bluuebunny/crossref_metadata_2025 huggingface dataset:
Theory of phase equilibria and critical mixing points in binary lipid bilayers
Authors: Risbo Jens , Sperotto Maria M. , Mouritsen Ole G.
Published: July 2002

The fundamental problem of determining the phase equilibria of binary mixtures is discussed in the context of two-component phospholipid bilayer membranes of saturated phospholipids with different acyl-chain lengths. Results are presented from mean-field calculations and Monte Carlo simulations on a statistical mechanical model in which the interaction between lipid acyl chains of different length is formulated in terms of a hydrophobic mismatch. The model permits a series of binary phase diagrams to be determined in terms of a single ‘‘universal’’ interaction parameter. The part of the free energy necessary to derive phase equilibria is determined from the simulations using distribution functions and histogram techniques, and the nature of the phase equilibria is determined by a finite-size scaling analysis which also permits the interfacial tension to be derived. Results are also presented for the enthalpy and the compositional fluctuations. It is shown, in accordance with experiments, that the nonideal mixing of lipid species due to mismatch in the hydrophobic lengths leads to a progressively nonideal mixing behavior as the chain-length difference is increased. Moreover, indications are found that a phase transition in a strict thermodynamic sense may be absent in some of the short-chain one-component lipid bilayers, but a transition can be induced when small amounts of another species are mixed in, leading to a closed phase separation loop with critical points. The physical mechanism of inducing the transition is discussed in terms of the molecular properties of the lipid acyl chains. The results of the numerical model study are expected to have consequences for the interpretation of experimental measurements on lipid bilayer systems in terms of phase diagrams.
Interfacial tension of phase-separated polymer solutions and relation to their equation of state
Authors: Enders S. , Wolf B. A. , Binder K.
Published: July 2002

Using an effective (coarse-grained) thermodynamic potential describing the excess free energy of mixing of a polymer solution and fitting its parameters to measured critical point data, we obtain the ‘‘hump’’ ε(τ) of this potential in the two-phase region (τ being the reduced distance from the critical temperature T of unmixing). For 30 different systems (varying the degree of polymerization r as well as choosing different polymer–solvent pairs) it is shown that the data are reasonably well represented by a power law, ε(τ)=ε̂ττζ. While mean field theory implies ζ=5/2 and scaling theory ζ=3ν+β≊2.22 (using Ising model exponents ν≊0.63,β≊0.325), the ‘‘effective’’ exponent extracted from the data mostly falls in between these limits (ζeff≊2.4). Since the interfacial tension satisfies a similar power law, σ(τ)=σ̂ττμ (with μ=3/2 in mean field theory or μ=2ν≊1.26 in scaling theory), we also consider a relation between interfacial tension and free energy hump, σ(ε)=σ̂εεφ. While mean-field theory implies φ=3/5 and scaling theory φ=2/(3+β/ν)≊0.57, the empirical exponent lies in the range 0.5≲φeff≲0.6. We present estimates of molecular weight dependencies of critical amplitude prefactors ε̂τ,σ̂τ,σ̂ε and of related quantities for many different systems. We also discuss whether the critical amplitude combination (ε̂τ/B̂τ)2/3/σ̂, where B̂τ describes the coexistence curve {φcoex(2)−φcoex(1)=B̂ττβ} is universal. Contrary to some theoretical expectations, our data imply that this combination is not universal, and hence it cannot be used to predict interfacial tensions from equation of state data.
Mean molecular potentials in a model lipid bilayer: A molecular dynamics simulation
Authors: Xiang Tian-xiang , Anderson Bradley D.
Published: July 2002

Various mean-field potentials in a model lipid bilayer are calculated by means of molecular dynamics (MD) simulation. The bilayer assembly consists of 200 chain molecules. The anisotropic united atom model is employed for nonbonded interactions and is extended to allow bond length to vary with time. The interfacial translational order is systematically varied and found to correlate strongly with the chain orientational order. A new torsional potential is developed and shown to give order parameters in better agreement with experiment than the Padilla–Toxvaerd potential. Nonbonded interaction reduces the trans–gauche and gauche–gauche transition barriers by 0.9—1.5 kcal/mole. The mean trans–gauche energy difference near the chain tail is close to that in liquid hydrocarbons but 0.34 kcal/mol lower than that in the highly ordered chain region. In contrast to the Marčelja model, both mean intermolecular dispersive and repulsive energies depend exponentially on the chain orientational parameter and the repulsive component has a poor and inverse correlation with the reciprocal of the chain end-to-end displacement along the bilayer normal. Inclusion of spatial heterogeneity effects of the interaction energy, a treatment similar to the Gruen model [Biochim. Biophys Acta 367, 165 (1980)], does not give a better description of the mean intermolecular interaction. A new and unified model for the mean intermolecular interaction energy is developed based on our present MD simulation data. Various possible chain configurations which are responsible for these results are discussed. Finally, our MD results suggest that, consistent with the ‘‘wobble in a cone’’ model, a chain molecule can rotate freely within an angular range without being subjected to a strong potential force.
Tracer-diffusion coefficients for both localized and nonlocalized adsorption: Theory and molecular-dynamics simulation
Authors: Raut Janhavi S. , Fichthorn Kristen A.
Published: July 2002

We present a quantitative analysis of tracer diffusion in a molecular-dynamics simulation of the adsorption of an isolated ethane molecule on Pt(111). In particular, we examine the deviations between the tracer diffusion of ethane in the simulations and the assumptions of the nearest-neighbor adsorbate-hopping model at temperatures for which the kinetic energy of the molecule approaches and exceeds the diffusion-barrier energy. Our method of analysis can be implemented experimentally, with techniques such as scanning-tunneling microscopy. We show that the adsorbate-hopping model cannot accurately describe tracer diffusion at any of the temperatures probed. This is because ethane exhibits very long flights with flight times that are not negligible compared to the time required for the molecule to escape from a binding site. We propose a new formula for the diffusion coefficient that includes the influence of non-nearest-neighbor jumps with non-negligible flight times. In the limit of low temperatures, this expression reduces to a hopping model while, at high temperatures, our model predicts that the diffusivity becomes analogous to that for a two-dimensional gas. We show that our model quantitatively describes the tracer diffusion of ethane on Pt(111) in molecular-dynamics simulations over a wide temperature range, spanning both localized and nonlocalized adsorption. We comment on future research directions that may lead to a quantitative model of tracer diffusion in other similar systems.
Fully ab initio investigation of bound and predissociating states of the NeOH(X) complex
Authors: Yang Moonbong , Alexander Millard H.
Published: July 2002

New ab initio potential energy surfaces (PESs) are reported for the interaction of Ne(1S0) with the OH radical in its ground (X 2Π) electronic state. These are then used in the variational calculation of the bound vibrational states of the NeOH(X) complex. The calculated dissociation energy (D0) is 26.2 cm−1, which lies within the experimental estimate (23–30 cm−1). The ab initio PESs are also used to determine the positions and widths of the metastable levels of the complex which correlate with the first excited rotational state (j=5/2, ω=3/2) and the first excited spin–orbit state (j=1/2, ω=1/2) of OH(X 2Π). The predissociation rates are strongly dependent on the bending motion, the intermolecular stretching vibrational quantum number and the parity. The predissociation lifetimes are in good agreement with estimates from stimulated emission pumping experiments of [Chuang, Andrews, and Lester, J. Chem. Phys. 103, 3418 (1995)]. A quantum flux method is used to study the redistribution of the predissociation flux as a function of the fragment separation.
Interrelation between density functional and self-consistent-field formulations for inhomogeneous polymer systems
Authors: Freed Karl F.
Published: July 2002

We provide a bridge between the density functional and self-consistent-field formulations for inhomogeneous polymer systems by deriving the self-consistent-field equations from a density functional approach. The density functional theory employs the zeroth-order inhomogeneous model of Gaussian chains in the presence of interacting interfaces (or more generally of chains whose single chain distribution functions are derivable from a diffusion equation). Nonideality is represented, for simplicity, using a random mixing model, and an implicit formal solution is used for the ideal free energy functional. Application of the standard density functional variational principle produces the self-consistent-field equations and provides a method for generating analytical approximations both to the density functional and to the self-consistent-field equations. The final density functional emerges in the form of a Landau-type expansion about an analytically tractable zeroth-order inhomogeneous reference system, and the important presence of chain connectivity contributions is quite evident. We illustrate the theory by analytically computing the leading contribution to the inhomogeneous density profile induced by the presence of a polymer–surface interaction in a polymer melt that is confined by an impenetrable surface. Future works will extend these analytical computations to treat surface segregation in multicomponent polymer systems with interacting impenetrable interfaces.
The simulation of an unusual magnetic circular dichroism spectrum: The 5p→6s 1Σ+→3Π2 transition of HI
Authors: Rupnik K. , McGlynn S. P.
Published: July 2002

The absorption and MCD (magnetic circular dichroism) spectra of the origin band of the 1Σ+0→3Π2 5p→6s Rydberg transition of HI are presented at the optimal resolution consonant with the generation of a reliable MCD spectrum. The MCD spectrum is complicated and not readily interpretable. The absorption and MCD spectra have been simulated at three levels of resolution which involved calculation of the appropriate intensities (i) for each individual roelectronic transition, (ii) convolution of these in (i) with a shape function, and (iii) convolution of those in (ii) with an apparatus function. It is shown that excellent agreement of experiment and theory occurs, and it is suggested that the majority of MCD spectra of the rovibronic transitions of gas phase molecules may be amenable to such analysis.
Binary diffusion coefficients of helium/hydrogen isotope mixtures
Authors: Taylor William L. , Hurly John J. , Meyer Brad A. , Dunlop Peter J.
Published: July 2002

Mass diffusion coefficients have been measured for the six binary mixtures formed from 3He and 4He with hydrogen, deuterium, and tritium. An intermolecular potential of the Hartree–Fock-dispersion form is presented which, when employed in the quantum mechanical theoretical evaluation of the diffusion coefficient, correlates experimental results for all six systems within the experimental uncertainty. No isotopic effect is displayed by the correlating potential.
Phase space distribution function formulation of the method of reactive flux: Memory friction
Authors: Kohen Daniela , Tannor David J.
Published: July 2002

The Grote–Hynes transmission coefficient for the rate of barrier crossing in the presence of memory friction is rederived here using the method of reactive flux. By combining the methodology developed in an earlier paper [D. J. Tannor and D. Kohen, J. Chem. Phys. 100, 4932 (1994)] with the non-Markovian Fokker–Planck equation of Adelman [S. Adelman, J. Chem. Phys. 64, 124 (1976)] we are able to obtain not only the asymptotic rate constant but the behavior of the rate constant at all times. The salient features of the time dependent rate constant, k(t), are interpreted in terms of the time evolution of the representative distribution functions that originate at the top of the barrier. The short time behavior of the rate constant is very different in the dynamic and static limits, with close analogies to the stochastic theory of spectral line shapes. The dependence of the ‘‘plateau time’’—the time for the rate constant to reach its steady state value—on the memory kernel is explored numerically, and analytical expressions are obtained in the dynamic and static limits.
Monte Carlo simulations of a layering transition in hard parallelepipeds
Authors: Casey Alison , Harrowell Peter
Published: July 2002

We describe a new layered phase, the discotic smectic, found in Monte Carlo simulations of hard parallelepipeds. This phase differs from the existing smectic A phase in that the shortest particle axis, rather than the longest, is aligned perpendicular to the layers. A range of particle shapes with dimensions varying from 5×1×1 (‘‘rods’’) to 5×1×5 (‘‘tiles’’) has been studied. We identify the important role played by correlations arising from the stacking of flat surfaces in stabilizing the layered order. We find that 5×1×1 rods can also exhibit such correlations through orientational disorder. Over this range of particle shapes no nematic phase is observed.
Emission spectroscopy of the predissociative excited state dynamics of acrolein, acrylic acid, and acryloyl chloride at 199 nm
Authors: Arendt Michael F. , Browning Paul W. , Butler Laurie J.
Published: July 2002

The emission spectroscopy of acrolein (C3H4O), acrylic acid (C2H3COOH), and acryloyl chloride (C2H3COCl) excited at 199 nm elucidates the dominant electronic character of the excited state reached by the optical transition at this wavelength. Progressions in the C=C and C=O stretching overtones and various combination bands suggest the antibonding orbital has mixed π*(C=C)/π*(C=O) character. We interpret the results in conjunction with ab initio calculations at the configuration interaction singles level to identify the influence of resonance in the excited state of these conjugated molecules. The results on acrylic acid are of particular interest as excitation in this absorption band produces the HOCO intermediate of the OH+CO→H+CO2 reaction that is important in combustion.
Picosecond spectroscopy of the HgAr van der Waals complex
Authors: Krim L. , Soep B. , Visticot J. P.
Published: July 2002

We have performed the time-dependent spectroscopy of the B 3P1 state of the HgAr van der Waals complex using a two-color picosecond pump-probe experiment. Recurrences can be observed for a short delay between pump and probe pulses and the oscillations disappear after 100 ps as a result of the large anisotropy of the weak B van der Waals well. The time-resolved approach is well adapted to the observation of vibrational levels close to the dissociation limit. Moreover, the spectral bandwidth of the picosecond laser allows for a selective excitation of groups of levels in various parts of the potential well.
Theory for the phase behavior of a mixture of a rodlike colloid and a rodlike polymer
Authors: Sear Richard P. , Jackson George
Published: July 2002

A suspension of rodlike colloidal particles and rodlike liquid crystalline polymers is modeled as a mixture of thick (colloidal) and thin (polymeric) rods. The model mixture is studied by means of an Onsager type theory, the colloidal particles are represented by hard rods and the polymer by hard rods with zero diameter. Extensive immiscibility is observed for the colloid plus polymer model mixture and if the polymer is more than 3 times as long as the colloid coexistence between two isotropic phases is observed. For all polymer lengths phase separation into an almost pure ideal gas of polymer and a dense nematic phase of colloid is observed at high polymer fugacity. The phase separation is driven by excluded volume effects; the volume available to the polymer is maximized by separating from the colloid.
Deuteron spin diffusion and spin lattice relaxation in amorphous solids
Authors: Hinze G. , Diezemann G. , Sillescu H.
Published: July 2002

The effect of spin diffusion on the decay of longitudinal magnetization in deuterated glassy systems is investigated. The decay of longitudinal magnetization was measured for different dilutions of amorphous deuterated toluene in protonated toluene. For increasing dilution the magnetization was found to decay more slowly, indicating the importance of spin diffusion also in deuteron spin systems. A strong correlation between the apparent width of the distribution of spin lattice relaxation times and the absolute value of the average spin lattice relaxation time was found. This correlation can be explained in terms of spin diffusion. A simple model is able to reproduce the main features of the observed phenomena. The initial decay rates are shown to be independent of spin diffusion. Conclusions concerning the frequency dependencies of observed average spin lattice relaxation times and width parameters are drawn.
Classical and quantum mechanical studies of ice Ih near the melting temperature
Authors: Gai Huadong , Schenter Gregory K. , Garrett Bruce C.
Published: July 2002

Classical and path integral Monte Carlo methods have been used to study the structure and energetics of ice Ih. The water–water interaction is described by the SPC water model. We compute the change in average intermolecular potential energy, radial distribution function, and structural factor as a function of temperature. It is found that near 280 K, the structural and energetic properties from quantum and classical simulations are quite different.
Quantum mechanical calculation of the CO vibrations in CO/Cu(100)
Authors: Park Seung C. , Bowman Joel M. , Jelski Daniel A.
Published: July 2002

We report a calculation of the vibrational energies of CO/Cu(100) focusing on anharmonic coupling between the six CO–Cu modes, for Cu treated as a rigid, multilayer slab. A realistic many-body potential [J. C. Tully, M. Gomez, and M. Head-Gordon, J. Vac. Sci. Technol. A 11, 1914 (1993)] is used to obtain a fourth-order force field in normal coordinates. The vibrational eigenvalue problem is solved using the vibrational self-consistent field method, and the fundamental frequencies are obtained for a thermal distribution of hot bands. The absorption spectra for the CO stretch, the CO–Cu stretch, the CO frustrated rotation, and the CO frustrated translation are calculated at two temperatures. All spectra are significantly broadened due to thermal effects of intermode coupling. Agreement with experiment is generally quite good.
A quasiclassical trajectory study of energy transfer in benzene–benzene collisions
Authors: Lenzer Thomas , Luther Klaus
Published: July 2002

Collisional energy transfer from large, highly vibrationally excited molecules has been studied by quasiclassical trajectory calculations and results for large polyatomic colliders are reported for the first time. Highly excited benzene molecules in a thermal bath gas of benzene were investigated using a sum of atom–atom Lennard-Jones interactions for the intermolecular potential. Excellent agreement with experiment has been found for the first moment of energy transfer 〈ΔE〉 and its energy dependence. 〈ΔE〉 is increasing with a slight leveling off at high energies. The results suggest that vibrational energy loss of the excited molecule is dominated by V–V transfer.
Cavity ringdown laser absorption spectroscopy and time-of-flight mass spectroscopy of jet cooled platinum silicides
Authors: Paul J. B. , Scherer J. J. , Collier C. P. , Saykally R. J.
Published: July 2002

The cavity ringdown technique (CRLAS) has been employed to measure the gas phase absorption spectrum of the platinum silicide molecule in the 350 nm region. All nine of the measured rovibronic bands are assigned to a single 1Σ–1Σ electronic transition, with a ground state vibrational frequency of ωe″=549.0(3) cm−1, and a bond length of r0″=2.069(1) Å. The results of this study are compared with experimental data for the coinage metal silicides. Additionally, time-of-flight mass spectrometric results indicate that a variety of polyatomic metal silicides are formed in our molecular jet expansion.
Embedded density functional approach for calculations of adsorption on ionic crystals
Authors: Stefanovich Eugene V. , Truong Thanh N.
Published: July 2002

We present an embedded density functional approach to study adsorption on crystalline surfaces. Following ideas suggested by Cortona, Wesolowski, and Warshel, we divide the total system into a quantum cluster and the surrounding lattice whose density is assumed to be the same as in the ideal crystal. In this case the Kohn–Sham Hamiltonian for electrons in the cluster contains additional terms corresponding to the Coulomb, exchange, correlation, and ‘‘nonadditive kinetic energy’’ potentials from the environment. Test calculations for the He and Ar dimers, X–H2O molecular complexes (X=Li+, Na+, K+, F− or Cl−) and water adsorption on the (001) surface of the NaCl crystal suggest that this model provides a promising alternative for cluster models employed earlier for calculations of defects and adsorption on ionic crystals.
Accurate high-pressure and high-temperature effective pair potentials for the systems N2–N and O2–O
Authors: Thiel M. van , Ree F. H.
Published: July 2002

Statistical mechanical chemical equilibrium calculations of N2 and O2 show that these molecules dissociate behind strong shock waves. Our determination of accurate intermolecular potentials has required the consideration of the dissociation products N and O. Our previous theoretical efforts to predict the thermodynamic properties of these molecules relied in part on corresponding states theory and shock wave data of argon, without consideration of the dissociation products. Recent high-pressure Hugoniot measurements, however, allowed a more accurate determination of the potentials and the explicit inclusion of the dissociation products. The best fit to the data is obtained with the exponential-6 coefficients, for O2–O2: ε/k=125 K, r*=3.86 Å, α=13.2; for O–O: ε/k=700 K, r*=2.40 Å, α=11.0; for N2–N2: ε/k=293 K, r*=3.91 Å, α=11.5; and for N–N: ε/k=600 K, r*=2.47 Å, α=10.0. The unlike pair interactions are obtained from these like interactions with a modified Lorentz–Berthelot rule. The coefficients in the modified Lorentz–Berthelot equations are k/l/m=1/1/0.93 for O2–O– and k/l/m=1/1/0.90 for N2–N interactions.
Further direct evidence for stepwise dissociation of acetone and acetone clusters
Authors: Buzza S. A. , Snyder E. M. , Castleman A. W.
Published: July 2002

The (pre)dissociation of acetone and acetone clusters after excitation to states corresponding to upper {S1,T1} and 3s Rydberg states of the acetone monomer are investigated through femtosecond pump–probe experiments coupled with molecular beam time-of-flight mass spectrometry techniques. Upon excitation to either state, [(CH3)2CO]n* dissociates rapidly. Acetyl fragments, [(CH3)2CO]n−1CH3CO+ may arise from ionization of an excited species formed by (pre)dissociation of intact precursors or by dissociation after the intact species has been ionized. The method employed to separate these two channels is discussed herein; the resulting transients are fit to a kinetic model to elucidate intermediate lifetimes and dissociation mechanisms. The present experiments establish that a stepwise dissociation mechanism is operative upon excitation to the 3s Rydberg state for the acetone monomer and dimer, with their corresponding acetyl fragments having lifetimes on the order of picoseconds. Larger cluster species, [(CH3)2CO]n (n≳2), do not exhibit the predissociative behavior evident in the monomer and dimer. Conversely, dissociation upon excitation to the {S1,T1} state exhibits an initial loss of a methyl unit, with the acetyl intermediate being considerably more stable than those created by dissociation of acetone from higher lying states. A strong dependence on the internal energy available after the dissociation event is noted.
Intracluster reactions: The formation of hydrazine complexes from ammonia clusters following ArF excimer laser excitation
Authors: Färber Matthias , Huisken Friedrich
Published: July 2002

We have found a novel intracluster reaction proceeding in large ammonia or ammonia/argon clusters upon excitation with ArF excimer laser radiation which leads to the formation of unprotonated and prototaned hydrazine clusters, (N2H4)+n and (N2H4)nH+ (n=1–4). To our knowledge it is the first time that the production of bare clusters of a different chemical species has been observed as a result of an intracluster reaction. Besides neat hydrazine complexes also mixed cluster ions containing hydrazine and ammonia molecules were detected, both as unprotonated and protonated species. However, the bare hydrazine clusters were found to be more abundant. It is assumed that the wavelength of the exciting laser radiation is of great importance for inducing the observed reaction.
Calculation of nuclear magnetic resonance order parameters in proteins by normal mode analysis
Authors: Sunada Shinji , Go Nobuhiro , Koehl Patrice
Published: July 2002

An analytic formula is developed for calculating the generalized NMR order parameters in a protein from the normal mode analysis (NMA). The generalized order parameter, S̃2, is given as thermal ensemble average of a Taylor series in powers of Δ, the displacement of internuclear vector from its mean. Henry and Szabo developed a method to calculate the ensemble average based on the NMA carried out in the Cartesian coordinate space (CCS). However, atomic motions in each individual CCS normal modes are linear in the three-dimensional space, which may cause interatomic distances even between covalently bonded atoms to change significantly. In this situation Henry and Szabo proposed to use a special formula for S̃2 which includes a trick to compensate such changes. We showed that by carrying out the NMA in the dihedral angle space (DAS) and by interpreting each DAS normal mode into curved atomic motions, S̃2 can be calculated reliably for spin pairs separated up to about 10 intervening covalent bonds by a natural formula without any trick.
A simplified approach to optimally controlled quantum dynamics
Authors: Botina Jair , Rabitz Herschel , Rahman Naseem
Published: July 2002

A new formalism for the optimal control of quantum mechanical physical observables is presented. This approach is based on an analogous classical control technique reported previously [J. Botina, H. Rabitz, and N. Rahman, J. Chem. Phys. 102, 226 (1995)]. Quantum Lagrange multiplier functions are used to preserve a chosen subset of the observable dynamics of interest. As a result, a corresponding small set of Lagrange multipliers needs to be calculated and they are only a function of time. This is a considerable simplification over traditional quantum optimal control theory [S. Shi and H. Rabitz, Comp. Phys. Comm. 63, 71 (1991)]. The success of the new approach is based on taking advantage of the multiplicity of solutions to virtually any problem of quantum control to meet a physical objective. A family of such simplified formulations is introduced and numerically tested. Results are presented for these algorithms and compared with previous reported work on a model problem for selective unimolecular reaction induced by an external optical electric field.
Quantum beats in cyanogen from singlet/singlet and singlet/triplet superposition states
Authors: Hemmi Naoki , Cool Terrill A.
Published: July 2002

Fluorescence quantum beats, observed among hyperfine substates of the N=8, 10, 15, 18, 19, 22, and 38 rotational levels of the 41 Ã(1Σ−u) state of C2N2, are discussed. The Zeeman splittings of some beats, for N=8, 19, and 22, are too small to arise from the spin–orbit coupling of a singlet/triplet pair of states, and are attributed to singlet/singlet interactions. A simple two-state singlet/triplet model, successfully used for a previous analysis of beats from N=12, accurately accounts for magnetic modulations observed for beats from the N=8 and N=18 rotational states.
On the direct complex scaling of matrix elements expressed in a discrete variable representation: Application to molecular resonances
Authors: Museth Ken , Leforestier Claude
Published: July 2002

We present an extension of a method initially proposed by Moiseyev and Corcoran [Phys. Rev. A 20, 814 (1978)] to a direct continuation of the matrix elements of a real Hamiltonian operator expressed in a contracted, discrete variable representation type basis set. It is based on the identity which relates the matrix elements of a complex scaled potential between real basis set functions to those of the unscaled potential between backward scaled basis functions. The method is first applied to the study of the resonances of a one dimensional model by means of complex scaling. It is shown that the resulting matrix elements of the scaled potential are no longer diagonal in the DVR. This paradox is discussed and shown to be of no practical consequence in the formulation. The scheme is then extended to the direct complex scaling of a two dimensional Hamiltonian operator expressed in a contracted basis set built through the successive adiabatic reduction method of Bac̆ić and Light. Results show that, due to the use of a numerical continuation, slightly larger grids have to be used as compared to the case of an analytic continuation where the potential is available.
Quasiclassical trajectory calculations of photodissociation of Ar–H2O(X̃–Ã) and H2O(X̃–Ã)
Authors: Christoffel Kurt M. , Bowman Joel M.
Published: July 2002

We present results of full-dimensional quasiclassical trajectory calculations of the photodissociation of H2O(3νOH,X̃–Ã) and Ar–H2O(3νOH,X̃–Ã) at 243 and 218 nm, and compare the resulting OH rotational distributions, and also relate them to recent experiments of Nesbitt and co-workers [D. F. Plusquellic, O. Votava, and D. J. Nesbitt, J. Chem. Phys. 101, 6356 (1994)]. The dynamics calculations make use of a new six degree-of-freedom potential for Ar–H2O(Ã), which is reported here. The potential is based on a previously reported ab initio H2O Ã-state potential, a semiempirical Ar–OH(2Π) potential, and a semiempirical Ar–H potential, together with an appropriate switching function to ensure permutation symmetry with respect to the two H atoms. Initial conditions for the trajectories are obtained from a product of a Husimi phase-space density for the Ar–H2O(X̃) intermolecular modes and a Wigner/classical phase-space density for the H2O(X̃) intramolecular modes. The Husimi phase-space density is derived from the ground-state wave function for Ar–H2O(X̃), using a previous spectroscopically empirical potential. To assess the accuracy of the trajectory approach, trajectory calculations are also reported for X̃–Ã photodissociation of H2O in the ground vibrational state at 166 nm and compared with the corresponding full-dimensional quantum wave packet calculations of von Dirke and Schinke. To further assess the accuracy of the  Ã-state potential surface for H2O, calculations for H2O(4νOH,X̃–Ã) are also reported at 218 nm and compared with experiment. Rotation/vibration distributions of the OH fragment are also calculated for photodissociation of Ar–H2O(4νOH,X̃–Ã) at 218 nm.
On the realism of the re-engineered simple point charge water model
Authors: Chialvo Ariel A.
Published: July 2002

The realism of the recently proposed high-temperature reparameterization of the simple point charge (SPC) water model [C. D. Berweger, W. F. van Gunsteren, and F. Müller-Plathe, Chem. Phys. Lett. 232, 429 (1995)] is tested by comparing the simulated microstructure and dielectric properties to the available experimental data. The test indicates that the new parameterization fails dramatically to describe the microstructural and dielectric properties of water at high temperature; it predicts rather strong short-range site–site pair correlations, even stronger than those for water at ambient conditions, and a threefold smaller dielectric constant. Moreover, the resulting microstructure suggests that the high-temperature force-field parameters would predict a twofold higher critical density. The failure of the high-temperature parameterization is analyzed and some suggestions on alternative choices of the target properties for the weak-coupling are discussed.
Comment on ‘‘Long-time tails in angular momentum correlations’’ [J. Chem. Phys. 103, 1582 (1995)]
Authors: Cichocki B. , Felderhof B. U.
Published: July 2002

It is shown that the shape-dependence of the coefficient of the long-time tail of the angular velocity autocorrelation function of a Brownian particle, as predicted for both two and three dimensions from the fluctuation–dissipation theorem and linearized hydrodynamics, agrees with recent computer simulations. The disagreement with a mode-coupling calculation suggests that the latter should be revised.
Magnetic field induced alignment–orientation conversion: Nonlinear energy shift and predissociation in Te2 B1u state
Authors: Auzinsh M. , Stolyarov A. V. , Tamanis M. , Ferber R.
Published: July 2002

The paper analyzes magnetic field induced alignment–orientation conversion (AOC) phenomenon caused by simultaneous effect of quadratic terms in Zeeman energy shift and magnetic predissociation (PD), producing asymmetry either in energy splitting ωMM±1≠ω−M∓1−M or in relaxation of coherence ΓMM±1≠Γ−M∓1−M between coherently excited M, M±1 magnetic sublevels. The AOC is registered via the appearance of circular polarization (C) of fluorescence under linearly polarized excitation. The unified perturbation treatment of a molecule in external magnetic field B is presented, accounting for magnetic and intramolecular perturbations via interaction with bonded or continuum states, considering Hund’s (c)-case coupling and dividing the intramolecular perturbation operator into homogeneous (ΔΩ=0) and heterogeneous (ΔΩ=±1) parts. Explicit expressions up to B2 terms are given for energy shift and PD rate, adapted to 1u state in conditions relevant to the B 3Σ−u complex of Te2 molecule. Numeric simulation revealed that nonlinear magnetic energy shift and heterogeneous magnetic PD produce dispersion type fluorescence circularity signals C(B) of different sign. Fitting of experimental data on B1−u, v(J)=2(96) state of 130Te2 molecule allowed to determine the electronic matrix element of paramagnetic Hamiltonian (Ω=0|Ĥpm|Ω=1)≡G±=2.7, as well as the natural Cvhet=±6 s−1/2 and the magnetic αvhet=∓9×103 s−1/2 T−1 rate constants of heterogeneous PD, supposing that the B1−u state PD takes place through 0−u state continuum. As a result, magnetic AOC represents a sensitive method to investigate molecular structure and intramolecular interaction between both bonded and continuum states. Additionally, it has been shown that the magnetic PD effect leads to strong amplification of nonzero field level crossing signals caused by B2 terms in Zeeman energy shift.
Ab initio finite oligomer method for nonlinear optical properties of conjugated polymers: Nonresonant frequency dispersion in polyacetylene
Authors: Hasan Muhammad , Kim Seung-Joon , Toto Joseph L. , Kirtman Bernard
Published: July 2002

Nonlinear optical properties of the linear polyenes C4H6 through C30H32 are calculated at frequencies below the first resonance using ab initio time-dependent Hartree–Fock theory. The results are used to determine accurate values for the infinite polymer, polyacetylene, by means of a new extrapolation method. Dispersion coefficients describing the low frequency behavior are obtained for the largest oligomer.
The isotope effect in solvation dynamics and nonadiabatic relaxation: A quantum simulation study of the photoexcited solvated electron in D2O
Authors: Schwartz Benjamin J. , Rossky Peter J.
Published: July 2002

Quantum nonadiabatic molecular dynamics simulations are used to explore the molecular details surrounding photoexcitation of solvated electrons in deuterated water. The results are compared to previous studies in normal water [B. J. Schwartz and P. J. Rossky, J. Chem. Phys. 101, 6902, 6917 (1994)] to elucidate the nature of the isotope effect on both the solvation and nonadiabatic relaxation dynamics. The solvent spectral density couples differently to the individual energy levels than to the quantum energy gap, indicating the importance of the symmetry of both the ground and excited states in determining the resulting solvent response. The solvation dynamics are characterized by a Gaussian plus biexponential decay. Deuteration has little effect on the Gaussian component or long time exponential decay of the solvent response function, but a ∼20% isotope effect is observed on the faster exponential decay. The solvent response following nonadiabatic relaxation is found to be much more rapid than that following photoexcitation, reflecting the importance of short range mechanical forces and molecular shape in solvation dynamics. Simulated spectral dynamics of the individual ground state bleach, excited state absorption, and stimulated emission components in deuterated water are presented and the results compared to those in normal water. The spectral isotope dependence results principally from the difference in calculated nonadiabatic relaxation rates, which are a factor of ∼2 slower in D2O than H2O. Using the fact that a separate analysis of the quantum decoherence times for the electron suggests that the nonadiabatic transition rates in the two solvents should be identical, calculated spectral transients are corrected for the case of identical nonadiabatic lifetimes and show essentially identical behavior in light and heavy water, in agreement with current experimental results. The small isotope effect on the solvation response should be observable with higher time resolution.
Polarized infrared attenuated total reflection spectroscopy for three-dimensional structural analysis on long-chain compounds
Authors: Kaneko Fumitoshi , Miyamoto Hideki , Kobayashi Masamichi
Published: July 2002

Two simple techniques using single-crystal specimens in polarized infrared attenuated total reflection (ATR) spectroscopy were developed for structural studies of molecular crystals, in particular of long-chain compounds. The first technique relates to a rotatory change of sample setting around the normal of the sampling plane of an internal reflection element (IRE) in the p-polarized ATR measurement. This is a simple method to distinguish whether a transition moment is parallel to a certain crystal face. The inclination manner of hydrocarbon chains constructing an orthorhombic polyethylene subcell can be easily determined. Another technique relates to the employment of intermediate polarizations between p and s settings of incident infrared radiation, which allows three-dimensional structural analysis. The evanescent electric field is estimated for a simple model system. Linearly polarized incident radiation generates an elliptical electric field in a specimen. By combining a rotatory change of sample setting, the long axis of the elliptical electric field can be directed in any direction in a fixed coordinate system located on a specimen. Transition moments nearly parallel to the long axis are emphasized in spectra. The phase difference between the p and s components of evanescent waves has a large influence on the elliptically polarized electric field. For a multiple reflection IRE, the polarization of ATR spectra depends on the sampling position, which can be ascribed to a phase jump of the guided infrared radiation at every reflection.
State-to-state time-dependent quantum calculation for reaction H2+OH→H+H2O in six dimensions
Authors: Zhu Wei , Dai Jiqiong , Zhang John Z. H. , Zhang Dong H.
Published: July 2002

We report benchmark quantum mechanical results of state-to-state reaction probabilities for the title reaction in full dimensions (6D) using the widely used Schatz–Elgersma potential energy surface. The time-dependent wave packet is propagated using the diatom–diatom Jacobi coordinates and the energy-specific state-to-state reaction probabilities are obtained by asymptotic flux calculation. All results reported here are for the reaction resulting from the ground state of H2+OH to various product states of H+H2O for total angular momentum.
Determination of Λ-doublet resolved cross-sections for inelastic scattering of OH by para- and normal-H2
Authors: Schreel K. , ter Meulen J.J.
Published: July 2002

In this paper we report the measurement of Λ-doublet resolved state-to-state cross sections for inelastic collisions of OH by H2 at a translational energy of 595 cm−1. Experimental values are obtained for transitions from both the upper (f) and the lower (e) Λ-doublet substates of the lowest rotational state (J=3/2) of OH(2Π3/2) to almost all other states within the range of the collision energy. Cross sections for scattering by both para- and normal-H2 have been determined. The main difference between para- and normal-H2 scattering is seen in the Λ-doublet cross section. The results are compared to He-scattering which shows that H2 (J=0) scattering behaves similar to He scattering. When averaged over the Λ-doublet states, the cross sections are in good agreement with the measurements of Andresen et al. [J. Chem. Phys. 81, 571 (1984)], although the conclusions with regard to collisional pumping of interstellar OH masers are different. The quantum calculations of Offer et al. [J. Chem. Phys. 100, 362 (1994)] show a surprisingly good correspondence with the measured values. In this comparison the full initial state distribution of the OH and H2 beams has been taken into account.
Periodic Hartree–Fock calculations on crystalline HCN
Authors: Platts J. A. , Howard S. T.
Published: July 2002

Periodic Hartree–Fock calculations on the tetragonal, high temperature, form of solid HCN are presented. Using HF/6-31G** methodology, the lattice energy per molecule is predicted to be 21.8 kJ mol−1, in good agreement with an experimental value of 19 kJ mol−1. Atoms-in-molecules decomposition of the resulting charge distribution, based on Bader’s zero-flux partitioning scheme, yields an in-crystal dipole moment of 1.734 a.u., a 38% increase over the isolated molecule. It is shown that this dipole moment enhancement is due solely to increased interatomic charge transfer, since the atomic dipoles are actually reduced by the crystal field. The intermolecular interactions in the crystal are characterized by means of the topology of the charge density; each chain is bound by relatively strong N⋅⋅⋅H hydrogen bonds, while the chains are linked by weaker C⋅⋅⋅N interactions. The effects of the crystal field on topological properties of ρ and ∇2ρ are explored, and are found to cause changes of up to 10% compared to the gas phase.
Quantum mechanical geometry optimization in solution using a finite element continuum electrostatics method
Authors: Cortis Christian M. , Langlois Jean-Marc , Beachy Michael D. , Friesner Richard A.
Published: July 2002

We present a new algorithm for performing ab initio solution phase geometry optimizations. The procedure is based on the self consistent-reaction-field method developed in our laboratory which combines electronic structure calculations with a finite element formulation of the continuum electrostatics problem. A gradient for the total solution phase free energy is obtained by combining different contributions from the gradient of the classical polarization free energy and the derivatives of the quantum mechanical energy. The method used in obtaining the classical gradient is based on exact linear algebra relations and a Green function formalism due to Handy and Schaefer. Both the classical and quantum mechanical gradients are validated by comparison with energy finite differences. The result of applications to a number of small organic compounds are discussed. Comparisons between the predicted location and depth of the various solution phase minima of the Ramachandran map for the alanine dipeptide and those reported by Gould et al. are also presented.
Photofragments CH3(X̃ 2A2″)+HCO(X̃ 2A′) from acetaldehyde: Distributions of rotational states and preferential population of K doublets of HCO
Authors: Lee Shih-Huang , Chen I-Chia
Published: July 2002

The relative rotational populations of HCO dissociated from acetaldehyde excited to the S1 state at photolysis energies 31 412, 31 771, 32 165, 32 728, and 33 239 cm−1 are obtained. Fluorescent spectra B̃ 2A′−X̃ 2A′(0,0) of HCO were detected; the intensity is corrected for both the fluorescent quantum yield and the effect of axis switching to obtain the populations of rotational states of HCO. All measured populations of rotational states of HCO, displaying a Gaussian-type pattern, cannot be described according to a Boltzmann distribution. The average energy partitioned to rotation of HCO along the b and c axes is [(270–465)±25] cm−1, about 11% of the available energy, but the average energy partitioned along the a axis remains the same (32±2) cm−1 for all photolysis energies. The rotational energy of HCO is less than that predicted according to an impulsive model calculated with the ab initio transition state structure. Preferential population of Ka=1 doublet states Nu1 and Nl1 is observed. The ratio of population varies with photolysis energy, with a maximum 12u1/12l1=3.7 near 31 800 cm−1. Distributions among the rotational state of HCO indicate that the energy released appears to be affected by the dynamics of the exit channel.
Response to ‘‘Comment on ‘Fluid wetting on molecularly rough surfaces’ ’’ [J. Chem. Phys. 105, 4889 (1996)]
Authors: Harris Jonathan G.
Published: July 2002

We revise our calculations of contact angles of a simple fluid against corrugated surfaces as suggested by Bruin and Nijmeijer in their comment on our work. The corrected formula predicts different contact angles for some cases, but the trends observed in our original work remain unchanged.
Cation concentration and local ligand field of polyether–salt complexes
Authors: Videira A. L. L. , Carlos L. D.
Published: July 2002

Picturing the ion-chain local configuration of polyether–salt materials as forming oxygen-lined helical turns, with the cation located within the polymer cavity, the local environment of each cation is described in terms of a first coordination shell, characterized by the cation and by its nearest oxygen ligands. The interaction energy between nearest shells, ε, for the eutectic concentration of europium-based poly(ethylene oxide), PEO, electrolytes, is calculated by two separate processes: One relating the empirical value of the nearest-ligands local-field energy with the variation of Eu3+ concentration, n, and the other involving a two-electron polarization potential. This last procedure, besides determining the minimum intershells distance for the eutectic phase, permits the number of Eu3+–nearest oxygens to be fixed. The value obtained by the first procedure was ε=554.2 cm−1; while by the second, were ε=520.3 cm−1, for 10 nearest oxygens, and ε=572.4 cm−1, for 11 oxygens, for a mean radius of the first coordination shell R̄=2.4 Å and a minimum distance between nearest shells R0=5.3 Å. This model of chains of interacting coordination shells is extended to other polyether–salt complexes modified by mono and divalent cations. The corresponding two-electron interaction potential is calculated for a few monovalent-based crystalline PEO, complexes, while for noncrystalline divalent electrolytes only an upper limit is estimated.