bluuebunny/crossref_metadata_2025 huggingface dataset:
Statistics of wormlike chains. I. Properties of a single chain
Authors: Zhao S. R. , Sun C. P. , Zhang W. X.
Published: July 2002

Functional integrals in quasimomentum space (normal mode analysis) is used to study configurational statistics of wormlike chains (WLCs) and properties of WLCs in an external field, with which various correlation functions are defined and can be expressed in terms of simple analytical forms. From the correlation functions, some statistical properties of WLCs, such as gyration radius and scattering functions, have been obtained. The difference between correlation functions and end to end distance distribution functions are discussed, and by varying a parameter the WLC exhibits crossover from an ideal Gaussian chain to a rodlike chain.
Toward a molecular theory of vapor-phase nucleation. V. Self-consistency in the decoupled dimer limit
Authors: Oh K. J. , Zeng X. C. , Reiss H.
Published: July 2002

The i/v cluster theory of vapor phase nucleation has been presented in a series of papers of which the present paper is No. V. The theory has experienced some evolution throughout this series. In the first paper an attempt was made to use the classical cluster of Lee, Barker, and Abraham (LBA cluster) that is so attractive for computer simulation. Among other things, that theory uses a so-called “shell molecule” that does not interact with the remaining molecules of the cluster. We show that this “decoupling” of the shell molecule leads to a result that is not internally consistent in the sense that dimers (which are really single vapor molecules) do not have the right concentration. We show how to repair this inconsistency so that the decoupling approximation can be used and, with it, the useful LBA cluster.
Exact results and mean field approximation for a model of molecular aggregation
Authors: Duque Daniel , Tarazona Pedro
Published: July 2002

We present a simple one-dimensional model with molecular interactions favoring the formation of clusters with a defined optimal size. Increasing the density, at low temperature, the system goes from a nearly ideal gas of independent molecules to a system with most of the molecules in optimal clusters, in a way that resembles the formation of micelles in a dilution of amphiphilic molecules, at the critical micellar concentration. Our model is simple enough to have an exact solution, but it contains some basic features of more realistic descriptions of amphiphilic systems: molecular excluded volume and molecular attractions which are saturated at the optimal cluster. The comparison between the exact results and the mean field density functional approximation suggests new approaches to study the more complex and realistic models of micelle formation; in particular it addresses the long-standing controversy surrounding the separation of internal degrees of freedom in the formulation of cluster association phenomena.
Quasibound states in long-range alkali dimers: Grid method calculations
Authors: Dulieu Olivier , Kosloff Ronnie , Masnou-Seeuws Françoise , Pichler Goran
Published: July 2002

A local minimum is found in the 0g+ long range potential curves of the K2 and Rb2 alkali dimers. This well-of magnitude 42 cm−1 for K2 and 93 cm−1 for Rb2—is located above the first ns+n2P3/2 dissociation limit and metastable states could be populated using laser light blue detuned compared to the resonance line. To compute the previously unknown energies and lifetimes of these quasibound states, two grid methods are employed. One method is based on diagonalizing a Fourier grid Hamiltonian, the other uses a propagation technique in imaginary time to filter out vibrational eigenfunctions. Equivalent results are given by both methods. Then the lifetimes are extracted from the correlation function obtained by propagation in real time of these numerical vibrational wave functions. The methods are employed both in adiabatic representation with one electronic potential curve and in diabatic representation with two potential curves. Two quasibound states are found for K2, and three for Rb2 above seven stable bound states. Their lifetimes vary from 20 ps to 3 ns.
Highly excited vibrational states of HCP and their analysis in terms of periodic orbits: The genesis of saddle-node states and their spectroscopic signature
Authors: Beck Christian , Keller Hans-Martin , Grebenshchikov S. Yu. , Schinke Reinhard , Farantos Stavros C. , Yamashita Koichi , Morokuma Keiji
Published: July 2002

We present quantum mechanical bound-state calculations for HCP(X̃) using an ab initio potential energy surface. The wave functions of the first 700 states, corresponding to energies roughly 23 000 cm−1 above the ground vibrational state, are visually inspected and it is found that the majority can be uniquely assigned by three quantum numbers. The energy spectrum is governed, from the lowest excited states up to very high states, by a pronounced Fermi resonance between the CP stretching and the HCP bending mode leading to a clear polyad structure. At an energy of about 15 000 cm−1 above the origin, the states at the lower end of the polyads rather suddenly change their bending character. While all states below this critical energy avoid the isomerization pathway, the states with the new behaviour develop nodes along the minimum energy path and show large-amplitude motion with H swinging from the C- to the P-end of the diatomic entity. How this structural change can be understood in terms of periodic classical orbits and saddle-node bifurcations and how this transition evolves with increasing energy is the focal point of this article. The two different types of bending motion are clearly reflected by the rotational constants. The relationship of our results with recent spectroscopic experiments is discussed.
A quantum molecular similarity analysis of changes in molecular electron density caused by basis set flotation and electric field application
Authors: Simon Sı́lvia , Duran Miquel
Published: July 2002

Quantum molecular similarity (QMS) techniques are used to assess the response of the electron density of various small molecules to application of a static, uniform electric field. Likewise, QMS is used to analyze the changes in electron density generated by the process of floating a basis set. The results obtained show an interrelation between the floating process, the optimum geometry, and the presence of an external field. Cases involving the Le Chatelier principle are discussed, and an insight on the changes of bond critical point properties, self-similarity values and density differences is performed.
Folding a 20 amino acid αβ peptide with the diffusion process-controlled Monte Carlo method
Authors: Derreumaux Philippe
Published: July 2002

In this study we report on the application of the diffusion process-controlled Monte Carlo method to a 20 amino acid αβ peptide (Ac-E-T-Q-A-A-L-L-A-A-Q-K-A-Y-H-P-M-T-M-T-G-Am). The polypeptide chain is represented by a set of 126 particles, the side chains are modeled by spheres, and the backbone dihedral angles φ and ψ of each of the amino acid residue are essentially restricted to a set of ten high probability regions, although the whole φ-ψ space may be visited in the course of the simulation. The method differs from other off-lattice Monte Carlo methods, in that the escape time from one accepted conformation to the next is estimated and limited at each iteration. The conformations are evaluated on the basis of pairwise nonbonded side chain energies derived from statistical distributions of contacts in real proteins and a simple main chain hydrogen bonding potential. As a result of four simulations starting from random extended conformations and one starting from a structure consistent with NMR data, the lowest-energy conformation (i.e., the αβ fold) is detected in ∼103 Monte Carlo steps, although the estimated probability of getting the αβ motif is ∼10−12. The predicted conformations deviate by 3.0 Å rms from a model structure compatible with the experimental results. In this work further evidence is provided that this method is useful in determining the lowest-energy region of medium-size polypeptide chains.
A global A-state potential surface for H2O: Influence of excited states on the O(1D)+H2 reaction
Authors: Schatz George C. , Papaioannou Anastasios , Pederson Lisa A. , Harding Lawrence B. , Hollebeek Timothy , Ho Tak-San , Rabitz Herschel
Published: July 2002

In this article a global potential energy surface for the 1A′′ state of H2O based on application of the reproducing kernel Hilbert space interpolation method to high quality ab initio results is presented. The resulting 1A′′ surface is used in conjunction with a previously determined 1A′ surface to study the O(1D)+H2(HD,D2) reaction dynamics, with emphasis on the influence of the 1A′′ excited state on measurable properties such as the reactive cross sections, rate coefficients, and product state distributions. There is a reactive threshold of about 2 kcal/mol on the 1A′′ surface, and even at 5 kcal/mol, the 1A′′ reactive cross section is only a small fraction (∼20%) of the barrierless 1A′. However, the 1A′′ surface populates very specific product vibrational states (v=3–4) and gives strongly backward peaked differential cross sections, so certain types of measurements are quite sensitive to the presence of this excited state. In particular, better agreement is found with experimental vibrational and angular distributions with 1A′′  included, especially at translational energies of 4 kcal/mol and above. A rough estimate of the influence of the 2A′ surface indicates that this state also makes an important contribution to vibrational and angular distributions at high energies. The 1A′′ and 2A′ surfaces also measurably affect the energy dependence of the integral cross sections at energies above 2 kcal/mol, and the temperature dependence of thermal rate coefficients for O(1D)+H2 at temperatures of 1000 K and above.
Computer simulation studies of anisotropic systems. XXVII. The direct pair correlation function of the Gay–Berne discotic nematic and estimates of its elastic constants
Authors: Stelzer J. , Bates M. A. , Longa L. , Luckhurst G. R.
Published: July 2002

Monte Carlo simulations have been performed for a discotic liquid crystal composed of Gay–Berne particles. On the basis of these simulations for the nematic phase, a subset of the spherical harmonic expansion coefficients of the direct pair correlation function (DPCF) were determined from the pair distribution function (PDF) by solving the Ornstein–Zernike (OZ) equation. This was achieved by generalizing the Wiener–Hopf factorization scheme for the numerical solution of the OZ equation. Only the expansion coefficients gl1,l2,l(r) (lα⩽4) of the PDF in the laboratory frame were used when solving the OZ equation; this means that the DPCF so obtained is equivalent to that for a nematic in which the director is randomly distributed. From the DPCF, the scaled Oseen–Zöcher–Frank elastic constants K11*, K22*, and K33*, as well as the surface constant K13*, have been calculated from the subset of expansion coefficients. Generally, we find that K33*<K11*<K22*, in agreement with what is expected and found for discotic nematics. These results are quantitatively but not qualitatively different from those calculated with the help of analytical approximations for the same spherical harmonic expansion coefficients of the direct pair correlation function. For example, the values of the bulk elastic constants determined via the OZ equation are about three times larger than the bulk elasticity obtained with the low density approximation.
Diffusion quantum Monte Carlo calculation of nondifferential properties for atomic ground states
Authors: Langfelder Peter , Rothstein Stuart M. , Vrbik Jan
Published: July 2002

An algorithm to sample the exact (within the nodal error) ground state distribution to find nondifferential properties of multielectron systems is developed and applied to first-row atoms. Calculated properties are the distribution moments and the electronic density at the nucleus (expected value of the δ operator). For this purpose compact trial functions are developed and optimized, and a new estimator for the δ is formulated. A comparison is made with results of highly accurate post-Hartree-Fock calculations, thereby illuminating the nodal error in our estimates. In general, we obtain more accurate estimates for the distribution moments than those obtained previously using Monte Carlo methods, despite the relative crudeness of our trial functions. We confirm the literature values for the electron density at the nucleus for the lighter atoms (Li-C), but disagree with previous (Monte Carlo) estimates for the heavier ones (N-Ne).
Inversion of absorption spectral data for relaxation matrix determination. I. Application to line mixing in the 106←000 overtone transition of HCN
Authors: Boyd Robert , Ho Tak-San , Rabitz Herschel , Romanini Daniele , Lehmann Kevin
Published: July 2002

A new method of extracting the relaxation matrix directly from absorption spectral data is formulated and applied to R-branch line mixing in HCN. The formulation makes use of a general iterative inversion algorithm based upon first-order sensitivity analysis and Tikhonov regularization. The recovered relaxation matrices describe line mixing much better than those derived from the fitting laws currently in use, and the inversion algorithm usually converges within just three iterations. This formulation presents the first known method for extracting the imaginary, off-diagonal elements of the relaxation matrix.
Light and neutron scattering studies of the OH stretching band in liquid and supercritical water
Authors: Ricci M. A. , Nardone M. , Fontana A. , Andreani C. , Hahn W.
Published: July 2002

The hydrogen projected OH stretching density of states has been determined by an inelastic neutron scattering experiment in liquid and supercritical water. The results, compared with new measurements of the isotropic Raman spectra at the same state conditions, support the interpretation of the Raman spectra in terms of superposition of the allowed ν1 band with the overtone of the ν2 band.
Charge separation processes of highly charged fullerene ions
Authors: Senn G. , Märk T. D. , Scheier P.
Published: July 2002

Using high sensitivity two sector field mass spectrometric techniques (in particular MIKE scans) we have studied quantitatively (and systematically) the energetics of the superasymmetric spontaneous decay reactions (involving either C2+ or C4+ loss) of triply, quadruply, quintuply, and sextuply charged carbon clusters ions Cnz+ in the size range from n=36 up to n=70. From the kinetic energy release data determined, the apparent intercharge distance has been derived using different models including the simple point charges model, the movable charges model and the charged conducting sphere model. As in earlier but less extensive studies the intercharge distance obtained is for all three models used larger than the cage radius of the respective precursor fullerene ion. It is shown that this and other experimental results are only compatible with the recently suggested auto charge transfer (ACT) reaction as the decay mechanism responsible for the superasymmetric charge separation reactions, whereas two other conceivable decay mechanisms (ball-chain-propagation and decay of charged conducting liquid sphere) are not consistent with all of the experimental fingerprints observed.
Infrared study of water in benzene at high temperatures and pressure
Authors: Furutaka Seiya , Ikawa Shun-ichi
Published: July 2002

Infrared spectra of isotopic mixtures of water dissolved in benzene at temperatures in the 373–533 K range and at constant pressure of 100 bar have been measured. The absorption intensities of the OD and OH stretching bands of HDO increases greatly with increasing temperature, indicating increase in solubility of water in benzene. Furthermore, the band shapes exhibit remarkable temperature dependence. This is due to increase in hydrogen bonded species relative to a monomeric species with increasing temperature. Concentrations of HDO in benzene have been estimated from the observed intensities by use of empirical relationships between molar absorption intensities and band peak wave numbers for hydrogen-bonded HDO. The result indicates that the solubility of water in benzene at 533 K and 100 bar is about 40 times larger than that at 293 K and 1 bar. The major part of the hydrogen-bonded species are attributed to dimeric species from comparison of the peak wave numbers of component bands with those reported for water in the gas phase and in low temperature matrices. Assuming a monomer–dimer equilibrium, it is shown that the rate of increase in solubility of water with increasing temperature is larger than the rate of decrease in the equilibrium constant due to an entropy effect.
Shear thinning vs shear thickening in associating fluids
Authors: Evans G. T.
Published: July 2002

When one-bond Frenkel dumbbells are allowed to dimerize, as analyzed using the Wertheim theory of association, the resulting fluid exhibits shear thinning or shear thickening behaviors. In contrast, when Gaussian chains undergo shear-assisted association, only shear thickening is possible. The present theory of shear-induced association of non-Newtonian fluids is applied to interpret the shear thickening seen in equimolar solutions of the CTAB (cetyl trimethyl ammonium bromide) micelles and sodium salicylate. Also derived in this study are the frequency dependent Newtonian viscosity and the non-Newtonian viscosity for a dilute Brownian suspension of two-bond freely jointed Frenkel dumbbells.
Experimental study of dynamics of topological defects in nematic polymer liquid crystals
Authors: Wang Wei , Shiwaku Toshio , Hashimoto Takeji
Published: July 2002

The dynamics of stringlike defects and the shrinkage and annihilation processes of individual defect loops in a nematic polymer liquid crystal have been experimentally investigated. In 40<t<500 s, the line density of defects ρ scales as ρ(t)∼t−1, as expected by theory and as found in the experimental studies of some low molecular weight (LMW) liquid crystals. For individual loops, they first change their complex shape into a circle, resulting in a rapid decrease in length, and then the circular loops continuously shrink. Before a complete annihilation, the relation between the radius R(t) of the circular loop and time t0−t, where t0 is the time to annihilation, is described by R2(t)=2Γ(t0−t)(t⩽t0). The kinetic constant Γ determined from this polymer liquid crystal is 0.34∼0.38 μm2/s, much smaller than those (200∼300 μm2/s) obtained in some LMW liquid crystals. The small Γ is ascribed to the high viscosity of this polymer liquid crystal, compared with LMW liquid crystals.
Tert-butyl nitrite surface photochemistry: The transition from submonolayer to multilayer behavior
Authors: Jenniskens Hans G. , Philippe Laurent , van Essenberg Wouter , Kadodwala Malcolm , Kleyn Aart W.
Published: July 2002

The photochemistry of tert-butyl nitrite, (CH3)3CONO, adsorbed on Ag(111) at 355 nm has been studied using time-of-flight and reflection adsorption infrared spectroscopy (RAIRS) for coverages ranging from 0.1 to 50 ML. For all coverages a thermal and hyperthermal NO desorption distribution is observed due to photodissociation. The thermal component originates from hyperthermal NO molecules from the first and second layer, which were trapped by the silver substrate. The cross section for photodissociation is independent of coverage and matches the gas phase value. This is consistent with a direct excitation into the S1 state. For coverages exceeding 5 ML the outermost layers still dissociate, but RAIRS shows that in the more inward layers caging is preventing dissociation. Due to caging only isomerization of tert-butyl nitrite is allowed, as is evident from the RAIRS.
Ab initio dynamic multipole polarizabilities and hyperpolarizabilities of H2O and the long-range interaction coefficients for its dimer
Authors: Spelsberg Dirk , Meyer Wilfried
Published: July 2002

Perturbed states representing the response to an external electric field are obtained from multi-reference configuration interaction wavefunctions. The multi-configurational reference function is designed to represent the ground state as well as the perturbed and even the first excited states on equal footing, i.e., orbital optimization is performed in a perturbed multi-reference self-consistent field procedure. For various perturbing operators, several perturbed wavefunctions for different frequencies of the perturbing field are obtained simultaneously with the corresponding lowest excited state. From these sets of perturbed and excited states, reduced spectra are derived which represent effective oscillator strength distributions and serve to calculate dynamic polarizabilities, hyperpolarizabilities, and long-range interaction coefficients. The results for both the dynamic polarizabilities and the interaction coefficients are found to be in excellent agreement with experimental data and other accurate theoretical values.
Monte Carlo study of core–valence separation schemes
Authors: Staroverov Viktor N. , Langfelder Peter , Rothstein Stuart M.
Published: July 2002

All-electron partitioning of wavefunctions into products ΨcoreΨval of core and valence parts in orbital space results in the loss of core-valence antisymmetry, uncorrelation of motion of core and valence electrons, and core–valence overlap. These effects are studied with the variational Monte Carlo method using appropriately designed wavefunctions for the first-row atoms and cations. It is shown that the loss of antisymmetry with respect to an interchange of core and valence electrons is a dominant effect which increases rapidly through the row, while the effect of decorrelating the core and valence electrons is significantly smaller. Orthogonality of the core and valence parts partially substitutes the exclusion principle and is absolutely necessary for meaningful calculations with partitioned wavefunctions. Core–valence overlap may lead to nonsensical values of the total energy. Even relatively crude core–valence partitioned wavefunctions generally can estimate ionization potentials and electron affinities with better accuracy than that of the traditional, non-partitioned ones, provided that they achieve maximum separation (independence) of core and valence shells accompanied by high internal flexibility of Ψcore and Ψval. Our best core–valence partitioned wavefunction of that kind estimates the ionization potentials with an accuracy comparable to the most accurate theoretical determinations in the literature.
Fully relativistic density functional calculations of the ground and excited states of Yb, YbH, YbF, and YbO
Authors: Liu Wenjian , Dolg Michael , Li Lemin
Published: July 2002

Fully relativistic density functional calculations using the recently developed Beijing 4-component density functional program package (BDF) were performed for a large number of excited states of the ytterbium atom and the spectroscopic constants of the ground and some excited states of the diatomic molecules YbH, YbF and YbO. It is shown that in a relativistic framework based on the Dirac–Coulomb Hamiltonian modern (nonrelativistic) density functionals work fairly well even for the rather compact 4f shell, i.e., they quantitatively reproduce the excitation energies due to the occupation changes in the 4f shell, in contrast to previous statements made by other authors. The nondegeneracy induced by the approximate density functionals to the degenerate open 4f shell is found to be almost independent of the occupancy of outer shells as well as Hund’s coupling and of the same order as that for the first transition metals. After subtracting the unphysical nondegeneracy we obtain reasonable patterns of excited states due to different occupations of the 4f spinors for the molecules studied here. Although the spectroscopic constants for YbH and YbF obtained by this and other theoretical work are all in good agreement with available experimental data, the theoretical results for YbO show remarkable disagreement with each other and experiment. The present calculations favor a Ω=0+ ground state with a leading f14σ0 configuration, which is in agreement with the interpretation of experimental data.
Experimental and theoretical study of the AlNe complex
Authors: Yang Xin , Dagdigian Paul J. , Alexander Millard H.
Published: July 2002

The laser fluorescence excitation spectrum of the AlNe complex, in the vicinity of the Al atomic 3d←3p and 5s←3p atomic transitions, is reported. Transitions out of the v=0 vibrational levels of both lower-state spin-orbit levels, X1 2Π1/2 and X2 2Π3/2, to vibrational levels of the C 2Δ, D 2Π, and H 2Σ+ AlNe electronic states were observed. From observations of the onset of excitation to the Al(3d)+Ne dissociation continuum, dissociation energies for the various AlNe electronic states were determined. Ab initio calculations of AlNe electronic states correlating with the ground Al(3p)+Ne atomic asymptote were also carried out. The X1 2Π1/2 and X2 2Π3/2 binding energies computed using the calculated AlNe(X 2Π, A 2Σ+) potential energy curves were in reasonable agreement with the experimental determinations. The experimentally determined dissociation energy for the X2 2Π3/2 level is significantly larger than that of the ground X1 2Π1/2 level (D0=32.3±0.3 and 14.1±0.3 cm−1, respectively).
Orientational ordering and dynamics in the columnar phase of a discotic liquid crystal studied by deuteron NMR spectroscopy
Authors: Shen X. , Dong Ronald Y. , Boden N. , Bushby R. J. , Martin P. S. , Wood A.
Published: July 2002

We report on a deuteron NMR study of quadrupolar splittings and spin-lattice relaxation times T1Q and T1Z as a function of temperature and at two different Larmor frequencies in the columnar phase of hexakis(n-hexyloxy)triphenylene (HAT6). The additive potential method is used to model the quadrupolar splittings, from which the potential of mean torque is parameterized, and the order parameter tensor for an “average” conformer is determined. The small-step rotational diffusion model is used to find the rotational diffusion constants D∥ and D⊥ for the spinning and tumbling motions of the molecular core. It is found that D⊥ is slightly larger than D∥ in contrast with the findings in calamitic liquid crystals. The decoupled model of Dong for correlated internal rotations in the end chains is used for the first time in a discotic liquid crystal. Both jump constants for one- and three-bond motions are nearly independent of temperature, while the jump constant for two-bond motion is thermally activated. The rotational speeds D∥ and D⊥ are some two orders of magnitude slower than a typical charge hopping frequency between the aromatic cores of adjacent molecules in the columns. Thus, to a migrating charge, the “lattice” appears static with disorder being due to the instantaneous displacement of the cores with respect to each other.
Discrete energy representation and generalized propagation of physical systems
Authors: Chen Rongqing , Guo Hua
Published: July 2002

This work discusses the discrete energy representation based on generalized propagation of a physical system. Here, the propagation is defined as a recursion scheme which generates a series of system states from a given initial state. Examples of such schemes include the time propagation and polynomial recursion. It is argued that each propagation determines a set of energy points, which form the discrete energy representation. A unitary transformation can be established between the discrete energy representation and the generalized time representation, much like the well-known transformation between the discrete variable representation and the finite basis representation. Such a collocation approach can be useful in calculating many properties that are local in the energy domain. Numerical examples are presented to demonstrate the utility in filter diagonalization.
Theoretical study of the dipole-bound anion (H2O…NH3)−
Authors: Skurski Piotr , Gutowski Maciej
Published: July 2002

The adiabatic electron detachment energy for (H2O…NH3)− has been found to be 109 cm−1 at the coupled-cluster level of theory with single, double, and noninteractive triple excitations (CCSD(T)), to be compared with the recent experimental result of 123–129 cm−1 obtained by Abdoul-Carime et al. [Z. Phys. D 40, 55 (1997)]. The stationary points on the potential energy surface of the neutral and anionic dimer have been determined at the second-order Mo/ller-Plesset level of theory. Our results indicate that the second-order dispersion interaction between the loosely bound electron and electrons of the neutral dimer is as important as the electrostatic electron-dipole stabilization. The higher-order electron correlation corrections are also very important and the CCSD(T) electron binding energy is approximately four times larger than the Koopmans theorem estimation. In addition, the hydrogen bond in H2O…NH3 is susceptible to a deformation upon attachment of an electron. This deformation enhances both the electrostatic and dispersion components of the electron binding energy. The calculated Franck-Condon factors indicate that neutral dimers formed in electron photodetachment experiments may be vibrationally excited in both soft intermolecular and stiff intramolecular modes. The theoretical photoelectron spectrum based on the calculated Franck-Condon factors is reported.
Improved Feynman’s path integral method with a large time step: Formalism and applications
Authors: Drozdov Alexander N.
Published: July 2002

We describe an efficient path integral scheme for calculating the propagator of an arbitrary quantum system, as well as that of a stochastic system in special cases where the Fokker–Planck equation obeys strict detailed balance. The basic idea is to split the respective Hamiltonian into two exactly solvable parts and then to employ a symmetric decomposition of the time evolution operator, which is exact up to a high order in the time step. The resulting single step propagator allows rather large time steps in a path integral and leads to convergence with fewer time slices. Because it involves no system-specific reference system, the algorithm is amenable to all known numerical schemes available for evaluating quantum path integrals. In this way one obtains a highly accurate method, which is simultaneously fast, stable, and computationally simple. Numerical applications to the real time quantum dynamics in a double well and to the stochastic dynamics of a bistable system coupled to a harmonic mode show our method to be superior over the approach developed by the Makri group in their quasiadiabatic propagator representation, to say nothing about the propagation scheme based on the standard Trotter splitting.
Separability of spin–orbit and correlation energies for the sixth-row main group hydride ground states
Authors: DiLabio Gino A. , Christiansen Phillip A.
Published: July 2002

The spin–orbit energy contributions to the ground state potential energy curves for the main group hydrides, TIH through AtH are estimated by differencing multireference, single promotion, configuration interaction (MRS-CI) energies with and without the spin–orbit operator. The spin–orbit contributions are then summed into the energies determined at the λ−s MRSD-CI level (both single and double promotions). The agreement between the resultant curves and those obtained using intermediate coupling MRSD-CI is within 1.2 kcal/mol over a range of internuclear separations. This suggests that, contrary to previous arguments, spin–orbit coupling and correlation energies are very nearly separable for the main group hydride ground states. Furthermore, the computational effort expended by this separate evaluation is up to 12 times less than that for a comparable intermediate coupling CI. The analysis of some properties of these hydrides indicates that bond length shifts due to spin–orbit coupling are small (0.03 Å) while harmonic vibrational frequencies decrease by up to 9%. Dissociation energies are predicted to change considerably in the presence of the operator in agreement with previous findings.
Vibronic coupling and Jahn–Teller effect in negatively charged benzene and [18]annulene
Authors: Yoshizawa Kazunari , Kato Takashi , Yamabe Tokio
Published: July 2002

The vibronic (vibrational–electronic) interactions and Jahn–Teller distortions in the mono- and trianions of benzene and [18]annulene, which show delocalized D6h structures in their neutral states, are discussed. E2g modes of vibration remove the orbital degeneracies in these highly symmetric molecules to lead to D2h distortions. We calculate the electronic structures, vibrational modes, and linear vibronic coupling constants of benzene and [18]annulene as well as their deutero-forms using the B3LYP method, a hybrid (Hartree–Fock/density functional theory) method. The C–C stretching E2g mode of 1656 cm−1 and the C–H stretching E2g mode of 3184 cm−1 give large vibronic coupling constants in the mono- and trianions of benzene. On the other hand, in [18]annulene the lowest E2g mode of 116 cm−1 affords extremely large coupling constants in its mono- and trianions to contribute to the Jahn–Teller distortions. The lowest mode of [18]annulene is related to a deformation of the carbon ring of [18]annulene and analogous to acoustic mode of phonon in solid. Thus, the different types of E2g mode play an important role in the Jahn–Teller distortions in negatively charged benzene and [18]annulene.
Investigation of the rotational motion of self-assembled fatty acid films: An electron paramagnetic resonance line shape analysis
Authors: Risse T. , Hill T. , Schmidt J. , Abend G. , Hamann H. , Freund H.-J.
Published: July 2002

We have investigated the electron paramagnetic resonance (EPR) of self-assembled stearic acid films adsorbed on an Al2O3-film. Doping the film with spin labels at different positions of the alkyl chain in order to make the films accessible for EPR spectroscopy provides an opportunity to investigate the rotational motion of the molecule along the alkyl chain. The temperature dependent EPR spectra show a strong dependence of the rotational motion of the molecules with variation of the location of the spin label along the chain. We study the rotational motion by means of the EPR line shape analysis.
Numerical evaluation of the internal orbitally resolved chemical hardness tensor: Second order chemical reactivity through thermal density functional theory
Authors: Grigorov Martin G. , Weber Jacques , Vulliermet Nathalie , Chermette Henry , Tronchet Jean M. J.
Published: July 2002

In a previous work we have presented a numerical procedure for the calculation of the internal chemical hardness tensor at the molecular orbital resolution level from standard density functional calculations. In this article we describe an improvement of our method using the thermal extensions of density functional theory. Furthermore, new concepts are introduced in the orbitally resolved theory of chemical reactivity. Traditional molecular orbital theories of chemical reactivity are based only on considerations concerning the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) of molecules, supposed to describe the behavior towards electrophiles, respectively, nucleophiles. By applying our methodology to two test molecular systems, namely water and ferrocene, we show how chemical reactivity can be differentiated against hard and soft electrophiles (acids) and hard and soft nucleophiles (bases). As a by-product of the numerical algorithms being used, a self-consistent method for calculating the molecular chemical potential is also described.
Direct reaction of gas-phase atomic hydrogen with chemisorbed chlorine atoms on a silicon surface
Authors: Kim Y. H. , Ree J. , Shin H. K.
Published: July 2002

The collision-induced reaction of gas-phase atomic hydrogen with chlorine atoms chemisorbed on a silicon (001)-(2×1) surface is studied by use of the classical trajectory approach. The model is based on reaction zone atoms interacting with a finite number of primary system silicon atoms, which are coupled to the heat bath. The potential energy of the H⋯Cl interaction is the primary driver of the reaction, and in all reactive collisions, there is an efficient flow of energy from this interaction to the Cl–Si bond. All reactive events occur in a single impact collision on a subpicosecond scale, following the Eley–Rideal mechanism. These events occur in a localized region around the adatom site on the surface. The reaction probability is dependent upon the gas temperature and largest near 1000 K, but it is essentially independent of the surface temperature. Over the surface temperature range of 0–700 K and gas temperature range of 300 to 2500 K, the reaction probability lies below 0.1. The reaction energy available for the product state is small, and most of this energy is carried away by the desorbing HCl in its translational and vibrational motions. The Langevin equation is used to consider energy exchange between the reaction zone and the surface.
Collisional energy loss in cluster surface impact: Experimental, model, and simulation studies of some relevant factors
Authors: Christen W. , Even U. , Raz T. , Levine R. D.
Published: July 2002

Measurements of the collisional energy transfer of size and energy-selected ammonia cluster ions (NH3)nH+, n=1–10, impacting a silicon wafer coated with p-type diamond film are reported. The transfer from translational energy of the incident cluster ions to kinetic energy of intact scattered cluster ions has been studied as a function of impact energy, surface composition, and size of the impinging cluster cations. For low impact energies (<2.5 eV/molecule), cluster ions scattered off the target surface lost most of their initial kinetic energy, while for higher impact energies the elasticity of the cluster–surface collision is surprisingly high: Typically 75% of the impact kinetic energy is retained by the scattered parent clusters. Larger cluster ions are scattered less elastically and a large fraction of them shatter to small(est) fragments. The molecular dynamics simulations examine the two energy disposal regimes, deep inelasticity and shattering. Deep inelastic scattering occurs already below the lowest impact energies probed by the experiment. At higher collision energies, the energy loss continues to increase but a point is reached where most clusters shatter. Those few clusters that rebound intact have lost a disproportionately low fraction of their initial energy. The simulations also explore the cluster size effects, the role of the attraction to the surface, and the importance of the anisotropic forces between the molecules in the cluster. The experimental results and the simulations are discussed using the hard cube model with special reference to collective effects.
Asymmetric water-oil-amphiphile mixtures: Lamellar phases and droplet microemulsions
Authors: Guerra C. , Somoza A. M. , Telo da Gama M. M.
Published: July 2002

A continuum microscopic model for symmetric amphiphilic mixtures, is generalized by considering explicitly water-oil asymmetry, through the interactions between amphiphiles and water and oil. The phase diagram, including lamellar phases, and the properties of water-oil interfaces are studied, using an approximate free energy density-functional, for a wide range of amphiphilic interactions. We also consider the structure and stability of spherical micelles and study the dilute micellar regime. By combining the microscopic density-functional description with the phenomenologic Helfrich elastic free energy, we calculate the elastic properties of the amphiphilic film. Our results for the elastic constant, ks=2k+k̄, are compared with experimental data.
Rotational excitation of OH in collisions with CO, N2, and CO2
Authors: van Beek M. C. , Schreel K. , ter Meulen J. J.
Published: July 2002

Relative state-to-state cross sections are obtained for OH colliding with CO, N2, and CO2. Hexapole state selection is used to prepare a beam of OH molecules in the upper Λ-doublet component of the lowest rotational state. The collision induced rotational energy transfer is monitored state selectively by means of LIF (laser induced fluorescence) spectroscopy. A study is made of the symmetry effects in the obtained cross sections. The results are compared with previously reported cross sections for Ar, He, and H2. A general propensity is found for parity conserving transitions to the Π3/2, J=52 and Π1/2, J=12 states.
Quantum Monte Carlo simulations of Arn–CO2 clusters
Authors: Severson Mark W.
Published: July 2002

Potential-energy functions for CO2–Arn clusters are constructed using the pairwise-additive approximation from the Ar–Ar potential of Aziz [J. Chem. Phys. 99, 4518 (1993)] and three different CO2–Ar potentials which have been reported recently. These are used to find minimum-energy structures and to carry out rigid-body diffusion Monte Carlo simulations of the ground vibrational state for CO2–Arn clusters with n up to 30, as well as the first excited state for n=1. From these results, the CO2 ν3 redshift is estimated. For all values of n, the Ar atoms tend to surround the CO2 molecule. A complete first solvation shell is first found for n=14, and the largest complete first solvation shell is found for n=17. Although the most recent semiempirical CO2–Ar potential function of Hutson et al. [J. Chem. Phys. 105, 9130 (1996)] gives more accurate predictions of spectroscopic properties for n=1 than the best available ab initio potential function [Marshall et al., J. Chem. Phys. 104, 6569 (1996)], both potential functions give quite similar predictions for structures and approximate CO2 ν3 redshifts for larger values of n.
A comparison of ground- and excited-state properties of gas phase and crystalline ruthenocene using density functional theory
Authors: Gilardoni F. , Weber J. , Hauser A. , Daul C.
Published: July 2002

The ground- and excited-state properties of both gas phase and crystalline ruthenocene, Ru(cp)2, are investigated using density functional theory. A symmetry-based technique is employed to calculate the energies of the multiplet splittings of the singly excited triplet states. For the crystalline system, a Buckingham potential is introduced to describe the intermolecular interactions between a given Ru(cp)2 molecule and its first shell of neighbors. The overall agreement between experimental and calculated ground- and excited-state properties is very good as far as absolute transition energies, the Stokes shift and the geometry of the excited states are concerned. An additional energy lowering in the B23 component of the 5a1′→4e1″ excited state is obtained when the pseudolinear geometry of Ru(cp)2 is relaxed along the low-frequency bending vibration.
Ab initio based potential energy surfaces, microwave spectrum, and scattering cross section of the ground state Ne–Cl2 system
Authors: Naumkin F. Y. , McCourt F. R. W.
Published: July 2002

The high-level ab initio potential energy surface (PES) for NeCl2 in the ground electronic state predicts the energy minimum in the linear geometry (L-well) to be slightly deeper than that in the T-shaped geometry (T-well). The experimental D0 and R0 values are reproduced within uncertainties of measurements by both adding the calculated perturbation of the Ne–Cl interactions due to intramolecular forces in Cl2 to empirical NeCl potentials, and by linearly extrapolating or simply scaling the ab initio PES. These procedures lead to equal or even reversed relative depths of the two wells, in accord with both predictions of an atom-atom model using equivalently accurate ab initio NeCl potentials and variation of the ab initio PES with increasing accuracy of calculations. The D0 value for the L-well is predicted to be less than that for the T-well by 2.4 to 5.2 cm−1 for different scaling schemes. The calculated lowest energy rovibrational states associated with each of two conformers show negligible mutual influence, while the effect of the L-well on the rovibrational wave functions for the next vibrational states associated with the T-well is found to be rather important. Microwave spectra are predicted for each PES obtained, and include portions originating from the L-well. The calculated scattering cross section reproduces well the experimental data and is found to be significantly contributed by the L-well.
Intermolecular interaction potential of the carbon dioxide dimer
Authors: Tsuzuki Seiji , Uchimaru Tadafumi , Mikami Masuhiro , Tanabe Kazutoshi
Published: July 2002

The energy profile of the lowest energy path connecting the slipped parallel (C2h) and T-shaped (C2v) dimers of carbon dioxide was investigated using a reasonably large 6-311+G(2df) basis set with the MP2 level electron correlation correction and with the counterpoise correction. There was no inversion barrier and the T-shaped dimer was a transition state connecting the two slipped parallel dimers. The calculated interaction energies of the two dimers (−1.290 and −1.072 kcal/mol) were larger than those reported previously. The calculated energy difference of the two dimers (0.218 kcal/mol) was also larger than the previous values (less than 0.1 kcal/mol). The effects of basis set, electron correlation, and basis set superposition error were studied in detail. The second-order Mo/ller–Plesset (MP2) interaction energies of the two dimers were close to the coupled cluster calculations using single and double substitutions including noniteratively triple excitations [CCSD(T)] ones. The choice of the basis set greatly affected the calculated interaction energies. Small basis sets underestimated the attractive interaction energies. Diffuse polarization functions were essential to evaluate the attractive interaction. The calculated MP2 interaction energies of the two dimers with the counterpoise correction increased by the augmentation of the diffuse polarization functions to the 6-311G* basis set. These values were close to the estimated MP2 interaction energies at the complete basis set (−1.362 and −1.140 kcal/mol). The 6-311+G(2df) basis set was reasonably large to evaluate the attractive interactions. Although the basis set augmented with diffuse polarization functions had large basis set superposition error (BSSE), the calculated interaction energies with the counterpoise correction were close to those calculated with the nearly BSSE free cc-pV5Z and aug-cc-pVQZ basis sets and to the expected values at the complete basis set.
Resolution of alkane molecular polarizabilities into atomic terms
Authors: Ferraro M. B. , Caputo M. C. , Lazzeretti P.
Published: July 2002

Two additive schemes for resolving average molecular electric dipole polarizabilities into atomic contributions, based on the acceleration gauge for the electric dipole, are outlined. Extended calculations have been carried out for a few terms of the alkane series to test the reliability of the partition method. Gross atomic isotropic contributions evaluated for carbon, αAvC≈5.7 a.u., and hydrogen, αAvH≈2.7 a.u., are actually transferable from molecule to molecule, and can be used to predict fairly accurate average polarizabilities of higher homologous molecules in the alkane series.
Neutron diffraction study of high density supercritical water
Authors: Botti Alberto , Bruni Fabio , Ricci Maria Antonietta , Soper A. K.
Published: July 2002

New neutron diffraction data on supercritical water are presented, from which the site–site radial distribution functions are calculated, with the aim of providing additional experimental evidence about the degree of hydrogen bonding in bulk water under these thermodynamic conditions. Compared to water under ambient conditions severe modifications to the three site–site radial distribution functions are found, with little evidence for a distinct hydrogen bond peak in the OH function, at densities in the range 0.58 g/cm3 ⩽ρ⩽0.78 g/cm3, along the isotherm at T=673 K. The results highlight the loss of tetrahedral coordination in supercritical water, thus confirming the conclusions of a previous neutron diffraction experiment. These new results are relevant to the current debate about the suitability of rigid body, pairwise additive, effective potentials for predicting the behavior of water far away from ambient conditions.