering and straightened bonds. Our theory predicts that the partial stiffening of the flexible bonds can be a major cause of miscibility gaps showing a bent chimney shape and triple points. We also discuss the extension of chains in a nematic phase.
Classical-quantum correspondence in the Redfield equation and its solutions
Authors: Kohen D. , Tannor D. J.
Published: July 2002

In a recent paper we showed the equivalence, under certain well-characterized assumptions, of Redfield’s equations for the density operator in the energy representation with the Gaussian phase space ansatz for the Wigner function of Yan and Mukamel. The equivalence shows that the solutions of Redfield’s equations respect a striking degree of classical-quantum correspondence. Here we use this equivalence to derive analytic expressions for the density matrix of the harmonic oscillator in the energy representation without making the almost ubiquitous secular approximation. From the elements of the density matrix in the energy representation we derive analytic expressions for Γ1n(1/T1n) and Γ2nm(1/T2nm), i.e., population and phase relaxation rates for individual matrix elements in the energy representation. Our results show that Γ1n(t)=Γ1(t) is independent of n; this is contrary to the widely held belief that Γ1n is proportional to n. We also derive the simple result that Γ2nm(t)=|n−m|Γ1(t)/2, a generalization of the two-level system result Γ2=Γ1/2. We show that Γ1(t) is the classical rate of energy relaxation, which has periodic modulations characteristic of the classical damped oscillator; averaged over a period Γ1(t) is directly proportional to the classical friction, γ. An additional element of classical-quantum correspondence concerns the time rate of change of the phase of the off diagonal elements of the density matrix, ωnm, a quantity which has received little attention previously. We find that ωnm is time-dependent, and equal to |n−m|Ω(t), where Ω(t) is the rate of change of phase space angle in the classical damped harmonic oscillator. Finally, expressions for a collective Γ1(t) and Γ2(t) are derived, and shown to satisfy the relationship Γ2=Γ1/2. This familiar result, when applied to these collective rate constants, is seen to have a simple geometrical interpretation in phase space.
Further partitioning of the reactant-product decoupling equations of state-to-state reactive scattering and their solution by the time-independent wave-packet method
Authors: Althorpe Stuart C. , Kouri Donald J. , Hoffman David K.
Published: July 2002

The reactant-product decoupling (RPD) equations are a rigorous formulation of state-to-state reactive scattering recently introduced by Peng and Zhang. For an N-arrangement reaction there are a total of N RPD equations, each of which describes the dynamics in just one region of coordinate space. One of the regions (the r-region) encloses the reactant channel and the strong interaction region; each of the other N−1 regions encloses one of the product channels. In this paper we develop a suggestion later made by Kouri and co-workers: that the original RPD equations can be further partitioned into a set of new RPD equations, in which the original r-region is now partitioned into three regions—two enclosing the reactant channel, and one enclosing the strong interaction region. After introducing the new RPD equations, we derive the time-independent wave-packet (TIW) form of the equations, and show how to solve them using an extended version of the Chebyshev propagator. We test the new RPD equations (and the method) by calculating state-to-state reaction probabilities and inelastic probabilities for the three-dimensional (J = 50) H+H2 reaction.
Vibrational deactivation of N2(v=1) by inelastic collisions with He3 and He4: An experimental and a theoretical study
Authors: Reid J. P. , Thakkar A. J. , Barnes P. W. , Archibong E. F. , Quiney H. M. , Simpson C. J. S. M.
Published: July 2002

A new N2–He intermolecular potential with vibrational coordinate dependence is presented. Rate constants for the vibrational deactivation of N2(v=1) by He in the gas phase have been calculated over the temperature range 5–300 K. Accurate values of the rate constants for this process are known down to 100 K. We have now extended these measurements down to 70 K for the deactivation of N214(v=1) by He4 and down to 50 K for the deactivation of N215(v=1) by He3. Agreement between the theoretically calculated and the experimentally determined rate constants is excellent with the calculated values reproducing the experimental measurements within their error bars. An investigation of the low impact energy regime is also presented. While this focuses on collision energies of less than 20 cm−1 and yields rate constants which are in a temperature region inaccessible to our experimental method, it gives further insights into the influence of the attractive well on vibrational energy transfer processes.
Molecular motion and solvation of benzene in water, carbon tetrachloride, carbon disulfide and benzene: A combined molecular dynamics simulation and nuclear magnetic resonance study
Authors: Laaksonen Aatto , Stilbs Peter , Wasylishen Roderick E.
Published: July 2002

Benzene, dissolved in several solvents of varying viscosities and as a neat liquid, is investigated using both MD simulation techniques and NMR spectroscopy. Molecular motions—linear translation, angular rotation and reorientational motion—are studied and related to intermolecular interactions and to the microscopic solvation structure obtained from MD simulation. Self-diffusion coefficients are measured by Fourier transform pulse-gradient spin–echo NMR techniques. The reorientational rotational motion of benzene is studied using 2H NMR T1 relaxation measurements. The 2H T1 measurements also include benzene in solvents: hexane, cyclohexane, chloroform, dodecane and hexadecane. A special emphasis is on the anisotropy of benzene. Attempts are made to interpret the anisotropic behavior in terms of specific solute–solvent interactions and the solvation structure around benzene. Concerning the three solvents—water, carbon tetrachloride and carbon disulfide—a fairly clear and consistent picture emerges from both experimental and theoretical studies. The reorientational motion of benzene is found to be highly anisotropic in water, slightly less anisotropic in carbon tetrachloride, while it is nearly isotropic in carbon disulfide solution. MD simulations show some evidence for hydrogens of water approaching the ring region of benzene.
Structure and phase diagram of mixtures of hard spheres in the limit of infinite size ratio
Authors: Vega Carlos
Published: July 2002

The phase diagram of a mixture of hard spheres for the particular case of infinite size ratio is considered. This is equivalent to the problem of a mixture of points and hard spheres, with the condition that the points can not penetrate the spheres. The equation of state and free energy of this system can be related to that of a pure hard sphere system. This is true for the fluid and for the solid phase. In this way it is shown that in the limit of infinite size ratio, no fluid-fluid separation is possible for binary mixtures of hard spheres and that freezing occurs at the same volume fraction than for pure hard spheres.
A coupled cluster study of the 1 1A1g and 1 1B2u states of benzene
Authors: Christiansen Ove , Stanton John F. , Gauss Jürgen
Published: July 2002

A theoretical investigation of the equilibrium structures and harmonic frequencies of the 1 1A1g and 1 1B2u states of benzene is presented. The performance of coupled cluster singles (CCS), the recently proposed CC2 model, and coupled cluster singles and doubles (CCSD) is compared. The CC2 ground and excited states frequencies are a significant improvement of the CCS results and are relatively close to the CCSD results. A comparative analysis of the vibrations in the two electronic states of both C6H6 and C6D6 is presented. The reliability of predicted shifts in harmonic frequencies between the two states and isotopic shifts is estimated on the basis of the convergence in the CCS, CC2, and CCSD hierarchy of models, and through comparison with related theoretical work. The shifts are used in a critical comparison with experiment. Inconsistent experimental assignments have been scrutinized on the basis of the predicted shifts. The complete harmonic force fields are given for both states.
Exciton superradiance in aggregates: The effect of disorder, higher order exciton-phonon coupling and dimensionality
Authors: Potma Eric O. , Wiersma Douwe A.
Published: July 2002

In this paper a detailed theoretical analysis is presented of the temperature dependent radiative decay in aggregates of pseudoisocyanine (PIC). Our approach extends the original linear exciton-phonon coupling model used by Spano, Kuklinsky, and Mukamel [Phys. Rev. Lett. 65, 212 (1990)] including static disorder and second order exciton-phonon interactions. It is shown that for a one-dimensional exciton model neither of these additional effects alone or in combination with linear electron–phonon coupling can explain the steep rise in radiative lifetime at 40 K observed in the J-aggregate of PIC. However, when the aggregate assembles into a two-dimensional bricklike structure its radiative dynamics can be simulated, with linear exciton-optical phonon coupling as the only source for exciton scattering. Exciton-phonon scattering transfers oscillator strength from the k=0 state to other band states and also generates a nonequilibrium population among the exciton states, which persists during the superradiant decay. These effects together explain the marked temperature dependence of the radiative lifetime of the PIC J aggregate. When disorder limits the coherence length at low temperatures to a few molecules, as seems the case in several light harvesting complexes, the exciton population can equilibrate on the time scale of the superradiance. This situation pertains to the strong collision limit of the master equation, where the radiative decay is insensitive to details of the electron–phonon coupling, but only senses change in the thermal population among the exciton states.
Long range corrections to mixture properties of inhomogeneous systems
Authors: Guo Mingxue , Lu Benjamin C.-Y.
Published: July 2002

The Euler forward difference scheme applied to derive expressions for the long range corrections (LRC) to configurational energy, normal pressure, surface tension, and chemical potential of single-component inhomogeneous systems is extended to these properties of mixtures. The obtained expressions contain two parts and are consistent in their forms with those for pure fluids. Analytical expressions are also obtained for the first part when the Lennard-Jones potential is adopted in the evaluation. Simulations are carried out for four model binary mixtures (ε11=ε22=1.0, ε12=0.75, σ11=σ12=σ22=1.0, and P*=0.068, at T*=1.15 and one composition; ε11=1.0, ε22=0.597, ε12=0.773, σ11=1.0, σ12=0.884, and σ22=0.768, at T*=0.928 and three compositions) and mixtures of argon and krypton with εAr/k=119.8 K, εKr/k=163.1 K, σAr=3.405 Å, and σKr=3.63 Å, at T=119.8 K and three compositions. Comparisons of simulated results with those reported in the literature verify the applicability of the extension.
The total neutron scattering cross section of solid methane in phase II
Authors: Grieger S. , Friedrich H. , Guckelsberger K. , Scherm R. , Press W.
Published: July 2002

The total neutron scattering cross section per molecule σtot of solid methane in phase II was measured for incident neutron energies 0.5 meV⩽En⩽10 eV in the temperature range 0.3 K⩽T⩽19.5 K. The σtot depends on the rotational, translational and vibrational dynamics of the CH4 molecule and shows significant dependence both on En and T. For En above 165 meV, which corresponds to the first vibrational excitation level of CH4, the σtot is equal to 4σp(A/(A+1))2=80 b with proton mass A=1 and a bound cross section σp=80 b of a single proton. For En≈6.5 meV, which is approximately the librational energy of the CH4 rotor, σtot approaches 205 b at all temperatures. This is the cross section of a rigid H4 tetrahedron rotating freely about its fixed center; the effective mass is A=4. For En<6.5 meV, the interference effects of scattering from different protons within the same molecule, sensitive to nuclear spin symmetry, are observed. σtot∼〈I(I+1)〉T with total nuclear spin of the ortho, meta, para molecules I=2, 1 or 0, respectively. For En→0, two limiting cases can be distinguished. At high temperatures, the lower limit of σtot is given by the bound (incoherent) cross section of four protons 4σp=320 b. At low temperature, where only the ground state with I=2 is occupied, σtot=8σp=640 b. The experimental data compare well with a quantum-mechanical model and the mass-tensor approximation. Multiple rotational-translational-vibrational scattering contributions are discussed also.
Rotational spectra of the carbon chain free radicals C10H, C12H, C13H, and C14H
Authors: Gottlieb C. A. , McCarthy M. C. , Travers M. J. , Grabow J.-U. , Thaddeus P.
Published: July 2002

The four carbon chain radicals C10H, C12H, C13H, and C14H have been observed in a pulsed supersonic molecular beam with a Fourier transform microwave spectrometer. The radicals were produced in a discharge through a dilute diacetylene/neon mixture in the throat of a supersonic nozzle. All are linear with Π2 electronic ground states, and all except C14H have resolved lambda-type doubling. For each species at least ten rotational transitions, between 6 and 16 GHz, were measured in the lowest spin component, which is Π3/22 for C10H, C12H, and C14H, and Π1/22 for C13H. Only three spectroscopic constants in the standard Hamiltonian for a molecule in a Π2 state were required to reproduce the spectra to a few parts in 107: an effective rotational constant, a centrifugal distortion constant, and a lambda-type doubling constant. All of the chains here have abundances in the most intense part of the supersonic molecular beam of ⩾5×109 per gas pulse, which suggests that optical transitions of all four may be detectable with present laser techniques. For the carbon chain radicals with an even number of carbon atoms, there is very little change in relative abundance from C6H to C14H.
The Kauzmann paradox interpreted via the theory of frustration- limited-domains
Authors: Kivelson Daniel , Tarjus Gilles
Published: July 2002

The entropy, Sliq(T), of many supercooled liquids decreases strongly and the structural relaxation time increases dramatically as the temperature T is lowered below the melting point, Tm. Below the glass transition temperature, Tg, the system relaxes too slowly for supercooled liquids to equilibrate in the experimental times; however, if the data above Tg are extrapolated to lower T, one finds that the extrapolated Sliq(T)→Sxtal(T) as T→TK, Sxtal(T) being the entropy of the crystal. This phenomenon is known as the “Kauzmann paradox.” If the extrapolation is extended below TK, it may be that the extrapolated Sliq(T)→0 as T→TKc>0; we call this apparent violation of the third law, the “Kauzmann catastrophe.” We discuss these phenomena, as well as the general problem of the entropy of supercooled liquids, in terms of the theory of frustration-limited domains. The apparent vanishing of [Sliq(T)−Sxtal(T)] and the apparent violation of the third law that results from extrapolation to T’s below Tg are consistent with extension of the scaling result predicted by the theory to inappropriately low T’s.
Line-mixing effects in He- and N2-broadened Σ↔Π infrared Q branches of N2O
Authors: Bouanich J.-P. , Hartmann J.-M. , Blanquet G. , Walrand J. , Bermejo D. , Domenech J. L.
Published: July 2002

Two Q branches of N2O near 579.3 and 2798 cm−1 belonging to the 2ν20e−ν21f and ν2+ν3 bands, respectively, of Σ←Π and Π←Σ symmetry, have been studied for He and N2 perturbers at pressures ranging from 0.1 to 2 atm, using a tunable diode laser and a difference-frequency laser spectrometer. To interpret the line-mixing effects in these spectra, we have applied a model based on the energy corrected sudden approximation whose parameters have been only derived from line-broadening data for N2O–He and also from the measured absorption by the Q branches for N2O–N2. This model provides a satisfactory agreement with experimental band shapes, whatever the band, the perturber and the pressure considered. Significantly larger line-mixing effects are shown for N2O–He with respect to N2O–N2. Finally, the assumption made in the calculations to treat separately the couplings in the even and odd j levels appears to have a negligible influence on the resulting band shapes.
Simultaneous handling of dynamical and nondynamical correlation via reduced multireference coupled cluster method: Geometry and harmonic force field of ozone
Authors: Li Xiangzhu , Paldus Josef
Published: July 2002

The geometry and vibrational frequencies of the ozone molecule are studied using the reduced multireference coupled cluster method with singles and doubles (RMR CCSD) relying on a two-electron/two-orbital (2,2) active space. The role played by the choice of the reference space is analyzed in detail by considering both the spin adapted, two-configuration reference space and the complete three-configuration reference space based on the (2,2) active space. It is shown that while the two-reference RMR CCSD approach, using the C2v symmetry-adapted (2,2) active space, provides a better description of the geometry and harmonic frequencies of symmetric vibrational modes than the standard single-reference (SR) CCSD method, the three-reference approach is essential to achieve an accurate description of the harmonic frequency of the asymmetric stretching mode. A unique behavior of ozone in this regard is highlighted by a comparison with other symmetric triatomics. The comparison with results obtained by other methods, including the SR configuration interaction with singles and doubles (CISD), the standard SR CCSD, as well as two-reference and large reference space MR CISD methods, clearly demonstrates the superiority of the RMR CCSD approach. In fact, the performance of a small reference space RMR CCSD method is similar to that of a large reference space MR CISD, thus enabling a compact description of both dynamical and nondynamical correlation effects in states having a multireference character.
The first rotationally resolved spectrum of CH4+
Authors: Signorell R. , Merkt F.
Published: July 2002

The pulsed-field-ionization (PFI) zero-kinetic-energy (ZEKE) photoelectron spectra of CH4 and CD4 have been recorded in the region 100880–104100 cm−1. From the analysis of the photoelectron spectra the first adiabatic ionization potential of CH4 and CD4 has been determined to be (101773±35) cm−1 and (102210±25) cm−1, respectively. A one-dimensional model for the pseudorotation between three equivalent C2v equilibrium structures shows evidence for a fluxional behavior of CH4+.
Hard-sphere mixture excess free energy at infinite size ratio
Authors: Tukur Nasiru M. , Hamad Esam Z. , Mansoori G. Ali
Published: July 2002

This article presents the exact limiting value of the derivative of the excess Helmholtz energy, AE, with respect to molecular size at constant temperature, density and composition for a binary mixture of hard spheres with an infinite size ratio (σ11/σ22→∞); i.e., limσ22→0[(∂AhsE/RT)/∂σ22]T,ρ,x,σ11=(π/2)ρx1x2σ112/(1−(π/6)ρx1σ113). This limiting value is compared with the Mansoori–Carnahan–Starling–Leland (MCSL) and also used to test the limits of some commonly used models in estimating the excess free energy of solvents in mixtures or polymer solutions. The models evaluated include the van Laar, Wilson, Edmond–Ogston, Flory–Huggins, Lacome–Sanchez, Scott–Magat, and Chen et al. It is shown that while the MCSL equation of state produces the same limiting value as the exact value reported here the other mixture models deviate from the exact value. This expression may be utilized to correct the mixture theories at their infinite size ratio limits.
Photodissociation dynamics of H2O at 121.6 nm: Effect of parent rotational excitation on reaction pathways
Authors: Hwang D. W. , Yang X. F. , Harich S. , Lin J. J. , Yang X.
Published: July 2002

Photodissociation dynamics of H2O at 121.6 nm through the B̃ 1A1′ state have been studied using the high-resolution H atom Rydberg tagging technique. Experimental evidences show two different dissociation pathways to form the ground OH (X,v=0) products: dissociation through the B̃–X̃ conical intersection, and dissociation through B̃–Ã Coriolis coupling. By preparing the H2O molecules at higher rotational temperatures, dissociation through the B̃–Ã Coriolis coupling pathway can be enhanced.
Metastable eutectic gas to solid condensation in the Al2O3–SiO2 system
Authors: Rietmeijer Frans J. M. , Karner James M.
Published: July 2002

An analytical and transmission electron microscope study showed that nonequilibrium gas-to-solid condensation in an AlO–SiO vapor yielded solid units with well defined compositional peaks at 11.5 wt % Al2O3 and 41–47 wt % Al2O3. These peaks coincide with metastable eutectic points in the Al2O3–SiO2 phase diagram. The compositions of thermally annealed units support the presence of a (metastable) miscibility gap between ∼15–∼60 wt % Al2O3. The important finding in this analysis is the determinative role of metastable eutectics in nonequilibrium gas-to-solid condensation.
Relationships of shear-induced changes in the potential energy landscape to the mechanical properties of ductile glasses
Authors: Malandro Dennis L. , Lacks Daniel J.
Published: July 2002

The effects of shear on the potential energy landscape are examined with regard to the mechanical properties of ductile glasses. Shear strain is found to cause the disappearance of local potential energy minima, rendering the system mechanically unstable and forcing the system to move to alternate local minima. These mechanical instabilities are characterized by the decrease to zero of several features of the potential energy landscape: a barrier height, the curvature at both the barrier and the local minimum, and the distance from the barrier to the local minimum. From an analysis of the results of different size systems, the mechanical instabilities are shown to be localized to a small number of atoms. The consequences of the mechanical instabilities on the mechanical properties are stress drops which give rise to the yielding and plastic-flow behavior characteristic of ductile glasses; these stress drops are discontinuous at the molecular level, but become continuous at the macroscopic level. A simple model is developed which demonstrates that different stress–strain behaviors are obtained as the characteristics of the mechanical instabilities change. This simple model is also used to elucidate the effects of temperature on the mechanical properties.
Experimental and theoretical study of line mixing in methane spectra. I. The N2-broadened ν3 band at room temperature
Authors: Pieroni D. , Nguyen-Van-Thanh  , Brodbeck C. , Claveau C. , Valentin A. , Hartmann J. M. , Gabard T. , Champion J.-P. , Bermejo D. , Domenech J.-L.
Published: July 2002

Line-mixing effects have been studied in the ν3 band of CH4 perturbed by N2 at room temperature. New measurements have been made and a model is proposed which is not, for the first time, strictly empirical. Three different experimental set ups have been used in order to measure absorption in the 2800–3200 cm−1 spectral region for total pressures in the 0.25–2 and 25–80 atm ranges. Analysis of the spectra demonstrates the significant influence of line mixing on the shape of the Q branch and of the P and R manifolds. A model is proposed which is based on state-to-state collisional transfer rates calculated from the intermolecular potential surface with a semiclassical approach. The line-coupling relaxation matrix is constructed from these data and two additional parameters which are fitted on measured absorption. Comparisons between measurements and spectra computed accounting for and neglecting line mixing are made. They prove the quality of the approach which satisfactory accounts for the effects of pressure and of rotational quantum numbers on the spectral shape under conditions where modifications introduced by line mixing are important. For high rotational quantum number lines, the main features induced by collisions are predicted but some discrepancies remain; the latter may be due to improper line-coupling elements but there is strong evidence that the use of inaccurate line broadening parameters also contributes to errors in calculated spectra.
Effects of halogenation on the ionized and excited states of free-base and zinc porphyrins
Authors: Nguyen Kiet A. , Day Paul N. , Pachter Ruth
Published: July 2002

Ab initio electronic structure theory has been applied to study the effects of β-halogenation on the structure, ionization potentials (IP), and electronic spectra of free-base (PH2) and zinc porphyrins (ZnP). Electronic and infrared (IR) spectra of β-octahalogenated free-base (β-PX8H2; X=F, Cl, Br) and zinc porphyrins (β-ZnPX8; X=F, Cl, Br) are predicted using TDDFT, B3LYP, and CIS methods. Computations of the excitation energies and IPs are also carried out using structures obtained with the B3LYP density functional. Valence IPs of PH2 and ZnP increase significantly upon β-halogenation. Except for the fluoro group, all β-halogen auxochromes slightly redshift the ultraviolet (UV) and visible bands of the free-base and zinc porphyrin chromophores. β-halogenation significantly redshifts the B bands with increasing magnitude upon going down the period. In addition, we gauge the effects of β-halogenation on the thermodynamic stability of ZnP by calculating the metal binding energies for β-ZnPX8. We found that all β-ZnPX8 have lower metal binding energies than the corresponding binding energy predicted for ZnP. The metal binding energies for zinc porphyrins are significantly larger (∼2 eV) than the H2 elimination enthalpies of the corresponding free-base analogs.
Coupled-cluster ab initio investigation of singlet/triplet CH2S isomers and the reaction of atomic carbon with hydrogen sulfide to HCS/HSC
Authors: Ochsenfeld Christian , Kaiser Ralf I. , Lee Yuan T. , Head-Gordon Martin
Published: July 2002

The reaction of C(3P) with H2S to HCS/HSC isomers is investigated with high level ab initio methods. Besides reaction energies to form HCS/HSC, the energetics and properties of different singlet/triplet H2CS reaction intermediates are studied. The combination of ab initio theory and crossed molecular beams experiments leads to the conclusion that mainly HCS is formed under single collision conditions and that the reaction is indirect and proceeds through a thiohydroxycarbene intermediate.
Intermolecular interactions of liquid dichloromethane and equilibrium properties of liquid–vapor and liquid–liquid interfaces: A molecular dynamics study
Authors: Dang Liem X.
Published: July 2002

Extensive molecular dynamics simulations are carried out to study the molecular interactions, liquid states, and liquid/vapor properties of dichloromethane. The study is also extended to the equilibrium properties of the liquid/liquid interface of water-dichloromethane. The intermolecular interactions among water, dichloromethane, and water-dichloromethane are described using our polarizable potential models. The equilibrium properties of liquid dichloromethane, including the radial distribution functions, the intermolecular structural factor, the self-diffusion coefficient, and the dielectric constant, are evaluated. The dielectric constant is computed using Ewald summation techniques and the computed result compared reasonably well with the available experimental data. Properties such as surface tensions and density profiles of liquid/vapor dichloromethane are evaluated. We found that the computed surface tensions for several temperatures are in excellent agreement with experimental data. The computed density profile of the liquid/liquid interface of water-dichloromethane is averaged over 1 ns and we found the computed profile to be quite smooth and stable. The effect of polarization on the liquid/liquid interfacial equilibrium properties is evaluated by computing the dipole moments of water and dichloromethane molecules as a function of the distance normal to the interface. We found that these values deviated significantly from the simulations that are based on nonpolarizable potential models. We attribute these observations to the changes in the electric fields around the water and dichloromethane molecules near the interface.
Photofragment energy distributions and dissociation pathways in dimethyl sulfoxide
Authors: Thorson Gail M. , Cheatum Christopher M. , Coffey Martin J. , Fleming Crim F.
Published: July 2002

Photolysis of dimethyl sulfoxide in a molecular beam with 210 and 222 nm photons reveals the decomposition mechanism and energy disposal in the products. Using vacuum ultraviolet light and a time-of-flight spectrometer, we identify CH3 and CH3SO as primary fragments and CH3 and SO as secondary fragments. From CH3 quantum yield measurements, we find that secondary decomposition is minor for 222 nm photolysis, occurring in only about 10% of the fragments, but it increases to about 30% in the 210 nm photolysis. Laser-induced fluorescence measurements on the B3Σ−←X3Σ− transition of SO in the 235 to 280 nm region determine the internal energy of that photoproduct. We compare our results to a simple statistical model that captures the essential features of the decomposition, predicting both the extent of secondary decomposition and the recoil energy of the primary and secondary methyl fragments.
The Hermite correction method for nonadiabatic transitions
Authors: Adhikari Satrajit , Billing Gert D.
Published: July 2002

We have performed molecular dynamics simulations on a system where electronic transitions are allowed anywhere in configuration space among any number of coupled states. A classical path theory based on the Hermite correction to the Gaussian wave packet expansion, proposed by Gert D. Billing [J. Chem. Phys. 107, 4286 (1997)] has been used. The calculations are carried out on the same model used by J. C. Tully [J. Chem. Phys. 93, 1061 (1990)] and the transition probabilities agree well with corresponding exact quantum mechanical results.
Dipolar host/guest interactions and geometrical confinement at the basis of the stability of one-dimensional ice in zeolite bikitaite
Authors: Fois Ettore , Tabacchi Gloria , Quartieri Simona , Vezzalini Giovanna
Published: July 2002

One-dimensional chains of hydrogen-bonded water molecules are hosted in the noncrossing channels of bikitaite (Li2[Al2Si4O12]⋅2H2O), a rare natural lithium zeolite [C. S. Hurlbut, Jr, Am. Mineral. 42, 792 (1957); Ståhl et al., Zeolites 9, 303 (1989)]. We use first-principles methods to study such a mineral with different (Si, Al) ordering in the zeolite framework. The one-dimensional polymer of nondiffusing water molecules, one chain per channel, is arranged in such a way as to develop a permanent polarization parallel to the zeolite channel. We found that the one-dimensional and entropically unfavored water chain structure is stabilized by dipolar host/guest interactions between the dipolar water chain and the (reversed sign) dipolar framework. A ditrigonal distortion of the hexagonal arrangement of tetrahedra in the aluminosilicate bikitaite structure is probably at the origin of the net polarization of the framework, and is induced by the small-sized Li+ cations.
Molecular N2 chemisorption—specific adsorption on step defect sites on Pt surfaces
Authors: Tripa C. Emil , Zubkov Tykhon S. , Yates John T. , Mavrikakis Manos , Nørskov Jens K.
Published: July 2002

Infrared reflection-absorption spectroscopy and density functional theory, within the generalized gradient approximation, were used to investigate both experimentally and theoretically N2 chemisorption on stepped and smooth Pt surfaces. N2 chemisorption was observed to occur only on the edge atoms of step defect sites in atop configuration by both methods. The calculated vibrational frequency of N2 chemisorbed on Pt(112) step sites (2244 cm−1) is in good agreement with the frequency observed experimentally (2231–2234 cm−1) at saturation coverage on Pt(335) and Pt(779). The predicted small N2 binding energy confirmed its weak chemisorption on Pt surfaces claimed in previous studies. The calculations indicate that N2 decreases and CO increases the work function of the Pt(112) surface. N2 could be coadsorbed with CO below saturation coverage of the steps with CO and there is a charge transfer between the two adspecies through the substrate.
Molecular dynamics simulations in the grand canonical ensemble: Application to clay mineral swelling
Authors: Shroll Robert M. , Smith David E.
Published: July 2002

A grand canonical ensemble molecular dynamics (GMD) simulation method has been adapted to examine the thermodynamics of clay-mineral hydration. In the GMD method, the number of water molecules in the system is treated as a continuous variable for which an equation of motion is established. Fluctuations in the water content at constant chemical potential are investigated using trajectories of this particle number variable. A bias potential may be used to modify the free energy contour along the particle number coordinate. This catalyzes particle fluctuations and greatly improves simulation convergence. Adaptation of the GMD method to treat hydrated clay minerals included the introduction of a local-control technique that fixes the water chemical potential in the clay interlayer region. In addition, a bias-potential feedback algorithm was implemented to improve particle fluctuation efficiency. Information pertaining to the free energy contour, generated during the course of the simulation, was used periodically to enhance the bias potential. This allowed for the utilization of a single input bias potential under a broad range of simulation conditions. The method was used to investigate swelling of a cesium–montmorillonite clay. Measured disjoining pressures showed oscillations that are indicative of crystalline-swelling phase transitions. Integration of the disjoining pressures yielded a swelling free energy profile with distinct free-energy minima for the one- and two-layer hydrates. The results may be compared qualitatively with both clay swelling and surface force apparatus experiments, and with previous simulation studies of simple fluids in slit pores.
Thermodynamics of binary mixture glasses
Authors: Coluzzi Barbara , Mézard Marc , Parisi Giorgio , Verrocchio Paolo
Published: July 2002

We compute the thermodynamic properties of the glass phase in a binary mixture of soft spheres. Our approach is a generalization to mixtures of the replica strategy, recently proposed by Mezard and Parisi, providing a first principle statistical mechanics computation of the thermodynamics of glasses. The method starts from the interatomic potentials, and translates the problem into the study of a molecular liquid. We compare our analytical predictions to numerical simulations, focusing onto the values of the thermodynamic transition temperature and the configurational entropy.
Multicomponent semiconductor material discovery guided by a generalized correlated function expansion
Authors: Rabitz Herschel , Shim Kyurhee
Published: July 2002

A correlated function expansion (CFE) is presented to systematically sample an N-dimensional dual composition-processing variable space to efficiently guide the laboratory discovery complex materials with desirable properties. The CFE breaks down the material properties in terms of the independent, pair and higher order cooperative roles of the composition-processing variables. The CFE is expected to rapidly converge in the N-dimensional space of variables to specify (1) minimally sized hierarchical libraries of materials, and (2) how to utilize the observed properties of the library members to rapidly estimate the material properties throughout the entire composition-processing variable space. As an illustration the material properties (i.e., alloy bond length and the direct optical band gap E0) over the full composition space of the multicomponent semiconductor alloys, GaxIn1−xPyAs1−y, GaxIn1−xAsySb1−y, and GaxIn1−xPySbzAs1−y−z, are expressed through the CFE in terms of existing ternary experimental data. Band gap experimental results for GaxIn1−xPyAs1−y lattice matched to InP and for GaxIn1−xAsySb1−y lattice matched to GaSb are in good agreement with the CFE estimates from ternary input data alone. The alloy GaxIn1−xPySbzAs1−y−z is found to provide more diverse opportunities to achieve desired band gaps while still maintaining the lattice matching conditions by controlling the concentration of Sb at the anion site. For even broader classes of materials the CFE is generic tool designed to guide laboratory syntheses to aid in the discovery of new materials with desired properties.
Ab Initio calculations of [CoY6−nXn]2+ complexes
Authors: Rulı́s̆ek Lubomı́r , Havlas Zdenĕk
Published: July 2002

The complete active space self-consistent field (CASSCF) and multireference second order perturbation theory (CASPT2) calculations of [CoF6]4−, [Co(H2O)6]2+, [Co(NH3)6]2+, [Co(H2O)5X]2+, and [Co(H2O)4X2]2+ complexes (X=CH3OH, CH3SH, CH3NH2) are reported. The potential energy surfaces of 10 lowest quartet states of [Co(H2O)5X]2+ complexes near the equilibrium geometry are calculated and splitting of triple-degenerate T1g4(F), T2g4(F), and T1g4(P) electronic states of [Co(H2O)6]2+ complex induced by the substitution of one or two water ligands is characterized and quantified. It is shown that the energy differences between originally degenerate states are almost invariant to the changes of metal–ligand distances, and despite their proximity, the crossing does not occur. The coefficients of leading configuration of multireference wave functions of [Co(H2O)5X]2+ and [Co(H2O)4X2]2+ complexes are shown to approach unity and the usage of single-reference methods is justified. As a consequence, interaction energies of the studied functional groups with Co2+ are computed also at the HF (Hartree–Fock), DFT (density functional theory), and MP2 (second-order Moller–Plesset) levels. They are compared to CASSCF calculations and to the equivalent calculations done for Zn2+ and Ni2+ ions. The computational methodology for the accurate calculations of various cobalt (II) ionic complexes is described and the implications for the theoretical investigation of interactions of chemically and biologically important functional groups with Co2+ are discussed.
Inclusion behavior between molecular nanotubes and linear polymer chains in aqueous solutions
Authors: Ikeda Eiji , Okumura Yasushi , Shimomura Takeshi , Ito Kohzo , Hayakawa Reinosuke
Published: July 2002

The inclusion behavior between molecular nanotubes and linear polymer chains in aqueous solutions is experimentally investigated by measuring the optical absorption spectra of the solutions with iodine as a probe. It is found that the amount of polymer chains included into nanotubes increases with polymer length, which indicates a molecular recognition of the nanotube for the polymer length in the complex formation. Furthermore, the experimental results are compared with a theory based on the Flory–Huggins lattice model. They are in good agreement with each other and the inclusion energy between the molecular nanotube and the linear polymer chain is evaluated.
Ab initio treatment of electron correlations in polymers: Lithium hydride chain and beryllium hydride polymer
Authors: Abdurahman Ayjamal , Shukla Alok , Dolg Michael
Published: July 2002

Correlated ab initio electronic structure calculations are reported for the polymers lithium hydride chain [LiH]∞ and beryllium hydride [Be2H4]∞. First, employing a Wannier-function-based approach, the systems are studied at the Hartree–Fock level, by considering chains, simulating the infinite polymers. Subsequently, for the model system [LiH]∞, the correlation effects are computed by considering virtual excitations from the occupied Hartree–Fock Wannier functions of the infinite chain into the complementary space of localized unoccupied orbitals, employing a full-configuration-interaction scheme. For [Be2H4]∞, however, the electron correlation contributions to its ground state energy are calculated by considering finite clusters of increasing size modeling the system. Methods such as Mo/ller–Plesset second-order perturbation theory and coupled-cluster singles, doubles and triples level of theory were employed. Equilibrium geometry, cohesive energy and polymerization energy are presented for both polymers, and the rapid convergence of electron correlation effects, when based upon a localized orbital scheme, is demonstrated.
Low temperature measurements of the rate of association to benzene dimers in helium
Authors: Hamon Stéphanie , Le Picard Sébastien D. , Canosa André , Rowe Bertrand R. , Smith Ian W. M.
Published: July 2002

The association of benzene molecules to form dimers has been studied at temperatures between 15 and 123 K in a CRESU (Cinétique de Réactions en Ecoulement Supersonique Uniforme) apparatus with helium as the buffer gas. Second-order rate coefficients (k2) for the formation of dimers have been determined for temperatures between 15 and 88 K. The effective third-order rate coefficients (no study of fall off behavior has been undertaken) obtained by dividing the values of k2 by [He] show a strong negative temperature dependence. Further evidence for the formation of dimers at critical concentrations of benzene has been obtained from a study of the CH+C6H6 reaction and from LIF spectra. Modelling calculations indicate that any systematic errors in the results arising from the formation of higher oligomers are small. Based on a model in which it is assumed that the benzene molecules in the dimer are free to rotate, third-order rate coefficients have been calculated for dimerization and are in good agreement with the experimental values for T⩾30 K, where effects of falloff should not be too great.
Swelling of a model network: A Gibbs-ensemble molecular dynamics study
Authors: Aydt E. M. , Hentschke R.
Published: July 2002

We perform molecular simulations to study the swelling of a model network in contact with different chain-like solvents. The method is Gibbs-ensemble molecular dynamics, which is a molecular dynamics implementation of the concept underlying the Gibbs-ensemble Monte Carlo method. We simulate swelling isotherms for three coarse-grained solvents with one (S1), six (S6), and twelve (S12) centers in the high pressure regime at supercritical temperatures. The simulated swelling isotherms of the S1-solvent exhibit a maximum near the critical pressure which decreases and shifts towards higher pressures with increasing temperature. Based on a modified Flory–Huggins approach, a set of numerically soluble equations is derived in order to reproduce the simulated isotherms. The resulting theoretical isotherms are in very good qualitative agreement with the simulations. Comparison of the different solvents shows also that the swelling ratio decreases when the chain length of the solvent increases.
Ferroelectric polarization imaging using apertureless near-field scanning optical microscopy
Authors: Levy Jeremy , Hubert Charles , Trivelli Angelo
Published: July 2002

This paper reviews the technique of apertureless near-field scanning optical microscopy (ANSOM) and its use in mapping the inhomogeneous ferroelectric polarization in BaxSr1−xTiO3 thin films. A preliminary survey compares ANSOM with fiber-based near-field microscopy, highlighting the advantages and limitations of both methods. Interferometric ANSOM is described in detail, including a practical description of how ANSOM images are acquired. A discussion of the various contrast mechanisms in ANSOM is followed by a prescription for eliminating a certain class of topographic artifacts. For the imaging of polarization in ferroelectric thin films, the linear electro-optic effect provides the central contrast mechanism. High-resolution ANSOM images show the existence of polar nanodomains in BaxSr1−xTiO3 films, providing strong direct evidence of its relaxor character.
Monte Carlo hyperspherical description of helium cluster excited states
Authors: Blume D. , Greene Chris H.
Published: July 2002

The J=0 many-body Schrödinger equation for HeN4 clusters with N=3–10 is solved numerically by combining Monte Carlo methods with the adiabatic hyperspherical approximation. We find ground state and excited state energies for these systems with an adiabatic separation scheme that reduces the problem to motion in a one-dimensional effective potential curve as a function of the hyperspherical radius R. We predict the number of J=0 bound states for these clusters, and also the He+HeN−1 elastic scattering lengths up to N=10. For N=5–10, these are the first such calculations reported.