bluuebunny/crossref_metadata_2025 huggingface dataset:
Small-angle neutron scattering study on weakly charged poly(N-isopropyl acrylamide-co-acrylic acid) copolymer solutions
Authors: Shibayama Mitsuhiro , Tanaka Toyoichi
Published: July 2002

The scattered intensity for weakly charged polymer solutions of a poly(N-isopropyl acrylamide-co-acrylic acid) copolymer (NIPA/AAc) in deuterated water, I(q), was obtained by means of small-angle neutron scattering (SANS) as a function of temperature and polymer fraction, φ, where q is the magnitude of the scattering vector. The scattered intensity was then compared with those for the corresponding polymer gels. It was found that both structure factors were similar to each other and were well described with an Ornstein–Zernike-type scattered intensity function when the temperature was lower than the so-called Θ temperature, which is about 34.6 °C. The correlation length ξ for NIPA/AAc solution was given by ξ∼φ−3/8, indicating a weakly charged polymer solution in a semidilute regime. Above this temperature, both started to have a peak at q≊0.02 Å−1, which increased with temperature. However, a significant difference in I(q), due to the absence or presence of crosslinks, respectively for the solution and the gel, was observed at these temperatures. In the case of polymer solutions, a steep increase in I(q) appeared by approaching q=0, indicating a macroscopic inhomogeneity. On the other hand, I(q) for polymer gels had a finite value at q=0 due to the suppression of macroscopic inhomogeneity originated from the presence of crosslinks. A method to analyze these scattered intensity functions is proposed by comparing with the theory of Borue and Erukhimovich for polyelectrolytes in a poor solvent.
Molecular dynamics modeling of time-resolved fluorescence shifts in liquid solution
Authors: Brown Ross
Published: July 2002

This paper presents equilibrium and nonequilibrium molecular dynamics simulation of the time-dependent solvation shift of the fluorescence of 7-amino 3-methyl 1,4 benzoxazine 2-one in methanol between 200 and 300 K. The linear response theory of solvation is found to work well for this system, even long after the inertial response. The development of the solvation shift is remarkably well correlated with the rotational dynamics of the pure solvent, by an empirical relation due to Maroncelli, Kumar, and Papazyan [J. Phys. Chem. 97, 13 (1993)]. The solute–solvent radial distribution functions show evidence of hydrogen bonding of the solvent to the solute in its excited state. The roles of some factors commonly neglected in simulation of solvation are discussed, e.g., the influence of internal degrees of freedom of the solute.
Investigation of rare gas–oxygen exciplexes
Authors: Neeser S. , Voitik M. , Langhoff H.
Published: July 2002

Mixtures of rare gases (Rg) with oxygen donors at high pressures were excited by an electron beam. The fluorescence spectra in the ultraviolet were investigated. Besides the well-known continua of KrO (180 nm) and XeO (235 nm) several new transitions were identified. They are attributed to the decay of RgO and possibly of Rg2O. The study of the reaction kinetics yielded the rate constants for the most important buildup and decay reactions of KrO and ArO. The inverse lifetime of KrO was determined to be 3 × 106 s−1 for ArO to be <3 × 106 s−1.
Spectral patterns and dynamical bifurcation analysis of highly excited vibrational spectra
Authors: Svitak John , Li Zhiming , Rose John , Kellman Michael E.
Published: July 2002

Spectral patterns associated with recently proposed assignments of Fermi resonance systems are investigated with specific application to the 2:1 Fermi resonance fitting Hamiltonian. It is shown that the spectrum of a pair of resonant modes has characteristic patterns associated with the structure of the corresponding classical phase space. In particular, when a spectral fitting Hamiltonian has a separatrix structure in its classical phase space, the quantum Hamiltonian has an emblematic spectral pattern, a ‘‘dip’’ in the level spacings. This basic pattern is the starting point for an investigation of level patterns corresponding to the bifurcation and catastrophe map classification and associated dynamically based assignments of Fermi resonance Hamiltonians. The 2:1 Fermi resonance Hamiltonian is investigated in detail as a typical system. There are distinctive patterns for polyads from different zones of the catastrophe map classification of the 2:1 system. Conversely, when these patterns occur in an experimental spectrum, then in order to reproduce them in a reasonably behaved spectral fit, it is necessary and sufficient to invoke a resonant coupling term in the fitting Hamiltonian. Spectral fitting therefore gives reliable information about the phase space structure of a molecule. These considerations are used to address the interpretation of recent experimental and theoretical investigations of H2CO and benzophenone vibrational spectra.
Hydrogen dissociation on Pt{100}: Nonlinear power law in hydrogen induced restructuring
Authors: Pasteur A. T. , Dixon-Warren St. J. , King D. A.
Published: July 2002

Using molecular beam sticking probability measurements at desorption temperatures we have made a dynamical measurement of the (1×1) island growth rate and the simultaneous D coverage on the rotated quasihexagonal reconstructed phase (hex-R) during the D-induced hex-R→(1×1) phase transformation on Pt{100}. The island growth rate (rg) is described by a strongly nonlinear power law with respect to the local D coverage on the hex-R phase (θ Dhex) at surface temperatures in the range 150–250 K. The apparent rate is given by rg∝(θ Dhex)4.0±0.5. These kinetics are shown to be consistent with the dependence of the temperature programmed desorption spectra on the surface temperature and rate of D2 impingement during adsorption at ∼245 K. In particular the intensity of the α3 desorption peak is markedly sensitive to these parameters, consistent with its assignment to associative desorption from the (1×1) phase.
Correlation energy density from ab initio first- and second-order density matrices: A benchmark for approximate functionals
Authors: Süle Péter , Gritsenko Oleg V. , Nagy Ágnes , Baerends Evert Jan
Published: July 2002

A procedure has been proposed to construct numerically the exchange-correlation εxc(r) and correlation εc(r) energy densities of density functional theory using the correlated first- and second-order density matrices from ab initio calculations. εc(r) as well as its kinetic and potential components have been obtained for the two-electron He atom and H2 molecule. The way various correlation effects manifest themselves in the form of εc(r) has been studied. The εc(r) have been compared with some density functional local and gradient-corrected models εcmod(r). The investigation of the shape of the model energy densities εcmod(r) has been extended to the Be2 and F2 molecules and the corresponding correlation energies Ec have been calculated and discussed for a number of atomic and molecular systems. The results show the importance of a proper modeling of εc(r) in the molecular bond midpoint region.
Modification of thegaussian−2theoretical model: The use of coupled-cluster energies, density-functional geometries, and frequencies
Authors: Mebel A. M. , Morokuma K. , Lin M. C.
Published: July 2002

A family of modified GAUSSIAN−2 (G2M) calculational schemes have been proposed, based on geometry optimization and vibrational frequency calculations using the hybrid density-functional approach, and electron correlation evaluation using the coupled-cluster methods. The most accurate model, called G2M(RCC), gives the average absolute deviation of calculated atomization energies from experiment for 32 first-row compounds of 0.88 kcal/mol. The other two methods, called G2M(RCC,MP2) and G2M(rcc,MP2), exhibit the average absolute deviations of 1.15 and 1.28 kcal/mol, respectively, and can be used for the calculations of molecules and radicals of larger sizes containing up to six to seven heavy atoms. The G2M(rcc,MP2) model demonstrates an accuracy comparable to that of G2(MP2) and requires less intensive computations than the latter. The preference of the G2M(RCC) methods over the original G2 is expected to be particularly significant for the open shell systems with large spin contamination.
The microwave spectrum and structure of the methanol⋅SO2 complex
Authors: Sun Linghong , Tan Xue-Qing , Oh Jung Jin , Kuczkowski Robert L.
Published: July 2002

The rotational spectra of nine isotopomers of the methanol⋅sulfur dioxide van der Waals complex were observed with a pulsed molecular beam Fourier transform microwave spectrometer. Each rotational transition is split into an A-state (m=0) and an E-state (m=±1) transition due to methyl top internal rotation effects. The A and E transitions show an additional inversion splitting ranging from a MHz to a few tens of MHz in seven of the isotopomers. The inversion splitting is absent in the two S16O18O isotopomers. The center frequencies of the inversion doublets were used in a simultaneous fit of both the A- and E-state transitions, producing rotational constants which allowed a complete determination of the structure of the complex. Analysis of the moments of inertia indicate that the complex has a stacked structure. The center of mass distance between the two monomers is 3.08(5) Å. The effective torsional barrier height is V3=128.6(1) cm−1 based on the assumption that the methyl group rotates against a heavy frame. The dipole moment is μT=1.94(3) D. The inversion motion is discussed based on effects on the splitting associated with isotopic substitution and the transition dipole direction.
Fine-structure transitions in metastable Ne*(3P0,2) colliding with H2, HD, D2, O2, and H2O at thermal energies
Authors: Dehnbostel C. , Feltgen R. , Ferkel H. , Koch A. , Pikorz D.
Published: July 2002

The XUV photon emission following collision-induced fine-structure transitions Ne*(3P0,2)+M →Ne*(1,3P1)+M→Ne(1S)+M+hν(74 nm) has been measured for M=H2, HD, D2, O2, and H2O in a beam-cell experiment. The metastable Ne* velocity ranges from 500 to 1500 m/s. No emission was found for Ne*(3P0)+M. An appropriate detector system enables the determination of absolute 3P2→3P1 transition cross sections. The hierarchy of the cross sections measured parallels that of collision-induced 2P1/2→2P3/2 transition cross sections in Rb(5 2P) and Cs(6 2P) which have comparable fine-structure energy splittings.
All optical triple resonance spectroscopy of the A 1Σ+u state of 7Li2
Authors: Urbanski K. , Antonova S. , Yiannopoulou A. , Lyyra A. M. , Li Li , Stwalley W. C.
Published: July 2002

The A 1Σ+u state of 7Li2 has been studied using all optical triple resonance spectroscopy. Vibrational levels v=27–62 and rotational levels ranging from J=0 to 27 have been observed. This represents a region between previous data from single photon experiments and new data from cold atom photoassociative spectroscopy. Our data was fit to a Dunham expansion and the resulting molecular constants reproduce most observed energies to within 0.050 cm−1. A Rydberg–Klein–Rees potential curve has also been constructed and the calculated eigenvalues are in good agreement with observed energies.
Is the oxywater radical cation more stable than neutral oxywater?
Authors: Xie Yaoming , Allen Wesley D. , Yamaguchi Yukio , Schaefer Henry F.
Published: July 2002

Ab initio quantum mechanical methods have been applied to the distonic oxywater (H2OO+) and conventional hydrogen peroxide (HOOH+) cations. The investigation employed basis sets up to triple-ζ plus double polarization plus f functions (TZ2Pf) and levels of correlation up to coupled-cluster including single, double, and perturbatively treated connected triple excitations [CCSD(T)]. The HOOH+ cation, which is planar, has both trans (C2h) and cis (C2v) conformations, the former predicted to be 8 kcal mol−1 lower in energy. At the highest level of theory, the distonic H2OO+ structure is found to lie 23 kcal mol−1 above the conventional trans form. The barrier separating the oxywater cation from HOOH+ is about 33 kcal mol−1, roughly ten times larger than that for the neutral oxywater species. Accordingly, ionization greatly enhances the stability of the nonconventional oxywater structure. Harmonic vibrational frequencies and their infrared intensities are also reported for the H2O+2 species. Symmetry breaking of Hartree–Fock electronic wave functions is found in HOOH+, which adversely affects certain vibrational frequencies due to nearby singularities in related quadratic force constants. This problem is efficaciously overcome via Brueckner methods [BD and BD(T)].
Ab initio studies on the structures, vertical electron detachment energies, and fragmentation energies of CnN− clusters
Authors: Zhan Chang-Guo , Iwata Suchiro
Published: July 2002

In a recently published report on the time-of-flight mass spectrometry studies and ab initio RHF/3-21G calculations of the CnN− clusters (n=1 to 13), Wang, Huang, Liu, and Zhang claimed that all the CnN− clusters (n=1 to 13) should be linear chains. In the present work, we report the new results obtained from a series of ab initio calculations on the ground states of the CnN− cluster anions. The results calculated at different approximation levels reveal that the geometries of the CnN− determined at the levels without using the polarization functions is qualitatively incorrect, and that the diffuse functions of the two atoms at the two end points of the CnN− chains and the higher-order electron correlation are important in evaluating the vertical electron detachment energies (VDEs) of the CnN− anions. It is concluded from the calculated results that for the singlet ground states, only the linear structures of CN−, C2N−, C3N−, and C5N− are stable, and the linear structures of the other CnN− (n=4 and 6 to 13) are not at the local minima on the potential energy surfaces. The VDEs and fragmentation energies of CnN− (n=1 to 7) in their ground states are evaluated with the MP4SDTQ method at the MP2 geometry using 6-31G(d) except for two end atoms of each chain, to which a set of diffuse sp functions are augmented. The fragmentation energies are also evaluated with the QCISD(T) method using the same basis to verify the convergence of the perturbational results. The calculated VDEs and fragmentation energies all show that the CnN− clusters with odd n are more stable than those with even n, which is consistent with the observed even–odd alternation of the TOF signal intensities.
Damping of perturbation corrections in quasidegenerate situations
Authors: Surján P. R. , Szabados Á.
Published: July 2002

Shifting the pole of the energy denominators from the real axis to the imaginary results in a damping of the equation in degenerate cases. The term-by-term absolute value of the expression provides a simple and useful formula to treat quasidegenerate problems. The power of the new expression is illustrated on several examples calculating the correlation energy of some molecules. The Mo/ller–Plesset results are scarcely affected by the proposed modification in normal cases but significant improvement is observed in quasidegenerate situations. The proposed formula is size consistent and its evaluation does not need extra computational effort.
Temperature and density effects on the high temperature ionic speciation in dilute Na+/Cl− aqueous solutions
Authors: Chialvo Ariel A. , Cummings Peter T. , Simonson J. M. , Mesmer R. E.
Published: July 2002

Molecular simulation of dilute NaCl aqueous solutions is performed to study the Na+/Cl− ion-pair association and the constant of equilibrium between the solvent separated (shared) and contact ion pairs at high temperature. Using the simple point charge, the Pettitt–Rossky, and the Fumi–Tosi models for the water–water, the ion–water, and the ion–ion interactions, we determine the density dependence of the constants along the Tr=T/Tc=1.05 isotherm, and the temperature dependence along the ρr=ρ/ρc=1.5 isochore. The simulation results for the association constant are then compared with the predictions of two recent correlations based on conductance measurements at high temperature. The outcome of the comparison is interpreted in terms of the microstructural behavior of the solvent around the ions and the realism of its dielectric behavior.
Macrostates of classical stochastic systems
Authors: Shalloway David
Published: July 2002

The thermodynamic and dynamic properties of a stochastic system can be determined from the underlying microscopic description once appropriate macroscopic states (‘‘macrostates’’) have been identified. Macrostates correspond to temperature-dependent regions of conformation space that are effectively isolated by potential energy barriers. However, there is no rigorous procedure for defining them and they are generally specified by ad hoc temperature-independent prescriptions. This is inadequate for complicated multidimensional systems like proteins. Here we provide a rigorous definition of macrostates of diffusive stochastic systems by relating the eigenfunction expansion of the Smoluchowski equation to a macrostate expansion via a ‘‘Minimum Uncertainty Condition.’’ We develop a general computational bootstrap procedure for identifying macrostates in multiple dimensions and computing their thermal and dynamic properties. This employs nonlinear ‘‘characteristic packet equations’’ to identify anisotropic Gaussian packets that provide a coarse-grained representation of the equilibrium probability distribution. These provide starting points for a variational method for calculating transition rates between macrostates and for a perturbative method for describing relaxations within macrostates.
An adiabatic exponential perturbation theory for rotationally inelastic scattering
Authors: Curotto E. , Cross R. J.
Published: July 2002

We develop a perturbation theory to treat rotationally inelastic scattering using an adiabatic basis set. The results for Ar+N2 are twice as accurate as those using a diabatic basis set. The theory can be trivially extended to include closed channels. It can also be simply recast into the exact integration of a set of semiclassical coupled equations. In this mode it agrees to better than 1% with the exact quantal results.
Anomalous diffusion of small particles in dense liquids
Authors: Bhattacharyya Sarika , Bagchi Biman
Published: July 2002

We present here a microscopic and self-consistent calculation of the self-diffusion coefficient of a small tagged particle in a dense liquid of much larger particles. In this calculation the solute motion is coupled to both the collective density fluctuation and the transverse current mode of the liquid. The theoretical results are found to be in good agreement with the known computer simulation studies for a wide range of solute–solvent size ratio. In addition, the theory can explain the anomalous enhancement of the self-diffusion over the Stokes–Einstein value for small solutes, for the first time. Further, we find that for large solutes the crossover to Stokes–Einstein behavior occurs only when the solute is 2–3 times bigger than the solvent molecules. The applicability of the present approach to the study of self-diffusion in supercooled liquids is discussed.
A sequential formula for electronic coupling in long range bridge-assisted electron transfer: Formulation of theory and application to alkanethiol monolayers
Authors: Hsu Chao-Ping , Marcus R. A.
Published: July 2002

A recursion relation is formulated for the Green’s function for calculating the effective electron coupling in bridge-assisted electronic transfer systems, within the framework of the tight-binding Hamiltonian. The recursion expression relates the Green’s function of a chain bridge to that of the bridge that is one unit less. It is applicable regardless of the number of orbitals per unit. This method is applied to the system of a ferrocenylcarboxy-terminated alkanethiol on the Au(111) surface. At larger numbers of bridge units, the effective coupling strength shows an exponential decay as the number of methylene(–CH2–) units increases. This sequential formalism shows numerical stability even for a very long chain bridge and, since it uses only small matrices, requires much less computer time for the calculation. Identical bridge units are not a requirement, and so the method can be applied to more complicated systems.
Evidence of quinonoid structures in the vibrational spectra of thiophene based conducting polymers: Poly(thiophene), poly(thieno[3,4-b]benzene), and poly(thieno[3,4-b]pyrazine)
Authors: Cuff Lilee , Kertesz Miklos
Published: July 2002

By combining vibrational spectra and ab initio calculations, we obtained a consistent description of the IR and nonresonant Raman spectra, including intensities, of four thiophene based polymers—undoped and heavily doped poly(thiophene) (PTh), undoped poly(thieno[3,4-b]benzene) (PITN), and poly(thieno[3,4-b]pyrazine) (PThP) for the first time. Predicted spectra for poly(thiophene) agree with experiment very well. Based on the calculated force constants and Badger’s rule, we also estimated the average inter-ring bond lengths of undoped and doped PTh to be 1.47 and 1.42 Å, respectively. The latter leads to an estimated 33% quinonoid character on average for heavily doped PTh. The average inter-ring bond lengths of undoped PITN and PThP, that are consistent with their vibrational spectra, are estimated to be 1.41, and 1.42 Å, respectively. These values showed that undoped PITN and PThP have quinonoid character close to that of heavily doped PTh. Further, we also estimated that, upon doping the average bond lengths of PTh changed by −0.01, 0.11, and −0.05 Å for intra-ring Cβ–Cβ, Cα–Cβ, and inter-ring bonds, respectively. These bond length changes are significantly different from those of Hartree–Fock-type calculations, reflecting significant correlation contributions and are also in conflict with earlier empirical fits of the vibrational spectrum of the highly doped phase of PTh. However, our results are more in line with the generally accepted picture of an aromatic to quinonoid “transition” of the doping process. Furthermore, the counterintuitive downward frequency shifts in the vibrational spectra of PTh upon doping can be explained by the structural change from an essentially aromatic to a partially quinonoid form.
Hydrated proton clusters: Ab initio molecular dynamics simulation and simulated annealing
Authors: Wei Dongqing , Salahub D. R.
Published: July 2002

An ab initio molecular dynamics simulation technique is developed employing the Born–Oppenheimer (BO) approach in the framework of a Gaussian implementation of Kohn–Sham density functional theory (DFT). Simulation results for H5O2+ at 200 K are reported. The density profiles, autocorrelation functions and power spectra are presented. The anharmonic frequencies at 200 K are found to be close to the harmonic frequencies calculated directly from quantum methods at 0 K. Structures of large hydrated proton clusters are optimized. Simulated annealing techniques were employed to search for low energy structures and found to be very useful for clusters with 7–8 water molecules. A few very different structures with ground state energy 1–2 kcal/mol apart are shown. H3O+ is found to be the central unit of a few structures optimized. The ionic hydrogen bond was responsible for the stability of the H9O4+ unit in the large hydrated proton clusters. We also find structures with nascent H5O2+ units at the center whose energy is close to, sometimes even lower than that of the H3O+ centered structures. This can be used to explain the solvation facilitated proton transfer in clusters and in solution. The vibrational frequencies of the structures we optimized are tabulated and compared with the experimental results of Price et al. Questions are raised regarding their prediction of a new feature due to water molecules in the third solvation shell. Some new features have been observed for large clusters with heretofore unpredicted structures.
Phase diagram of the hard biaxial ellipsoid fluid
Authors: Camp Philip J. , Allen Michael P.
Published: July 2002

The phase diagram of fluids of hard biaxial ellipsoids with c/a=10 and b/a∈{1,10}, where a, b, and c are the semi-axes, has been studied using computer simulation. Four homogeneous phases are in evidence: isotropic (I), nematic (N+), discotic (N−) and biaxial (B). First-order isotropic-nematic and isotropic-discotic coexistence lines have been traced out using Gibbs–Duhem integration of the coexistence pressure with respect to the molecular biaxiality. We conclude that the isotropic-nematic transition is greatly weakened by a modest degree of molecular biaxiality, in agreement with several recent theories. The I–N+ and I–N− lines meet two second-order nematic-biaxial and discotic-biaxial lines at the Landau bicritical point. This point is predicted to occur at around the self-dual particle shape, b/a=c/a, and so extensive simulations have been performed at and around this point. Very sluggish behaviour is expected in this region of the phase diagram and so long simulations were required. An estimate of the location of the Landau bicritical point is reported. We also highlight the asymmetry of the phase diagram about b/a=c/a, where the ellipsoid is neither prolate nor oblate.
Manipulation of rovibrational wave packet composition in the Li2 E(1Σg+) shelf state using intermediate state selection and shaped femtosecond laser pulses
Authors: Papanikolas John M. , Williams Richard M. , Leone Stephen R.
Published: July 2002

The composition of a rovibrational wave packet in the E(1Σg+) state of Li2 is controlled using a combination of intermediate state selection and femtosecond optical pulse shaping techniques. Intermediate state selection is accomplished using a cw laser to prepare a steady state population of a single rovibrational level in the A(1Σu+) state of Li2, which acts as the initial state for subsequent ultrafast pump/probe studies in the “shelf” region of the E(1Σg+) state. An initial set of relative amplitudes of the wave packet eigenstates is defined by the intermediate level. Femtosecond optical pulse shaping, performed using a grating pair and a mechanical amplitude mask, is then used to further alter the wave packet composition. Quantitative changes in the Fourier transform of the pump/probe transient demonstrate that combining these two techniques can be a facile approach to manipulating wave packet composition, with the ultimate goal of controlling wave packet evolution into a desired final form.
Control of vector properties in vibrationally mediated photodissociation near asymmetric resonances
Authors: Lee Sungyul
Published: July 2002

Quantum mechanical analysis is presented on the control of the vector properties of the photoproducts by vibrationally mediated photodissociation of OH. The angular distributions and alignment of O(3P) fragments are calculated near isolated and overlapping asymmetric resonances. The vector properties depend very sensitively on the vibrational levels (νi=0–4) of the initial X 2Π state. The variations of the anisotropy parameters as a function of the excitation energy near the asymmetric resonances change markedly depending on νi. The widths of the variations tend to increase with increasing νi, which is very similar to the corresponding behaviors of the product branching ratios studied earlier [J. Chem. Phys. 104, 1912 (1996)], indicating that νi could be a useful tool for choosing the proper linewidths in the experiments for the control of the product branching ratios, angular distributions, and m distributions near asymmetric resonances. It is also found that the vector properties may exhibit splitting of the overlapping resonances for high νi, in contrast to scalar properties.
Erratum: “Lattice model of a hydrogen-bonded polymer blend” [J. Chem. Phys. 105, 738 (1996)]
Authors: Hobbie E. K , Han C. C.
Published: July 2002

[S0021-9606(97)00230-4]
Equation of state of a charged bilayer system: Measure of the entropy of the lamellar–lamellar transition in DDABr
Authors: Dubois M. , Zemb Th. , Fuller N. , Rand R. P. , Parsegian V. A.
Published: July 2002

The synthetic cationic double-chain surfactant didodecyldimethylammonium bromide shows two distinct thermodynamically stable lamellar phases; a dilute Lα phase stabilized predominantly by electrostatic forces, and a condensed Lα′ phase stabilized by “hydration” forces. Using six different experimental methods, applying osmotic stress from 102 to 109 Pa and varying temperature from 20 °C to 70 °C, we have measured the osmotic pressure vs interbilayer distance and thus mapped the phase diagram with an equation of state. In this binary system, the area per headgroup as well as bilayer thickness vary with concentration and temperature. Hence, lateral compressibility has to be taken into account in the free energy balance. The osmotic stress needed to effect the swollen-to-collapsed lamellar phase transition is determined as a function of temperature. From these data the entropy of the Lα–Lα′ transition is found to be a strong function of temperature. Below 40 °C, condensation from the dilute Lα phase, the change of entropy is negative as might be expected. Above 40 °C the entropy of condensation is positive, demonstrating that there is a release of degrees of freedom associated with the bilayer condensation. These data allow us to think far more critically about the forces that stabilize bilayer systems.
Variational calculation of macrostate transition rates
Authors: Ulitsky Alex , Shalloway David
Published: July 2002

We develop the macrostate variational method (MVM) for computing reaction rates of diffusive conformational transitions in multidimensional systems by a variational coarse-grained “macrostate” decomposition of the Smoluchowski equation. MVM uses multidimensional Gaussian packets to identify and focus computational effort on the “transition region,” a localized, self-consistently determined region in conformational space positioned roughly between the macrostates. It also determines the “transition direction” which optimally specifies the projected potential of mean force for mean first-passage time calculations. MVM is complementary to variational transition state theory in that it can efficiently solve multidimensional problems but does not accommodate memory-friction effects. It has been tested on model 1- and 2-dimensional potentials and on the 12-dimensional conformational transition between the isoforms of a microcluster of six-atoms having only van der Waals interactions. Comparison with Brownian dynamics calculations shows that MVM obtains equivalent results at a fraction of the computational cost.
Three-body collision contributions to recombination and collision-induced dissociation. II. Kinetics
Authors: T Pack Russell , Walker Robert B. , Kendrick Brian K.
Published: July 2002

Detailed rate constants for the reaction Ne+Ne+H⇌Ne2+H are generated, and the master equations governing collision-induced dissociation (CID) and recombination are accurately solved numerically. The temperature and pressure dependence are explored. At all pressures, three-body (3B) collisions dominate. The sequential two-body energy transfer (ET) mechanism gives a rate that is more than a factor of two too small at low pressures and orders of magnitude too small at high pressures. Simpler models are explored; to describe the kinetics they must include direct 3B rates connecting the continuum to the bound states and to the quasibound states. The relevance of the present reaction to more general CID/recombination reactions is discussed. For atomic fragments, the 3B mechanism usually dominates. For diatomic fragments, the 3B and ET mechanism are competitive, and for polyatomic fragments the ET mechanism usually dominates.
Ginzburg criterion for the crossover behavior of model fluids
Authors: Schröer Wolffram , Weiss Volker C.
Published: July 2002

The Ginzburg criterion, which is based on the three coefficients in the Landau–Ginzburg (LG) expansion of the Helmholtz free energy density of a nonuniform system, is believed to give a reasonable estimate for the temperature scale on which crossover occurs. To compute the contribution of the square-gradient term in the LG expansion, we extend the van der Waals theory of surface tension and, in contrast to our earlier treatment, account for the dependence of the pair distribution function on the spatially varying density. Via this approach we calculate and compare the Ginzburg temperatures of ionic, dipolar, and simple model fluids, namely the restricted primitive model (RPM), the Onsager model, and the square-well fluid (the second and third virial coefficients, for which we also present exact results). To compute the properties of the RPM, we employ the Fisher–Levin theory and its recent extension for Debye-shielded dipole–dipole interactions and a state-dependent dielectric constant that was developed by us. In contrast to the results of our earlier work and in accordance with the calculations of Fisher and Lee, we now find that the RPM has no exceptionally small region in which mean-field theory fails.
Stochastic complexity and statistical mechanics
Authors: Dewey T. Gregory
Published: July 2002

A statistical mechanical formalism based on stochastic complexity is presented. Stochastic complexity is a statistical estimation scheme that uses the principle of minimal description length (MDL). The basic concept is that the best form of statistical estimation is one in which both the data and the model’s structure and parameters are represented in the shortest binary string. This scheme is a generalization that encompasses both the maximum likelihood and maximum entropy methods. To apply this scheme to thermodynamic systems, the minimal number of bits required to describe both the observables and the phase space coordinates of the system is determined. The entropy of a microcanonical ensemble is associated with this minimum description length. This formulation provides a means for calculating partition functions for nonequilibrium systems that are strongly nonergodic. Because of the Bayesian properties of this estimation scheme, techniques for “image reconstruction” of phase space can be developed. These allow partition functions to be calculated from computer simulation data.
Resonance Raman intensity analysis of a dicyanovinyl-azaadamantane: Mode-specific reorganization energies for charge-transfer and locally-excited states
Authors: Lilichenko Mark , Tittelbach-Helmrich Dietrich , Verhoeven Jan W. , Gould Ian R. , Myers Anne B.
Published: July 2002

A resonance Raman intensity analysis is performed on the intramolecular charge-transfer molecule 1-aza-adamantane-4-ylidenemalononitrile in acetonitrile solution. We explore the extent to which changes in molecular structure upon charge transfer can be obtained from resonance Raman intensity analysis, and extend the analysis method for charge-transfer excitation to take into account the possible influence of nearby locally excited states. Absolute scattering cross sections are measured at five excitation wavelengths spanning both the charge-transfer band at 324 nm and the lowest locally excited band at 231 nm, and the absorption spectra and resonance Raman intensities are modeled self-consistently to obtain the mode-specific reorganization energies accompanying electronic excitation to both states. Interference effects between the two states are considered but are found to be of minimal importance for this particular charge-transfer molecule. The reorganization parameters in terms of dimensionless normal coordinates are converted to actual bond length and bond angle changes by making use of a previously developed ground-state normal mode analysis and by comparing with electronic structure calculations on models for the donor and acceptor ends to reduce the indeterminacy in the signs of the dimensionless displacements. The geometry changes upon excitation to the LE state are dominated by lengthening of the ethylenic C=C bond, while for CT excitation the distortions are distributed over the donor, acceptor, and adamantane bridge, with a smaller C=C bond length change.
Finite-size scaling of helix–coil transitions in poly-alanine studied by multicanonical simulations
Authors: Hansmann Ulrich H. E. , Okamoto Yuko
Published: July 2002

We report results from multicanonical simulations of poly-alanine. Homopolymers of up to 30 amino acids were considered and various thermodynamic quantities as a function of temperature calculated. We study the nature of the observed helix–coil transition and present estimates for critical exponents.
Comment on “Simple reversible molecular dynamics algorithms for Nosé–Hoover chain dynamics” [J. Chem. Phys. 107, 9514 (1997)]
Authors: Tuckerman Mark E. , Martyna Glenn J.
Published: July 2002

The leap-frog based integration algorithms for Nosé–Hoover chain dynamics recently presented by Jang and Voth [J. Chem. Phys. 107, 9514 (1997)] are shown to give rise to nonsecular growth in the conserved quantity at long times. Therefore, their use in molecular dynamics simulations should be approached with caution.
Ab initiostudy of the electronic spectrum of the CH3OCH2 radical
Authors: Liu Ruifeng , Maricq M. M. , Li Yumin , Francisco Joseph S.
Published: July 2002

Vertical excitation energies and transition moments have been computed for transitions to low-lying excited states of CH3OCH2 radical. The peak at 300 nm observed in the gas-phase absorption spectrum of CH3OCH2 is assigned to the Ã 2A state and the band observed at 205 nm may be attributed to an electronic transition from doubly occupied lone pair orbitals of oxygen to the singly occupied p orbital of carbon.
Forward–backward initial value representation for semiclassical time correlation functions
Authors: Sun Xiong , Miller William H.
Published: July 2002

The semiclassical (SC) initial value representation (IVR) for the time evolution operator e−iĤt/ℏ involves a phase space integral over the initial conditions of classical trajectories. It is shown in this paper how an IVR for the two time evolution operators in a typical quantum mechanical time correlation function, CAB(t)≡tr[ÂeiĤt/ℏB̂e−iĤt/ℏ], can be combined into one such phase space integral; i.e., time evolution from 0 to t and from t to 0 is combined into one overall SC-IVR propagation. This not only reduces the dimensionality of the phase space average, but the forward–backward (FB) nature of the net trajectory has a partial self-cancellation that reduces the oscillatory behavior of the integrand. Several applications of this FB-IVR to reactive flux correlation functions are presented to illustrate its possibilities.
Density-matrix-spectroscopic algorithm for excited-state adiabatic surfaces and molecular dynamics of a protonated Schiff base
Authors: Tsiper E. V. , Chernyak V. , Tretiak S. , Mukamel S.
Published: July 2002

Excited-state potentials of a short protonated Schiff base cation which serves as a model for the photoisomerization of retinal are computed by combining a semi-empirical ground-state adiabatic surface with excitation energies obtained using the time-dependent coupled electronic oscillator (CEO) approach. Excited-state molecular dynamic simulation of the in-plane motion of cis-C5H6NH2+ following impulsive optical excitation reveals a dominating 1754 cm−1 π-conjugation mode. A new molecular dynamics algorithm is proposed which resembles the Car–Parinello ground-state technique and is based on the adiabatic propagation of the ground-state single-electron density matrix and the collective electronic modes along the trajectory.
On exact and approximate exchange-energy densities
Authors: Springborg Michael , Dahl Jens Peder
Published: July 2002

Based on correspondence rules between quantum-mechanical operators and classical functions in phase space we construct exchange-energy densities in position space. Whereas these are not unique but depend on the chosen correspondence rule, the exchange potential is unique. We calculate this exchange-energy density for 15 closed-shell atoms, and compare it with kinetic- and Coulomb-energy densities. It is found that it has a dominating local-density character, but electron-shell effects are recognizable. The approximate exchange-energy functionals that have been proposed so far are found to account only poorly for the observed behaviors. Instead we use our results in proposing an alternative functional that depends on both first- and second-order derivatives of the electron density.
Lithium perchlorate ion pairing in a model of amorphous polyethylene oxide
Authors: Halley J. W. , Duan Y. , Curtiss L. A. , Baboul A. G.
Published: July 2002

We report a molecular dynamics study of pairing and dynamics of lithium cation and perchlorate anion in a previously reported model of amorphous polyethylene oxide. We are particularly interested in the question of whether these ions pair in the model, as previously reported experimentally. We calculate the potential of mean force between a lithium and perchlorate ion in the system for several temperatures when a pair of ions is at various separation distances in our model. We find evidence for two minima in the potential of mean force, one at lithium–chlorine separations of 3.5 Å and about 6.5 Å. We studied the same system with five ion pairs in the system and again find two minima at the same separation distances but in this case there is evidence of entropic effects in the binding free energy of the pairs at 3.5 Å. A study of radial distribution functions permits us to deduce information concerning the structure of the paired states.
Equation of state for thermodynamic properties of chain fluids near-to and far-from the vapor–liquid critical region
Authors: Jiang Jianwen , Prausnitz John M.
Published: July 2002

Upon incorporation of contributions from long-wavelength density fluctuations by a renormalization-group theory, a crossover equation of state is developed for describing thermodynamic properties of chain fluids. Outside the critical region, the crossover equation of state reduces to the classical equation; inside the critical region, it gives nonclassical universal critical exponents. The crossover equation of state correctly represents phase equilibria and pVT properties of chain fluids in both regions. Good agreement is obtained upon comparisons with computer simulations for square-well chain fluids. As obtained from experimental vapor–pressure and density data, the square-well segment–segment parameters for n-alkanes from ethane to eicosane are linear functions of molecular weight. Calculated thermodynamic properties agree well with experiment for n-alkanes from methane to hexatriacontane.
Fast multiquantum vibrational relaxation of highly vibrationally excited O2
Authors: Jongma Rienk T. , Wodtke Alec M.
Published: July 2002

A significant fraction of the initial population of highly vibrationally excited oxygen molecules X 3Σg−(v⩾23) prepared by stimulated emission pumping, relaxes to much lower vibrational levels (Δv≈−9). The time scale is much shorter than the known collisional lifetimes of the intervening vibrational levels and thus a sequential single-quantum relaxation mechanism can be explicitly ruled out. State-to-state measurements after preparation of v=28 and 30 provide the final-vibrational state population distribution resulting from relaxation of these two states. For v=28(30), at least 38%(7.9%) of the initially prepared population, undergoes multiquantum vibrational relaxation. The observed multiquantum relaxation explains, at least in part, the previously reported “dark channel” for relaxation of vibrational levels higher than v=26, but does not exclude the possibility of reactive scattering (forming ozone) for the remaining fraction of highly vibrationally excited molecules. We discuss possible explanations of this startling result including V–V energy transfer, V–E energy transfer, and complex formation.
An energy-resolved study of the partial fragmentation dynamics of Ar–HCl into H+Ar–Cl after ultraviolet photodissociation
Authors: Juanes-Marcos J. C. , García-Vela A.
Published: July 2002

The UV photolysis of Ar–HCl is simulated by an exact wave packet calculation. Partial fragmentation of the cluster into H and Ar–Cl fragments is studied by projecting out the asymptotic wave packet onto the product states, at several excitation energies in the range of the Ar–HCl absorption spectrum. The partial fragmentation pathway is found to dominate the photolysis process at very low excitation energies, and to be intense also at high energies. At medium excitation energies the other competing fragmentation pathway, namely total fragmentation into H, Ar, and Cl, dominates almost completely the photodissociation dynamics. The relative intensity of the two fragmentation pathways depends on the extent to which the hydrogen is initially blocked by Ar and Cl. The Ar–Cl radicals are produced with high rotational and low vibrational excitation at most of the Ar–HCl energies studied. The internal energy distributions of Ar–Cl show remarkable differences in shape depending on the regions of the absorption spectrum which are excited. This effect can be exploited to control both the efficiency of Ar–Cl generation and the internal excitation of the radical prepared, by changing the excitation energy of the parent cluster.
An energy-based mapping method for identifying the in-plane orientations of polypeptides and other macromolecules at crystalline interfaces
Authors: Dai Yaohua , Spencer Evans John
Published: July 2002

We present an energy-based algorithm, POINTER, which can determine the permissible alignments of a polypeptide (or other macromolecule) with respect to the lattice vectors of an interfacial surface (this alignment is defined by the angle θ). The algorithm represents both the interface and the macromolecule in three dimensions. For each value of θ, incremental moves of the macromolecule occur in the x, y, z direction along the θ orientation, as well as rotation (ω, γ, ζ) of either the macromolecular chain or the interfacial slab. We utilized a simple forcefield that consists of a dipole–dipole, dipole–charge, or charge–charge electrostatic interaction term and a Lennard-Jones attraction–repulsion term to describe the nonbonding interactions between macromolecular atoms and interfacial atoms. We benchmarked our method by modeling ice- and mineral-interaction polypeptides on various Miller planes of hexagonal ice and inorganic solids, respectively. In addition, we searched phase space for a simpler, nonpolypeptide system: The ice-nucleating C31 alcohol monolayer (comprised of 61 C31 molecules) in contact with the {001} plane of hexagonal ice. Our results indicate that the POINTER simulation method can reproduce the macromolecule orientation observed for each benchmark system. In addition, our simulations point to a number of factors—polypeptide binding site structure, the positioning of hydrophobic residues near the interface, and interface topology—which can influence the adsorption orientation of polypeptides on hexagonal ice and inorganic solids.
On a labeling for point group harmonics. I. Cubic group
Authors: Conte Robert , Raynal Jacques
Published: July 2002

Expressions for the Γ4, Γ6, and Γ8 representations are derived in terms of the Γ1 representations for neighboring angular momenta. Expressions for Γ5, Γ7, and again for Γ8 are derived in terms of the Γ2 representations. A third set of expressions for Γ8 is derived in terms of the Γ3 representations. With an arbitrary choice of orthonormal sets of the Γ1, Γ2, and Γ3 representations, orthonormal sets of all other kinds of representations are thus well defined and can be entirely labeled using the parent representations. All Clebsch–Gordan coefficients are expressed in terms of those between parent representations (and a few other ones). The Γ4 and Γ5 representations defined here are not those conventionally used, but they provide a simpler expression for the fictitious spin coefficients. Tables of the parent representations Γ1, Γ2, and Γ3 are given for quaternary and ternary axes of quantization. Using the usual Clebsch–Gordan coefficients of SU(2), these tables allow us to obtain any representation for integer or half-integer angular momenta up to j=25.