bluuebunny/crossref_metadata_2025 huggingface dataset:
Computer simulation of polymers in thin layers. II. Structure of polymer melt layers consisting of end-to-wall grafted chains
Authors: Pakula T. , Zhulina E. B.
Published: July 2002

The cooperative motion algorithm is applied to simulate static properties of a polymer melt confined between two parallel walls, where the melt consists of linear chains which are grafted by one end to one wall. The results characterizing the structure of layers, as well as the conformation and orientation of chains, are obtained for systems with various chain lengths (N=40, 80, and 160) and various grafting densities (σ=0.25 and 0.5). They are compared with respective characteristics of such layers predicted by the self-consistent-field (SCF) theory. A good agreement between the simulated results and the theoretical predictions is observed, especially for longer chains and higher grafting densities, i.e., for the situation closer to that assumed in the theory. The origin of small quantitative differences between simulated and theoretical results is discussed.
Reference interaction site model theory of polymeric liquids: Self-consistent formulation and nonideality effects in dense solutions and melts
Authors: Schweizer Kenneth S. , Honnell Kevin G. , Curro John G.
Published: July 2002

The reference interaction site model (RISM) integral-equation approach to polymeric liquids is generalized to allow a self-consistent determination of single-chain and intermolecular pair correlations. Nonlinear medium-induced effects on intrachain statistics are described at the level of self-consistent pair interactions. Tractable schemes to implement the self-consistency aspect are formulated for semiflexible and rotational isomeric state chain models, and applied numerically to concentrated solutions and melts of semiflexible polymers. Theoretical results are in good agreement with off-lattice molecular dynamics simulations, and a rich dependence of the renormalized persistence length on temperature, aspect ratio, density, and degree of polymerization is found. The general formalism for polymer alloys is sketched and the potentially important role of local density and concentration fluctuations as nonuniversal mechanisms for inducing conformational perturbations is emphasized. A detailed analysis is made of the connections between the polymer RISM integral-equation theory and the field theoretic psuedopotential formalism, and remarkable similarities in the predictions of the two distinct approaches for solutions of long thread-like polymers are demonstrated.
Osmotic pressure and salt exclusion in electrostatically swollen lamellar phases
Authors: Dubois Monique , Zemb Thomas , Belloni Luc , Delville Alfred , Levitz Pierre , Setton Ralph
Published: July 2002

Measurements of osmotic pressure, mainly of electrostatic origin, are reported in the diluted regime of charged bilayers. The interlamellar distances (100–1000 Å) are measured using small-angle neutron scattering. The electrostatic repulsive pressure is evaluated within the framework of the Poisson–Boltzmann model, taking into account the conditions of ionic conservation and electrochemical equilibrium, and checked by Monte Carlo simulations in the grand canonical ensemble. The predicted phenomenon of salt ejection from the bilayers is evidenced by direct chemical analysis. Implications concerning the phase behavior of double-tailed surfactants are discussed.
Monte Carlo study of polymers in equilibrium with random obstacles
Authors: Wu David , Hui Kenneth , Chandler David
Published: July 2002

We have performed Monte Carlo calculations for two-dimensional freely jointed polymers with no excluded volume in equilibrium with a quenched random lattice of obstacles. In addition to the obstacle density, there are two microscopic parameters in the problem: the obstacle side length a and the polymer step length l. Our Monte Carlo calculations extend to N=50 000 monomer polymer units. The calculations begin to exhibit standard Flory–Lifshitz scaling only at extremely large values of N. For example, when l≊a, nonuniversal behavior is found for N<104. For some choices of parameters, this behavior includes a nonmonotonic mean-square end-to-end length R2 as a function of N. These calculations are made feasible by exploiting an equivalence between annealed and quenched disorder valid when the polymer may equilibrate to the quenched material.
Systematic investigation of the phase behavior in binary fluid mixtures. II. Calculations based on the Carnahan–Starling–Redlich–Kwong equation of state
Authors: Kraska Thomas , Deiters Ulrich K.
Published: July 2002

Phase diagrams of binary fluid mixtures have been calculated from the Carnahan–Starling–Redlich–Kwong equation of state in connection with standard quadratic mixing rules. The phase diagrams were classified according to the system of van Konynenburg and Scott and then used to construct global phase diagrams showing the extent of the various phase diagram classes in the space of the parameters of the equation of state. For molecules of equal size, the global phase diagram is rather similar to that of the Redlich–Kwong or the van der Waals equation. For molecules of different sizes, however, a new tricritical line appears. Such a behavior is observed for cubic equations of state only if nonadditive covolumes are assumed. Along this new tricritical line, some unusual phase diagrams involving four-phase states and high-density instabilities can be found. The influence of molecular size ratios on the global phase diagrams and the relationship of the equation of state of this work to the ternary symmetric lattice gas and the van der Waals lattice gas are discussed.
Three-dimensional photodissociation dynamics of methyl iodide
Authors: Guo Hua
Published: July 2002

The photodissociation dynamics of methyl iodide is investigated on a three-dimensional grid using a time-dependent quantum-mechanical method. Two electronic states which correlate with I and I* fragments are explicitly included in the calculation. The potential-energy functions and the nonadiabatic coupling are adapted from a recent ab initio calculation by Morokuma and co-workers. The dynamically active degrees of freedom include the dissociation coordinate, the umbrella bend of the methyl group which is treated as a stretch between a hypothetic atom X (X=H3) and the carbon atom, and the H3–C–I bend. The discrete variable representation is used to describe the dynamics in the bending coordinate θ while the other two degrees of freedom are treated by a fast Fourier transform (FFT) based approach. The time propagation of the wave packet is carried out using the Chebychev expansion of the time propagator and the grid in the translational coordinate space is shifted during the propagation to avoid reflection at the end of the grid. Absorption spectra calculated from these ab initio surfaces are found to be blue shifted by a few thousand wave numbers and have broader widths when compared with experimental results. These discrepancies can be attributed to the topology of the ab initio potential-energy surfaces in the Franck–Condon region. Vibrational and rotational distributions for both the CH3 and CD3 fragments are calculated at several photon wavelengths. The agreement with experimental results is excellent. The vibration of the methyl fragment has a weak dependence on the photon wavelength and is found to be largely separable from its rotation. The rotational distribution of the methyl fragment is cold in the I* channel, but relatively hot for the lower I channel. The I* yield is strongly influenced by the initial excitation scheme. When both excited states are considered to be optically active, the calculated I* yield gives a better agreement with experimental observations. Our calculations are also compared with several previous theoretical works.
Structure and dynamics of the H2O–HCN dimer
Authors: Gutowsky H. S. , Germann T. C. , Augspurger J. D. , Dykstra C. E.
Published: July 2002

This report extends an earlier microwave study of the H2O –HCN weakly bonded dimer by Legon [Proc. R. Soc. London, Ser. A 396, 405 (1984)]. We have resolved the H–H(H2O) hyperfine structure (hfs) in rotational transitions of H2O–HC15N and the 17O hfs in H217O–HC15N, using a modified Balle/Flygare Fourier transform microwave spectrometer with a pulsed supersonic nozzle as the sample source. Also, the rotational constants of H2O–H13CN have been determined. The hfs, particularly that of 17O, and a substitution analysis, are used to clarify the dynamics of the dimer. The analyses support a pseudoplanar, H2O –HCN, C2v structure in which the H2O and HCN experience in-plane and out-of-plane bending vibrations of modest on average amplitude. The out-of-plane H2O bend is 20° and the in-plane is perhaps half that. The bending of the HCN is isotropic, with an amplitude of 9.4° in both directions. The molecular mechanics for clusters (MMC) model was used to explore the potential energy surface (PES) for the weak-bonding coordinates. The calculated equilibrium structure differs greatly from the experimental, with the H2O rotated out of plane by 60° in one direction and the HCN by 20° in the other (cis). The difference is shown by the 17O hfs and its dependence upon the H2O bending to be caused by the zero-point vibrational averaging of the structure, which extends over the shallow symmetric double minimum in the PES. The interaction energy is large (−1590 cm−1 ), but the PES is relatively flat in the bending coordinates over large regions between the equilibrium minima, making the vibrational averages differ substantially from the equilibrium values.
Ultrafast solvent dynamics: Connection between time resolved fluorescence and optical Kerr measurements
Authors: Cho Minhaeng , Rosenthal Sandra J. , Scherer Norbert F. , Ziegler Lawrence D. , Fleming Graham R.
Published: July 2002

The vibrational characteristics of liquid dynamics are used to describe the ultrafast relaxations observed in time-dependent fluorescence Stokes shift [J. Chem. Phys. 95, 4715 (1991)] and heterodyne detected optical Kerr effect measurements on acetonitrile, via a Brownian oscillator model. Introducing a frequency distribution of vibrational modes makes it possible to compare the two experiments. The ultrafast decays observed in the fluorescence Stokes shift and optical Kerr signals are produced by destructive superposition of the high frequency, underdamped modes.
Ab initio dispersion coefficients for interactions involving rare-gas atoms
Authors: Thakkar Ajit J. , Hettema Hinne , Wormer Paul E. S.
Published: July 2002

Calculations of the dynamic dipole, quadrupole, and octopole polarizabilities of Ne, Ar, Kr, and Xe are carried out using both time-dependent coupled Hartree–Fock and many-body perturbation theory methods. Dispersion coefficients are calculated for interactions involving these species. The dynamic polarizabilities are combined with previously published dynamic polarizabilities of H, He, H2, N2, HF, and CO to obtain dispersion coefficients for the interactions involving one of these species and one of Ne, Ar, Kr, or Xe. The dipole–dipole dispersion coefficients agree quite well with the best available semiempirical estimates. The isotropic higher multipole coefficients are in reasonable agreement with previous semiempirical estimates where available, and the anisotropic ones are, in most cases, the first reliable ones to appear in the literature. Nonadditive three-body dispersion coefficients for the Ne3, Ar3, Kr3, and Xe3 interactions are also calculated.
Towards an accurate molecular orbital theory for excited states: Ethene, butadiene, and hexatriene
Authors: Serrano-Andrés Luis , Merchán Manuela , Nebot-Gil Ignacio , Lindh Roland , Roos Björn O.
Published: July 2002

A newly proposed quantum chemical approach for ab initio calculations of electronic spectra of molecular systems is applied to the molecules ethene, trans-1,3-butadiene, and trans-trans-1,3,5-hexatriene. The method has the aim of being accurate to better than 0.5 eV for excitation energies and is expected to provide structural and physical data for the excited states with good reliability. The approach is based on the complete active space (CAS) SCF method, which gives a proper description of the major features in the electronic structure of the excited state, independent of its complexity, accounts for all near degeneracy effects, and includes full orbital relaxation. Remaining dynamic electron correlation effects are in a subsequent step added using second order perturbation theory with the CASSCF wave function as the reference state. The approach is here tested in a calculation of the valence and Rydberg excited singlet and triplet states of the title molecules, using extended atomic natural orbital (ANO) basis sets. The ethene calculations comprised the two valence states plus all singlet and triplet Rydberg states of 3s, 3p, and 3d character, with errors in computed excitation energies smaller than 0.13 eV in all cases except the V state, for which the vertical excitation energy was about 0.4 eV too large. The two lowest triplet states and nine singlet states were studied in butadiene. The largest error (0.37 eV) was found for the 2 1Bu state. The two lowest triplet and seven lowest singlet states in hexatriene had excitation energies in error with less than 0.17 eV.
The effect of nonadiabatic coupling in the predissociation dynamics of IBr
Authors: Guo Hua
Published: July 2002

A quantum time-dependent wave-packet method is used to investigate the predissociation dynamics of a model system (IBr). The dissociation is characterized by a two-state model. The effect of nonadiabatic coupling on the absorption and continuum Raman spectra is examined. The dissociation dynamics and nonadiabatic transitions are also studied in the time domain using femtosecond pulses.
Quadratic steepest descent on potential energy surfaces. I. Basic formalism and quantitative assessment
Authors: Sun Jun-Qiang , Ruedenberg Klaus
Published: July 2002

A novel second-order algorithm is formulated for determining steepest-descent lines on potential energy surfaces. The reaction path is deduced from successive exact steepest-descent lines of local quadratic approximations to the surface. At each step, a distinction is made between three points: the center for the local quadratic Taylor expansion of the surface, the junction of the two adjacent local steepest-descent line approximations, and the predicted approximation to the true steepest-descent line. This flexibility returns a more efficient yield from the calculated information and increases the accuracy of the local quadratic approximations by almost an order of magnitude. In addition, the step size is varied with the curvature and, if desired, can be readjusted by a trust region assessment. Applications to the Gonzalez–Schlegel and the Müller–Brown surfaces show the method to compare favorably with existing methods. Several measures are given for assessing the accuracy achieved without knowledge of the exact steepest-descent line. The optimal evaluation of the predicted gradient and curvature for dynamical applications is discussed.
Sound propagation in liquid water: The puzzle continues
Authors: Sciortino Francesco , Sastry Srikanth
Published: July 2002

Experimental and simulation studies of sound propagation in water have observed, at large wave vectors k (k≳0.25 Å−1), a longitudinal sound mode with a velocity of about 3500 m/s, more than twice the hydrodynamic sound velocity. The relation between the hydrodynamic sound mode and the high frequency mode has been the center of contrasting interpretations. In this paper, we report extensive molecular dynamics simulations designed ad hoc to explore the intermediate and low k part of the collective spectrum. We calculate the dispersion relations for longitudinal and transverse collective modes from 0.026 to 1 Å−1 for a range of temperatures. At all temperatures studied, the sound velocity increases with k. At the highest studied temperature, the sound velocity changes from values comparable to hydrodynamic sound velocity to ones observed by neutron scattering experiments. We show that the viscoelastic approximation describes the data satisfactorily. We also perform normal mode analysis of quenched liquid configurations to obtain further information about the behavior observed at intermediate frequencies (50–100 cm−1). We find further positive dispersion of the sound branch at these frequencies and indications which suggest the interaction of the sound branch with localized modes as the origin of such dispersion.
Investigation of the surface morphology of capped CdSe nanocrystallites by 31P nuclear magnetic resonance
Authors: Becerra L. R. , Murray C. B. , Griffin R. G. , Bawendi M. G.
Published: July 2002

Room temperature 31P magic angle sample spinning (MAS) nuclear magnetic resonance (NMR) is used to study the phosphine chalconide species coordinating the surface of CdSe nanocrystallites. Two surface species are identified: trioctylphosphine oxide (TOPO) and trioctylphosphine selenide (TOPSe). The TOPO coordinates Cd surface sites while trioctylphosphine (TOP) coordinates Se to produce surface TOPSe species. Chemical removal of TOPSe linkages produces CdSe nanocrystallites passivated exclusively with TOPO. A double resonance experiment (31P and 77Se) characterizes the TOPSe species. Quantitative studies indicate coordination to nearly all surface Cd atoms and allows the development of a model for the morphology of an average crystallite surface.
The molecular structure of C6: A theoretical investigation
Authors: Hutter Jürg , Lüthi Hans Peter
Published: July 2002

The question of the lowest energy structure of the C6 molecule has been addressed using high level of theory ab initio methods such as the coupled cluster and multiconfiguration perturbation theory approaches. The results show that the linear cumulenic chain and the monocyclic six-membered ring structure with D3h symmetry are very close in energy, indicating that both structures may be observed experimentally. The hexagonal six-membered ring structure of D6h symmetry was confirmed as a saddle point geometry. These results are in partial contradiction with two earlier ab initio studies that either find the distorted hexagon structure [Raghavachari, Whiteside, and Pople, J. Chem. Phys. 85, 6623 (1986)] or the linear cumulenic chain [V. Parasuk and J. Almlöf, J. Chem. Phys. 91, 1137 (1989)] to be the lowest energy structure.
A theoretical model of solvation in continuum anisotropic dielectrics
Authors: Mennucci Benedetta , Cossi Maurizio , Tomasi Jacopo
Published: July 2002

A new formulation of the polarizable continuum method (PCM) is presented to describe the electrostatic interaction between molecular solutes and anisotropic media (e.g., liquid crystals, crystalline matrices). The main features of the PCM (ab initio treatment of solute, cavity of arbitrary shape, dielectric reaction described with apparent charges) are maintained, though in this case polarization charges are present both on the cavity surface and in the bulk of the dielectric. A simplified method, saving about half of the computational time, is also presented.
Microscopic theory of liquid crystal rheology
Authors: Sarman Sten
Published: July 2002

We propose a new expression for the irreversible entropy production of a nematic liquid crystal subject to a velocity gradient. This is done by adding a contribution due to the streaming angular velocity, ω, which is distinct from the contribution from the angular velocity of the director, Ω. This removes the inconsistency between the isotropic fluid entropy production and the liquid crystal entropy production. The new entropy production means that the traditional viscosity coefficients must be replaced by a new set of coefficients. This can be done in a few different ways depending on how one defines the thermodynamic forces and fluxes. We derive equilibrium fluctuation relations for the viscosities by applying linear response theory. One finds that it is very important to select the proper equilibrium ensemble in order to obtain simple expressions, i.e., linear combinations of time correlation function integrals (TCFI’s), for the viscosities. It turns out that the thermodynamic forces must be given external parameters whereas the fluxes must be fluctuating phase functions. This means that one sometimes must use equilibrium ensembles where Ω and ω are constrained to be zero. Most TCFI’s are the same in those ensembles as in ordinary equilibrium ensembles such as the canonical or isokinetic ensemble. There are relations between those TCFI’s that are different. It is particularly convenient to constrain Ω to be zero because this makes a director based coordinate system an inertial frame.

It also prevents the director reorientation from affecting the tails of the time correlation functions. In order to test some of the fluctuation relations numerically, we have evaluated them for a nematic liquid crystal phase of an oblate version of the Gay–Berne fluid. We have compared the ordinary isokinetic ensemble to an ensemble where Ω has been constrained to be zero by performing equilibrium molecular dynamics (EMD) simulations. The results were either the same or satisfied relations between the TCFI’s in the two ensembles. We cross check these results by applying the SLLOD nonequilibrium molecular dynamics (NEMD) algorithm (so named because of its close relationship to the Dolls tensor algorithm) for planar Couette flow. The NEMD estimates and the EMD fluctuation results are consistent. Constraining Ω to be zero also makes it possible to fix the director at different angles relative to the stream lines. In particular, one can calculate the entropy production as a function of the alignment angle. It seems to be minimal very close to the preferred alignment angle.
Spectroscopic properties for the ground states of AuF, AuF+, AuF2, and Au2F2: A pseudopotential scalar relativistic Mo/ller–Plesset and coupled-cluster study
Authors: Schwerdtfeger Peter , McFeaters John S. , Liddell Michael J. , Hrušák Jan , Schwarz Helmut
Published: July 2002

Accurate spectroscopic properties are given for the molecules AuF, AuF+, and AuF2 derived from relativistic coupled-cluster calculations using an energy-adjusted relativistic pseudopotential for gold. Nonrelativistic and relativistic second-order Mo/ller–Plesset calculations are carried out for the dimer Au2F2 to investigate its stability in gas phase and to estimate the solid state stability of AuF. Relativistic effects are discussed for AuF at the coupled-cluster level by comparison with nonrelativistic results. For some of the properties relativistic effects are larger than electron correlation contributions leading to anomalies in the trend down the group 11 series of fluorides. Electric properties are calculated using the finite field method. Vibrational effects on molecular properties are discussed. The AuF coupled-cluster dissociation energy of 3.1 eV is in excellent agreement with the best estimate of Schwarz et al. [Angew. Chem. Int. Ed. Engl. 33, 212 (1994)] obtained from recent mass spectrometric studies (3.2 eV). The thermodynamic stability of AuF is discussed in detail. The calculated data suggest that AuF is not thermodynamically stable towards disproportionation into Au and AuF3.
The anharmonic force field of ethylene, C2H4, by means of accurate ab initio calculations
Authors: Martin Jan M. L. , Lee Timothy J. , Taylor Peter R. , François Jean-Pierre
Published: July 2002

The quartic force field of ethylene, C2H4, has been calculated ab initio using augmented coupled cluster, CCSD(T), methods and correlation consistent basis sets of spdf quality. For the 12C isotopomers C2H4, C2H3D, H2CCD2, cis-C2H2D2, trans-C2H2D2, C2HD3, and C2D4, all fundamentals are reproduced to better than 10 cm−1, except for three cases where the error is 11 cm−1. Our calculated harmonic frequencies suggest a thorough revision of the accepted experimentally derived values. Our computed and empirically corrected re geometry differs substantially from experimentally derived values: Both the predicted rz geometry and the ground-state rotational constants are, however, in excellent agreement with experiment, suggesting revision of the older values. Anharmonicity constants agree well with experiment for stretches, but differ substantially for stretch–bend interaction constants, due to equality constraints in the experimental analysis that do not hold. Improved criteria for detecting Fermi and Coriolis resonances are proposed and found to work well, contrary to the established method based on harmonic frequency differences that fails to detect several important resonances for C2H4 and its isotopomers. Surprisingly good results are obtained with a small spd basis at the CCSD(T) level. The well-documented strong basis set effect on the ν8 out-of-plane motion is present to a much lesser extent when correlation-optimized polarization functions are used. Complete sets of anharmonic, rovibrational coupling, and centrifugal distortion constants for the isotopomers are available as supplementary material to the paper via the World-Wide Web.
Nonequilibrium molecular dynamics study of shear and shear-free flows in simple fluids
Authors: Baranyai András , Cummings Peter T.
Published: July 2002

Nonequilibrium molecular dynamics simulations have been performed in order to compare the characteristics of planar Couette, planar elongation, uniaxial stretching, and biaxial stretching flows in simple fluids at different strain rates. After deriving the periodic boundary conditions for general flow fields and introducing some methodological improvements for elongation flow calculations we simulated the combination of shear and shear-free flows as well. We found that even at high strain rates where simple fluids exhibit strong non-Newtonian behavior (shear-thinning) it is a reasonable approximation to consider the two planar flows to be rotationally equivalent. This is because in planar Couette flow the in-plane normal stress difference of simple fluids is approximately zero even far from equilibrium. Similarly to planar Couette flow, the trace of the pressure tensor and the internal energy vary approximately as function of the 3/2 power of the strain rate in shear free flows. However, the individual diagonal elements of elongation flow pressure tensors deviate considerably from this approximation. In the extension direction the pressure seems to have a minimum in terms of the strain rate in every shear-free flow. We have discussed the implications of these results.
Extension of Gaussian-2 theory to molecules containing third-row atoms Ga–Kr
Authors: Curtiss Larry A. , McGrath Mark P. , Blaudeau Jean-Philippe , Davis Nancy E. , Binning Robert C. , Radom Leo
Published: July 2002

Gaussian-2 (G2) theory has been extended to molecules containing the third-row nontransition elements Ga–Kr. Basis sets compatible with those used in G2 theory for molecules containing first- and second-row atoms have been derived. Spin–orbit corrections for atoms and molecules having spatially degenerate states (2P, 3P for atoms and 2Π for molecules in this work) are included explicitly in the G2 energies. The average absolute deviation from experiment for 40 test reactions is 1.37 kcal/mol. In contrast to the situation for first- and second-row molecules, inclusion of spin–orbit effects is very important in attaining accurate energies for molecules containing third-row atoms. Without spin–orbit effects, the average absolute deviation is 2.36 kcal/mol.
Quantum and classical dynamics of a methyl group with tunneling frequency 3.4 MHz studied by low field NMR
Authors: Cleemput M. Van , Buekenhoudt A. , Gerven L. Van , Horsewill A. J.
Published: July 2002

Nuclear magnetic resonance (NMR) investigations of methyl tunneling in 2,5-dimethyl-1,3,4-thiadiazole are reported. The tunneling frequency (νt = 3.39 MHz) was obtained using low field NMR spectroscopy. By means of rapid field cycling irradiation and relaxation measurements the probabilities of the absorption transitions, responsible for the spectral lines in the low field NMR spectra, can be quantified. The results obtained agree well with the calculated probabilities of rf induced transitions between the eigenstates of a methyl rotor with νt = 3.39 MHz. Measurements of the temperature dependence of νt, the spin conversion time τcon and an analysis of the proton spin lattice relaxation time T1, the latter two revealing the correlation time τc, enabled the study of the methyl group dynamics over a temperature range encompassing both the quantum mechanical and the classical regimes. The dynamical data can be explained well with a threefold hindering barrier of height V3/kB = 1175 K and are compared with existing theoretical models.
An analytical six-dimensional potential energy surface for dissociation of molecular hydrogen on Cu(100)
Authors: Wiesenekker G. , Kroes G. J. , Baerends E. J.
Published: July 2002

A six-dimensional (6D) potential energy surface (PES) describing the molecule–surface interaction in the dissociative chemisorption system H2+Cu(100) is presented. The PES is based on slab calculations performed using the generalized gradient approximation (GGA) of density functional theory (DFT). To allow the use of the PES in dynamics calculations which can test the validity of the DFT/slab approach by comparing with available experiments on dissociative chemisorption, the PES was fit to an analytical form. The fit used describes the orientational dependence of the molecule–surface interaction above the high symmetry sites upto second order in spherical harmonics. The barriers to dissociation calculated for H2 approaching with its molecular axis parallel to the surface are all located in the exit channel. Also, for different impact sites and orientations, the height and the distance to the surface associated with the barrier correlate well with the chemisorption energy of the H-atoms in the sites to which dissociation takes place; the lowest barrier (0.48 eV) is found for dissociation over the bridge site into the hollow sites, the atomic chemisorption energy being highest in the hollow sites.
Free volume properties of a simulated lipid membrane
Authors: Marrink S. J. , Sok R. M. , Berendsen H. J. C.
Published: July 2002

In this paper, an extensive analysis of free volume related properties of a lipid membrane is given. Using percolation theory, and comparing the free volume properties to those of a soft polymer, additional insights are obtained. The analyses are discussed within the framework of the four region model. It is concluded that the four regions have very different free volume properties. The region containing the dense part of the lipid tails resembles a soft polymer membrane to a large extent. The middle part of the membrane is more similar to a low density alkane. The consequences of the computed free volume properties on the permeation process of small penetrants are discussed.
Contracted Gaussian-type basis functions revisited
Authors: Tatewaki Hiroshi , Koga Toshikatsu
Published: July 2002

The contracted Gaussian-type function (CGTF) sets by Huzinaga and co-workers are improved by an extensive optimization of exponents and contraction coefficients for the first-row atoms Li to Ne. The largest improvements in the total energy are 1.043, 0.096, 0.122, 0.23, 0.24, and 0.36 mhartrees, respectively, for the (33/3), (43/4), (53/5), (64/5), (64/6), and (74/7) sets. The virial ratios are considerably improved by the present optimization. The change in exponents and contraction coefficients amounts to 19%. Splitting the valence part of the CGTFs and adding polarization functions, we have examined the effect of the polarization functions on the properties of N2 molecule in self-consistent-field (SCF) and configuration interaction (CI) calculations. Referring to the results of a very large basis set, we confirmed that both in the SCF and CI calculations, polarization functions added to the present CGTF sets almost work as pure polarization functions; no basis set superposition error was found both in the SCF and CI calculations except for (321/21) and its family derived from (33/3) set in the SCF calculations.
Ab initio molecular orbital study of the HCO+O2 reaction: Direct versus indirect abstraction channels
Authors: Hsu C.-C. , Mebel A. M. , Lin M. C.
Published: July 2002

The reaction of HCO with O2 has been studied by ab initio molecular orbital and statistical theory calculations. Both the direct abstraction and the association–elimination processes have been considered. The direct abstraction of H by O2 producing the HO2+CO products was found to be unimportant below 2000 K. The association reaction occurs by the attack of O2 at the C atom to form a vibrationally excited complex, HC(O)OO°, which can undergo two reactions. The first possibility is H migration via TS2 forming HOOCO, which rapidly dissociates into either OH+CO2 via TS4 or HO2+CO via TS3; the latter is energetically less favorable. The second possibility is the direct production of HO2+CO from HC(O)OO° via TS5 in a concerted manner. The barrier of TS5 at the G2M level of theory is 23.5 kcal/mol relative to HC(O)OO; this is the major channel for the reaction. Variational transition state theory and Rice–Ramsperger–Kassel–Marcus calculations have been carried out for the direct abstraction and the indirect metathetical mechanisms, respectively. The calculated total rate constant at 1.5 Torr exhibits a small positive activation energy and its absolute values agree closely with experimental data.
Geometric isomerism in clusters: High resolution infrared spectroscopy of a noncyclic CO2 trimer
Authors: Weida Miles J. , Nesbitt David J.
Published: July 2002

High resolution infrared spectra of a previously unidentified noncyclic isomer of (CO2)3 have been obtained via direct absorption of a 4.3 μm diode laser in a slit jet supersonic expansion. Two vibrational bands (labeled νI and νIII) are observed, corresponding to the two most infrared active linear combinations of the three constituent CO2 monomer asymmetric stretches: νI is redshifted −5.85 cm−1 from the monomer vibrational origin and is predominately a c-type band of an asymmetric top, while νIII is blueshifted +3.58 cm−1 and is predominately an a-type band. Transitions with Ka+Kc=odd (even) in the ground (excited) state are explicitly absent from the spectra due to the zero nuclear spin of CO2; this rigorously establishes that the noncyclic isomer has a C2 symmetry axis. The vibrational shifts and relative intensities of the bands are interpreted via a resonant dipole interaction model between the high-frequency stretches of the CO2 monomers. Rotational constants are determined by fits of transition frequencies to an asymmetric top Hamiltonian. These results are used to determine vibrationally averaged structural parameters for the complex, which is found to be stacked asymmetric but with C2 symmetry about the b inertial axis. The structural parameters are then used to test several trial CO2–CO2 interaction potentials.
Assessment of Gaussian-2 and density functional theories for the computation of enthalpies of formation
Authors: Curtiss Larry A. , Raghavachari Krishnan , Redfern Paul C. , Pople John A.
Published: July 2002

A set of 148 molecules having well-established enthalpies of formation at 298 K is presented. This set, referred to as the G2 neutral test set, includes the 55 molecules whose atomization energies were used to test Gaussian-2 (G2) theory [J. Chem. Phys. 94, 7221 (1991)] and 93 new molecules. The G2 test set includes 29 radicals, 35 nonhydrogen systems, 22 hydrocarbons, 47 substituted hydrocarbons, and 15 inorganic hydrides. It is hoped that this new test set will provide a means for assessing and improving new theoretical models. From an assessment of G2 and density functional theories (DFT) on this test set it is found that G2 theory is the most reliable method both in terms of average absolute deviation (1.58 kcal/mol) and maximum deviation (8.2 kcal/mol). The largest deviations between experiment and G2 theory occur for molecules having multiple halogens. Inclusion of spin–orbit effects reduces the average absolute deviation to 1.47 kcal/mol and significantly improves the results for the chlorine substituted molecules, but little overall improvement is seen for the fluorine substituted molecules. Of the two modified versions of G2 theory examined in this study, G2(MP2,SVP) theory (average absolute deviation=1.93 kcal/mol) performs better than G2(MP2) theory (2.04 kcal/mol). The G2(MP2,SVP) theory is found to perform very well for hydrocarbons, radicals, and inorganic hydrides. Of the seven DFT methods investigated, the B3LYP method has the smallest average absolute deviation (3.11 kcal/mol). It also has a significantly larger distribution of error than the G2 methods with a maximum deviation of 20.1 kcal/mol.
Study of the electronic states of the allyl radical using spin-coupled valence bond theory
Authors: Oliva Josep M. , Gerratt Joseph , Cooper David L. , Karadakov Peter B. , Raimondi Mario
Published: July 2002

The spin-coupled valence bond method is used to study the doublet valence excited states and the n=3 Rydberg excited states of the allyl radical below the first ionization potential 8.13 eV. The calculations included three π electrons in the active space. Our results are close to those resulting from the most extensive MO-CI calculations reported to date. The spin-coupled VB approach has an advantage of providing a compact description of the various states.
Molecular theory of a charged particle in a polarizable nonpolar liquid
Authors: Lado F.
Published: July 2002

A generalization of liquid state theory that treats internal degrees of freedom on the same mathematical footing as orientational degrees of freedom is used to calculate the Maxwell field of a charged particle in a fluid of polarizable nonpolar molecules. The polarizable fluid is modeled by Lennard-Jones molecules with classical Drude oscillators. The electric field E(r) of the charged impurity is given in terms of a screening function that yields the correct dielectric constant at large r and can be computed using standard procedures for the pair correlation function of classical liquid state theory. Sample numerical results using the hypernetted-chain approximation are compared with simulation.
Second-order density-functional description of molecules and chemical changes
Authors: Liu Shubin , Parr Robert G.
Published: July 2002

Based on functional expansion methods recently developed, the total energy of an atomic and molecular system is expanded in terms of its chemical potential (electronegativity), hardness, softness, Fukui function, response function, and so on, up to the second order. Expansions are given within the four basic ensembles in density functional theory: the canonical, grand canonical, isomorphic, and grand isomorphic ensembles. The formulas obtained provide explicit relations among the important quantities related to the chemical reactivity of a system, and they provide a means to calculate one quantity when others are known. Furthermore, a favorable viewpoint of the maximum hardness principle follows from these formulas. At fixed electron number, external potential and chemical potential, the global hardness should go to a maximum as the total energy approaches a minimum. Arguments for the second-order truncation are given.
The water dimer interaction energy: Convergence to the basis set limit at the correlated level
Authors: Schütz Martin , Brdarski Steve , Widmark Per-Olof , Lindh Roland , Karlström Gunnar
Published: July 2002

The water dimer interaction energy and its convergence to the basis set limit was investigated, with electron correlation effects treated at the level of second order Mo/ller-Plesset perturbation theory (MP2). ANO-type and large uncontracted basis sets were used, spreading over a wide range in size; the biggest set included 1046 functions with angular momentum up to (l=7). Core correlation effects were treated accurately by augmenting the original valence basis with extended sets of core polarization functions. The MP2 dimer interaction energy at the basis set limit was determined to −4.94±0.02 kcal/mol, with a contribution due to core correlation of −0.04 kcal/mol. Furthermore, based on some elementary considerations from intermolecular perturbation theory, a simple procedure was devised, which brings the counterpoise corrected interaction energies of moderate basis set calculations closer to the basis set limit. The interaction energies so obtained turned out be surprisingly stable with respect to extensions of the basis set.
Extension of Gaussian-2 (G2) theory to molecules containing third-row atoms K and Ca
Authors: Blaudeau Jean-Philippe , McGrath Mark P. , Curtiss Larry A. , Radom Leo
Published: July 2002

Gaussian-2 (G2) theory has been extended to molecules containing the third-row nontransition elements K and Ca. Basis sets compatible with those used in G2 theory for molecules containing first- and second-row atoms, as well as the third-row elements Ga–Kr, have been derived. As for Ga–Kr, first-order spin–orbit corrections are included explicitly in the G2 energies of K- and Ca-containing atoms and molecules. In contrast to the procedure for Ga–Kr, the 3s and 3p orbitals need to be included in the correlation space for K- and Ca-containing molecules. The geometries obtained with the new basis sets are in satisfactory agreement with experiment. Assessment of the thermochemical predictions is limited because of the sparsity of accurate experimental data for molecules containing K and Ca. Ionization energies are generally in good agreement with experimental values, as are the atomization energies for K-containing molecules. However, there are larger differences between theoretical and experimental atomization energies for Ca-containing molecules.
The resonance Raman spectrum of CH3I: An application of the MCTDH approach
Authors: Gerdts Thorsten , Manthe Uwe
Published: July 2002

The resonance Raman spectrum of CH3I is investigated employing the multi-configurational time-dependent Hartree (MCTDH) approach. A general scheme for the calculation of Raman excitation profiles within a MCTDH representation is presented. This scheme is based on a previously described iterative diagonalization approach suitable for MCTDH-wavefunctions. An empirical four-dimensional potential energy surface is developed for the electronic ground state of methyl iodide. The vibrational states corresponding to excitations in these four modes are calculated up to an energy of about 4000 cm−1. Accurate MCTDH-calculations for the excited, vibronically coupled 3Q0/1Q1 surfaces are performed on the ab initio potential energy surface of Amatatsu, Morokuma, and Yabushita [J. Chem. Phys. 94, 4858 (1991)]. Based on these dynamical calculations, Raman excitation profiles are computed for the different relative polarizations of incident and scattered radiation. The dependence of the depolarization ratio on the amount of C–I stretching excitation is investigated and interpreted.
Tunneling dynamics of side chains and defects in proteins, polymer glasses, and OH-doped network glasses
Authors: Heuer Andreas , Neu Peter
Published: July 2002

Simulations on a Lennard-Jones computer glass are performed to study effects arising from defects in glasses at low temperatures. The numerical analysis reveals that already a low concentration of defects may dramatically change the low temperature properties by giving rise to extrinsic double-well potentials (DWP’s). The main characteristics of these extrinsic DWP’s are (i) high barrier heights, (ii) high probability that a defect is indeed connected with an extrinsic DWP, (iii) highly localized dynamics around this defect, and (iv) smaller deformation potential coupling to phonons. Designing an extension of the standard tunneling model (STM) which parametrizes this picture and comparing with ultrasound experiments on the wet network glass a-B2O3 shows that effects of OH-impurities are accurately accounted for. This model is then applied to organic polymer glasses and proteins. It is suggested that side groups may act similarly like doped impurities inasmuch as extrinsic DWP’s are induced, which possess a distribution of barriers peaked around a high barrier height. This compares with the structurlessly distributed barrier heights of the intrinsic DWP’s, which are associated with the backbone dynamics. It is shown that this picture is consistent with elastic measurements on polymers, and can explain anomalous nonlogarithmic line broadening recently observed in hole burning experiments in polymethylmethacrylate.
The benzene–argon complex: A ground and excited state ab initio study
Authors: Koch Henrik , Fernández Berta , Christiansen Ove
Published: July 2002

Equilibrium dissociation energies De of the benzene–argon van der Waals complex are calculated in the ground state S0 and in the excited state S1 using integral-direct coupled cluster methods. The results confirm previous investigations of S0, showing that high quality correlation consistent basis sets and connected triple excitations are imperative for a good description of the van der Waals complex. We estimate the CCSD(T) dissociation energy De=389±2 cm−1 for the ground state S0. Using the CCSD linear response approach the frequency shift (redshift) δνe=19 cm−1 is obtained. Accurate spectroscopic structural data and frequency shifts δν0 for the 601 band of the S1←S0 transition are available for most of the benzene–rare gas atom complexes. However, the experimental determination of absolute dissociation energies of these complexes is connected with much larger uncertainties. The theoretical result agrees very well with the experimentally available redshift, showing that integral-direct coupled cluster methods will become an important tool in the study of van der Waals complexes in the future.
Extended series expansions for random sequential adsorption
Authors: Gan Chee Kwan , Wang Jian-Sheng
Published: July 2002

We express the coverage (occupation fraction) θ in powers of time t for four models of two-dimensional lattice random sequential adsorption (RSA) to very high orders by improving an algorithm developed by the present authors [J. Phys. A 29, L177 (1996)]. Each of these series is, to the best of our knowledge, the longest at the present. We analyze the series and deduce accurate estimates for the jamming coverage of the models.
C
24
:
Ring or fullerene?
Authors: Jensen Frank , Koch Henrik
Published: July 2002

The energy difference between the ring and fullerene forms of C24 have been calculated by means of ab initio methods, and compared to density functional methods. The calculations strongly suggest that the fullerene form is favored by ∼80 kcal/mol over a monocyclic ring structure, which is at variance with experimental findings. Density functional results vary considerably, although functionals including exact exchange (B3LYP and B3PW91) give reasonable results when basis sets of at least triple zeta quality are employed.