bluuebunny/crossref_metadata_2025 huggingface dataset:
Basis set convergence study of the atomization energy, geometry, and anharmonic force field of SO2: The importance of inner polarization functions
Authors: Martin Jan M. L.
Published: July 2002

The total atomization energies, geometries, and anharmonic force fields of the SO and SO2 molecules have been studied at the augmented coupled cluster [CCSD(T)] level near the one-particle basis set limit. The effect of core correlation has been accounted for. The addition of high-exponent d and f “inner polarization functions” to the sulfur basis set was found to be essential for obtaining reliable molecular geometries. The differential effect of core correlation on computed properties is in fact much less important. The CCSD(T) one-particle basis set limit for the symmetric stretching frequencies appears to be higher than the exact value, while the antisymmetric stretching frequency benefits from an error cancellation. The basis set extension effects of diffuse functions and inner polarization functions appear to be nearly perfectly additive. Our best computed atomization energies and geometries agree to within 0.1 kcal/mol, 0.0004 Å, and 0.03 degrees with experiment: The errors in the fundamentals of SO2 are +3.9, −0.4, and +0.4 cm−1. The best computed quartic force field for SO2 should provide a good starting point for a new experimental refinement.
Breathing shell model in molecular dynamics simulation: Application to MgO and CaO
Authors: Matsui Masanori
Published: July 2002

Molecular dynamics (MD) simulation is used to calculate the elastic constants of both MgO and CaO at zero pressure, and their temperature dependences, as well as the temperature–pressure–volume equation of states of the two oxides. The interionic potential is taken to be the sum of pairwise additive Coulomb, van der Waals, and repulsive interactions. In order to account for the observed large departures from the Cauchy relation of the elastic constants of the two oxides, the breathing shell model (BSM) is introduced in MD simulation, in which the repulsive radii of O ions are allowed to deform isotropically under the effects of other ions in the crystal, with each core and breathing shell being linked by a harmonic spring with force constant k. Required energy parameters, including k, were derived empirically to reproduce the observed molar volumes and elastic constants of the two oxides at ambient conditions, and their temperature dependences as accurately as possible. The MD simulation with BSM is very satisfactory in reproducing accurately not only the observed large Cauchy violations,but also the measured molar volumes and individual elastic constants of the two oxides over wide temperature ranges, at zero pressure. In addition the MD simulation reproduces accurately the measured volume compression data of CaO up to 60 GPa at 300 K. Here we present the MD simulated temperature–pressure–volume equation of state of CaO as a useful internal pressure calibration standard at high temperatures and high pressures.
The role of adsorbed alkali metal atoms in the enhancement of surface reactivity: A scanning tunneling microscopy study of low coverage K/Si(111)7×7 surfaces
Authors: Gorelik D. , Aloni S. , Eitle J. , Meyler D. , Haase G.
Published: July 2002

Atomically resolved surface photovoltage imaging and tunneling barrier imaging were performed with a scanning tunneling microscope to elucidate the nature of the initial adsorption state of potassium on room temperature Si(111)7×7 surface, which appears as a missing center Si-adatom. No evidence was found to support the fact that this adsorption mode is charged. K islands start to grow (and accumulate at 7×7 domain boundaries) already at coverages as low as 4×1012 cm−2. These islands may be responsible for the previously reported work function lowering and enhanced chemical reactivity upon K deposition.
Theoretical study of the O-H stretching band in 3-hydroxy-2-methyl-4-pyrone
Authors: Alexandrov V. , Smith D. M. A. , Rostkowska H. , Nowak M. J. , Adamowicz L. , McCarthy W.
Published: July 2002

The infrared spectrum of 3-hydroxy-2-methyl-4-pyrone reveals an O-H stretching frequency roughly 200 cm−1 lower than that of a typical alcohol group. The frequency lowering results from intramolecular hydrogen bonding between the alcohol and ketone groups. In this work, the stretching and bending vibrations of the O-H group in 3-hydroxy-2-methyl-4-pyrone are studied with a theoretical methodology more rigorous than the conventional harmonic approximation. A two-dimensional potential energy surface in internal coordinates corresponding to different hydrogen positions in the plane of the molecule is calculated with the use of the second order Mo/ller-Plesset perturbation theory. To include all possible variations in kinetic energy in a large amplitude vibrational mode, g-matrix elements with variable values are employed. The analytical expression for the Hamiltonian matrix elements of the two-dimensional vibrational problem in a basis of shifted Gaussian functions is derived. Expectation values for the O-H stretch nuclear states are variationally determined with the use of shifted Gaussian functions as the basis set. The results of the calculations are compared with the recent matrix-isolation infrared (IR) spectroscopic results. The calculated transition frequency corresponding to the in-plane O-H stretching is found to be in good agreement with the experimental value.
Critical depletion of fluids in pores: Competing bulk and surface fields
Authors: Maciołek A. , Ciach A. , Evans R.
Published: July 2002

We investigate the effects of competing bulk and surface fields on the adsorption of near-critical fluids confined in model pores. Our study is motivated by recent measurements for SF6 adsorbed in a mesoporous glass which showed the phenomenon of critical depletion in that the adsorption excess first increases but then decreases rapidly to negative values as the temperature approaches the bulk critical temperature from above, following a near-critical isochore. We argue on the basis of (i) critical scaling ideas, (ii) exact transfer matrix calculations in two dimensions, and (iii) mean-field calculations, for the lattice gas confined between two planar walls, that critical depletion is a general phenomenon resulting from competition between positive adsorption (the walls or surface fields favor the dense liquid phase) and negative adsorption associated with the presence of a bulk field which favors the dilute gas phase. The temperature at which crossover from critical adsorption, of the type associated with a single wall, to depletion, depends on the magnitude of the bulk field and the width of the pore, and we identify the appropriate scaling relations. We discuss the extent to which results for our idealized model can account for the experimental observations and how these relate to the results of computer simulations of a Lennard-Jones fluid in a planar slit.
Transient nucleation on inhomogeneous foreign substrate
Authors: Kožı́šek Z. , Demo P. , Nesladek M.
Published: July 2002

Nucleation from the vapor phase on a foreign substrate with a low number of inhomogeneities is studied. Kinetic equations describing the 3D nucleation on active sites are solved numerically. It is shown that the cluster flux (i.e., the rate of formation of nuclei of a given size) reaches some extremal value at certain time and then tends to zero as time increases. In such a sense, formation of nuclei is a fully nonstationary process, in contrast to the standard nucleation theory. The total number of nuclei formed on a unit surface reaches for a sufficiently long time some stationary value, which is close to the number of nucleation active sites.
An accelerated Metropolis method
Authors: Stedman M. L. , Foulkes W. M. C. , Nekovee M.
Published: July 2002

We propose a stochastic method to reduce the autocorrelation time of a general Monte Carlo (MC) method and apply it to the variational quantum Monte Carlo (VMC) simulation of full-core atoms. We achieve a reduction in autocorrelation time of at least a factor of four compared with the standard method. Further, we find an approximate analytic fit to our results which gives a comparable reduction in autocorrelation time at essentially no cost. Our analytic form is independent of the geometry of the system being modeled and, therefore, can be easily applied to the VMC simulation of solids; it may also prove useful in any MC simulation where there are widely varying length scales. Results are presented for C, F, and Si.
Relaxation of a system with a conical intersection coupled to a bath: A benchmark 24-dimensional wave packet study treating the environment explicitly
Authors: Worth G. A. , Meyer H.-D. , Cederbaum L. S.
Published: July 2002

In this paper the phenomenon of the ultrafast relaxation of a system coupled to a small bath is treated. The system, representing the pyrazine molecule, contains 4 vibrational modes and 2 electronic states coupled via a conical intersection, and the bath is a set of up to 20 harmonic oscillators. The dynamics of the complete system are described by wave packet propagation using the multiconfiguration time-dependent Hartree method. By the use of multidimensional single-particle functions it was possible to obtain results of a high quality, even for the complete system with 24 degrees of freedom. The full wave function for the system and bath are analyzed to reveal the characteristics of the system-bath interaction, such as energy transfer to the bath, or the effect of the bath on the state populations. The results show that the damping due to the bath model adopted is not only nonhomogeneous but also selective: certain high frequencies are found to remain at long times.
Anion photoelectron spectroscopy of I2− and I2−⋅Arn (n=1–14, 16, 20) clusters
Authors: Asmis Knut R. , Taylor Travis R. , Xu Cangshan , Neumark Daniel M.
Published: July 2002

We report the mass-selected anion photoelectron spectra of I2− and the weakly bound clusters I2−⋅Arn (n=1–14, 16, 20) measured at a photon energy of 4.657 eV. The experiment yields size-dependent vertical and adiabatic detachment energies for the formation of the ground state and five valence-excited states of the neutral cluster, which correspond to the 1∑g+ (X), 3Π2u (A′), 3Π1u (A), 3Π0−u (B′), 1Π1u (B″), and 3Π0+u (B) states of bare I2. The detachment energies are successively blue-shifted with increasing cluster size, indicating a stronger stabilization of the anionic cluster relative to the neutral counterpart. The blue shift is of similar extent for the electronically excited states A′ and A and approximately 10% less for the X state. The I2− and I2−⋅Ar spectra are simulated employing a Franck–Condon analysis, from which we estimate the ion vibrational temperature and determine the I2−–Ar binding energy (D0=53±4 meV). The results are discussed with respect to possible cluster geometries and the evolution of the total and stepwise solvation energies. For I2−⋅Ar6 we present evidence that all Ar atoms are bound around the waist of the I–I bond.
Convergence analysis of the addition theorem of Slater orbitals and its application to three-center nuclear attraction integrals
Authors: Bouferguene Ahmed , Jones Herbert W.
Published: July 2002

The mathematical foundation of the methods using addition theorems to evaluate multicenter integrals over Slater-type orbitals is actually well understood. However, many numerical aspects of such approaches still require further investigations. In the framework of these methods, multicenter integrals are generally represented by infinite series which under certain circumstances are very slowly convergent. Accordingly, the determination of the convergence type of such series is of great importance since it allows one to choose adequately the convergence accelerator to be used in the summation procedure. In this work, the convergence of the two-range addition theorem proposed by Barnett and Coulson [Philos. Trans. R. Soc. London, Ser. A 243, 221 (1951)] is analyzed. The results obtained from this study are then applied to study the convergence of three-center nuclear integrals, and most importantly, to discuss the choice of the convergence accelerator to be used in the summation procedure.
Real-space renormalization group technique for low-lying energy states in chain folding
Authors: Tschöp Wolfgang , Noolandi Jaan
Published: July 2002

We apply a real-space renormalization-group (RG) technique to study the low-lying energy states of a strongly interacting copolymer chain on a three-dimensional simple cubic lattice. The chain is 27 beads long and consists of randomly chosen hydrophobic and polar components whereby the system favors interactions between similar beads. The RG scheme is applied by dividing the chain into different parts (conformons) and applying a variety of boundary conditions to find the embedded conformons with the lowest energy. The conformations of the low-lying energy states are found by combining the conformons of different parts of the chain. We show that the 27 beads chain folds into a 3×3×3 cube for the lowest energy state. We also obtain conformations which do not fold into a cube but have structural similarities to the lowest energy state. The model is static but can help to understand folding processes of polymeric molecules and can be extended to more complex situations.
A new interpretation of the bonding and spectroscopy of the tetraoxoferrate(VI) FeO42− ion
Authors: Al-Abdalla Abdalla , Seijo Luis , Barandiarán Zoila
Published: July 2002

In this paper we present an ab initio study of the absorption spectrum of the FeO42− ion. The wavefunctions and energies of the ground and excited states of the FeO42− cluster are calculated by means of the Restricted Active Space self-consistent-field method (RASSCF). The molecular orbitals of the cluster with main character Fe(3d) define a complete active space; all single, double, triple, and quadruple excitations from the molecular orbitals of main character O(2p) to those of main character Fe(3d) are allowed. The multiconfigurational expansions resulting from these ligands-to-metal excitations include between 50000 to 100000 configuration state functions. The results of the calculations lead to a new interpretation of the bonding and of the absorption spectra of FeO42− (which were observed in the solid state and in solution), both of them stem from the near degeneracy between Fe(3d) and O(2p) levels, which is ultimately due to the high and unstable oxidation state of Fe(VI) in the FeO42− complex. The analysis of the ground and excited state wavefunctions reveals that the electronic structure of FeO42− does not correspond to the ionic image of Ligand Field Theory [d2-Fe(VI)+closed-shell O2− ions], nor does it correspond to simple extensions of it which take into account ligands-to-metal 2p→3d single excitations, nor to any other simple image; on the contrary, it corresponds to the superposition of a large number of configurations with a very large weight of high-order ligands-to-metal excitations, which indicates a remarkable intra-cluster inwards delocalization of electron density away from the closed-shell ligands, impelled by the unstable high formal charge of Fe(VI). The calculated absorption spectrum allows for a thorough interpretation of the features observed in the experimental spectra measured in Fe(VI)-doped K2MO4 (M=S, Cr) and in 9 M KOH solution (absorption maxima, intensities, electronic origins, band shapes), which implies completely new assignments. This is particularly so for the broad intense bands observed between 10000–25000 cm−1, which, according to our calculations, are found to be associated to electronic transitions from the 3A2 ground state to increasingly dense sets of excited states that include not only spin singlet and triplet states (as expected for a d2 configuration from Ligand Field Theory), but also spin quintet electronic states, all of which can be understood as direct effects of the above-mentioned oxygens(2p)-iron(3d) near degeneracy.
Ionization potentials and bond energies of TiO, ZrO, NbO and MoO
Authors: Loock Hans-Peter , Simard Benoit , Wallin Sara , Linton Colan
Published: July 2002

The adiabatic ionization potentials of TiO, ZrO, NbO, and MoO have been measured using two-color photoionization efficiency (PIE) spectroscopy and mass-analyzed threshold ionization (MATI). From the sharp ionization thresholds in the PIE and MATI spectra the following ionization potentials were derived: IP(TiO)=6.8197(7) eV, IP(ZrO)=6.812(2) eV, IP(NbO)=7.154(1) eV, and IP(MoO)=7.4504(5) eV. These values have been combined with the ionization potentials of the metal atoms and the bond energies of the transition metal oxide cations, D0(MO+) [M. R. Sievers et al., J. Chem. Phys. 105, 6322 (1996)] to derive the bond energies, D0(MO), of the neutral metal monoxides; D0(TiO)=6.87(7) eV, D0(ZrO)=7.94(11) eV, D0(NbO)=7.53(11) eV, D0(MO)=5.44(4) eV. It is argued that these values are more accurate than the currently accepted values and hence are recommended for future work. Experimental evidence suggests that the ground state of MoO+ is the Σ−4 state arising from the δ2σ1 configuration.
Entropy-enthalpy compensation: Conformational fluctuation and induced-fit
Authors: Qian Hong
Published: July 2002

A localized change in a protein, which occurs as a result of either ligand binding or single amino acid substitution, necessarily encounters the conformational fluctuation of the rest of the protein. Both the entropy and the enthalpy associated with the change consist of contributions from fluctuations in the atoms surrounding the localized site, but they compensate. A novel thermodynamic ensemble with a fluctuating boundary is proposed for studying the energetics of localized changes in proteins. Using an ideal gas as illustration, it is shown that the entropy-enthalpy compensation reflects the flexibility of the surrounding structures—its fluctuations contribute a term to the entropy and the conformational change associated with the induced fit contributes a term to the enthalpy.
Size distributions of metal nanoparticles in polyelectrolyte gels
Authors: Svergun D. I. , Shtykova E. V. , Dembo A. T. , Bronstein L. M. , Platonova O. A. , Yakunin A. N. , Valetsky P. M. , Khokhlov A. R.
Published: July 2002

Small-angle x-ray scattering is used to study size distributions of noble metal nanoparticles embedded in polyelectrolyte hydrogels with oppositely charged surfactants. A procedure is proposed to subtract matrix scattering and to extract pure scattering due to the nanoparticles allowing to evaluate their size distribution functions by means of a regularization technique. Two kinds of collapsed gel-surfactant complexes were studied: a complex of a cationic gel of poly(diallyldimethylammonium chloride) with an anionic surfactant sodium dodecyl sulfate (PDADMACl/SDS), and that of an anionic gel of poly(methacrylic acid) with a cationic surfactant cetylpyridinium chloride (PMA/CPC). Addition of a gold compound (HAuCl4⋅3H2O) to the PDADMACl/SDS system forms the metal compound clusters and leads to a partial distortion of the gel structure. After subsequent reduction of the gold compound with sodium borohydride (NaBH4) ordering in the gel disappears and gold nanoparticles are formed. Their size distribution includes a fraction of small particles with approximately the same size as the compound clusters before reduction and a fraction of larger particles with the radii up to 40 nm. For the collapsed PDADMACl/SDS gels, aging does not change the size distribution profile; for the noncollapsed PDADMACl gels without surfactant, metal particles are found to grow with time. This suggests that the aggregation of metal colloids is prevented by the ordering in the collapsed gel-surfactant complex. The addition of HAuCl4⋅3H2O and the subsequent reduction of the metal ions in the PMA/CPC system does not distort the gel structure as the degree of incorporation of AuCl4− ions is very low. Particle sizes in the PMA/CPC system are found to be somewhat larger than those in the PDADMACl/SDS system. The PDADMACl/SDS gels loaded with the PtCl4 compound were also studied to analyze the influence of the reducing agent type on the particle size distribution distributions. Fast reduction with NaBH4 yielded mostly small particles with the radii around 2 nm grown from the compound clusters similar to those observed for the gold-loaded gels. In contrast, slow reduction with N2H4⋅H2O was found to produce larger nanoparticles and the size distribution function shows a major fraction of the particles with the radii up to 30 nm.
The transition dipole moment function of the chlorine E0g+(3P2)−B3Π0u+ system
Authors: Bibinov N. K. , Davydov V. K. , Fateev A. A. , Kokh D. B. , Lugovoj E. V. , Ottinger Ch. , Pravilov A. M.
Published: July 2002

Specific rovibrational levels of the E0g+ ion-pair state of Cl2 have been laser excited by optical–optical double resonance (OODR) via the B0u+ state. A two-color, single-laser visible+ultraviolet (VIS+UV) scheme was used, where the UV radiation was derived by second-harmonic generation from the VIS laser output. Spectrally resolved emission at 240–265 nm was observed. The E0g+→B0u+ spectra exhibited both discrete and continuous (Condon oscillation) parts, from which the E→B electronic transition moment curve Me(R) was derived. It is compared with corresponding results on ion-pair states of I2.
Strong-field optical control of vibrational dynamics: Vibrational Stark effect in planar acetylene
Authors: Liu Li , Muckerman James T.
Published: July 2002

We perform quantum mechanical simulations of vibrational excitation of planar (5D) acetylene (HCCH) with linearly polarized, intense but nonionizing, infrared laser pulses, exploring one particular pathway for exciting the HCCH molecule into a CC-stretching state via the fundamental excitation in a two-pulse scheme. We optimize the pulse widths, time centers, and carrier frequencies of the two pulses to achieve the maximal projection onto the target CC-stretching state, (0,3,0,0,0) A1, subject to penalties related to peak electric field and pulse duration. The influences of Fermi resonance, the vibrational Stark effect, and avoided crossings on the selective excitation are discussed. Different sizes of “essential-states” representation are used and checked against the underlying 299 475-point discrete variable representation (DVR) basis. We find that an essential-states basis consisting of 362 A1 and B2 eigenstates represents the results of a full-grid calculation for the excitation process under study. Moreover, we demonstrate that despite the complications associated with the higher dimensionality of the 5D model, we can nevertheless exert infrared laser control over the vibrational dynamics of selective excitation.
Highly charged colloidal suspensions in planar slits
Authors: Kim Young-Wha , Kim Soon-Chul , Suh Soong-Hyuck
Published: July 2002

A simple density functional approximation, which is based both on the density functional Taylor series expansion of the one-particle direct correlation function (DCF) and on the exact contact value theorem for a structureless hard wall with infinity wall distance, has been developed to study the structural properties of a highly charged colloidal suspension and compared with the computer simulation. Two kinds of two-particle DCFs, which are the rescaled mean spherical approximation (RMSA) and the Rogers-Young (RY) closure relation, have been used to calculate the local concentration profiles of a highly charged colloidal suspension confined in charged silt walls as input. The calculated results show that for a structureless hard wall the present approximation is better than the hypernetted chain (HNC) functional approximation and describes well the structural properties of a confined colloidal suspension compared with the computer simulation. However, the agreement between the computer simulation and the theory for the density profiles deteriorates with increasing the wall-particle interaction.
Photodissociation of OClO and Ar/OClO and H2O/OClO clusters studied by the resonance enhanced multiphoton ionization-time of flight method
Authors: Kreher Christoph J. , Carter Robert T. , Huber J. Robert
Published: July 2002

The photodissociation of jet-cooled OClO following excitation into the Ã 2A2 state at around 350 nm was investigated in homogeneous OClO and large heterogeneous Ar/OClO and H2O/OClO clusters (estimated cluster size n̄∼800–2600) by probing the O (3P) and ClO (X̃ 2Π) photofragments using the resonance enhanced multiphoton ionization-time of flight technique. Action spectra, photofragment excitation spectra and photofragment speed distributions were recorded and compared to those for monomer dissociation. OClO was found to occupy both surface and interior sites in the heterogeneous clusters with the percentage of surface and interior dissociation processes being ∼50% for large cluster sizes. Both O and ClO photofragments generated in the cluster interior are translationally thermalized with T∼300 K and the ClO fragments are strongly rotationally and vibrationally relaxed. This is most important for vibration as monomer dissociation yields ClO containing up to 8 vibrational quanta at this photolysis wavelength. Photodissociation on the cluster surface is found to proceed with little interaction with the cluster host. The distribution of counterfragment masses leads to a broadening of the speed distributions compared with monomer dissociation. In addition, cluster chemistry was found to occur in OClO-rich heterogeneous clusters as manifested by detection of O photofragments with velocities exceeding the highest thermodynamically possible value. This result, consistent with that from homogeneous OClO cluster dissociation, indicates the presence of small OClO aggregates on the surface and within heterogeneous clusters. From a standpoint of atmospheric chemistry, H2O/OClO clusters yield a substantial fraction of thermalized primary photofragments, in contrast to OClO monomer dissociation.
Microphase separation versus the vapor-liquid transition in systems of spherical particles
Authors: Sear Richard P. , Gelbart William M.
Published: July 2002

The conditions, i.e., the interparticle potentials, for which vapor–liquid coexistence is supplanted by microphase separation are examined. The liquid phase is found for potentials with a steeply repulsive core and an attraction of not-too-short a range. To this potential we add a repulsion of longer range than the attraction and find that it breaks up and mixes coexisting bulk vapor and liquid to form a single microphase-separated phase. Our spherical particles are perhaps the simplest continuum model to show a Lifshitz point and microphase separation. In the microphase-separated phase the density is modulated with a period typically ten times the size of the particle’s core. Microphase separation, apart from providing a rich and interesting phase behavior, may have a potential use in nanoelectronic applications.
Theoretical description of the interaction of CO adsorbed on a n(=1,2,⋯)×Ar/Pt(111) substrate: The transition from chemisorption to physisorption
Authors: Castells Victoria , Atabek Osman , Beswick Alberto
Published: July 2002

Potential energy calculations have been performed for the system CO/n×Ar/Pt where the argon atoms play the role of spacer layers. A detailed analysis of the construction of this multidimensional potential energy surface is presented and discussed. The change of the nature of the adsorbate–substrate bond going from chemisorption to physisorption is studied within the frame of a stepwise approach. First we investigate an incommensurate model in which no coupling between the argon and the platinum atoms is considered. Several convergence tests have been done concerning the size and the binding sites of the metal surface, the rare gas network, and the combined system in order to ensure the stabilization of the calculations. A structural analysis of this potential energy surface is made considering the minima of the potential interaction, the bending angle of the CO with respect to the normal to the surface, and the distance between the CO center of mass and the surface. In a second stage of our study the lateral Ar–Ar and the Ar–Pt corrugation interactions are included in order to consider commensurate criteria. A new analysis of the behavior of the main physical observables of the system is given and the dependence of the calculations on the variation of the argon lattice parameter is shown as a function of the number of argon spacer layers. The results show that the equilibrium value of the argon lattice parameter changes when the number of spacer layers increases. It is found that the main contribution to the change in the strength of the force field between the molecule and the metal surface is given by the introduction of the first two argon spacer layers. Additional layers produce a smooth variation within the physisorption regime.
A minimal subspace residual method for large-scale eigenvalue problems
Authors: Huang Youhong , Hoffman David K. , Kouri Donald J.
Published: July 2002

We present an iterative method for treating extremely large-scale eigenvalue problems. Based on an exact formula and the GMRES method, our approach generates a subspace which has the property that the residual of interior eigenpairs in the subspace is minimized. The result is that the corresponding large matrix is block-diagonalized iteratively. The accuracy of the final eigenpairs of interest is directly controlled by the accuracy of the GMRES procedure. Our method limits the number of Arnoldi iterations involved, and the dimension of the subspace, by including the residual in the subspace and minimizing it at each step of the iteration.
Simulation of gas diffusion and sorption in nanoceramic semiconductors
Authors: Skouras E. D. , Burganos V. N. , Payatakes A. C.
Published: July 2002

Gas diffusion and sorption in nanoceramic semiconductors are studied using atomistic simulation techniques and numerical results are presented for a variety of sorbate–sorbent systems. SnO2, BaTiO3, CuO, and MgO substrates are built on the computer using lattice constants and atomic parameters that have been either measured or computed by ab initio methods. The Universal force field is employed here for the description of both intramolecular and nonbonded interactions for various gas sorbates, including CH4, CO, CO2, and O2, pure and in binary mixtures. Mean residence times are determined by molecular dynamics computations, whereas the Henry constant and the isosteric heat of adsorption are estimated by a Monte Carlo technique. The effects of surface hydroxylation on the diffusion and sorption characteristics are quantified and discussed in view of their significance in practical gas sensing applications. The importance of fast diffusion on the response time of the sensitive layer and of the sorption efficiency on the overall sensitivity as well as the potential synergy of the two phenomena are discussed.
Polymer dynamics in bimodal polyethylene melts: A study with neutron spin echo spectroscopy and pulsed field gradient nuclear magnetic resonance
Authors: Rathgeber S. , Willner L. , Richter D. , Brulet A. , Farago B. , Appel M. , Fleischer G.
Published: July 2002

We have investigated the dynamics of polymers in bimodal polyethylene (PE) melts in the transition region from Rouse- to reptationlike behavior by varying the mass fraction Φt of long tracer chains (N≈3Ne or 4Ne) in a short-chain matrix (N≈Ne=entanglement segment number) over the full concentration range. At short times (ns) the dynamic structure factor for single-chain relaxation was investigated by neutron-spin-echo (NSE) spectroscopy. To obtain information about the long-time (ms) dynamics the tracer diffusion coefficient (DNMR) was measured by pulsed-field-gradient (PFG)-NMR. We discuss our NSE data within a mode analysis which includes the relaxation rates Wp of the independent normal modes of the internal chain dynamics and the center-of-mass diffusion coefficient DNSE as model parameters. Only modes exceeding the Φt-dependent length of a single entanglement strand Ne(Φt) are found to be strongly hindered by topological constraints. The DNSE are Φt-independent and systematically faster than the strong concentration-dependent DNMR, suggesting an effective time-dependent diffusion coefficient. The Hess model, which we have generalized for polydisperse melts, provides a time-dependent diffusion coefficient. Taking chain-end effects into account we get an excellent description of the NSE data. The mobility of the chain ends is much higher than the mobility of the inner segments resulting in an entanglement segment number which increases with decreasing tracer concentration. The concentration dependence of Ne(Φt), as obtained from the mode analysis and the Hess model, is in agreement with our calculation within a self-consistent modification of the model by Kavassalis and Noolandi for entanglement formation.
The Fe57 nuclear magnetic resonance shielding in ferrocene revisited. A density-functional study of orbital energies, shielding mechanisms, and the influence of the exchange-correlation functional
Authors: Schreckenbach Georg
Published: July 2002

The Fe57 nuclear magnetic resonance (NMR) shielding and chemical shift in ferrocene, Fe(C5H5)2, are studied using density functional theory (DFT) and gauge-including atomic orbitals (GIAO). Electronic factors contributing to the chemical shift are discussed in detail. It is shown that the chemical shift is entirely determined by paramagnetic contributions which in turn are dominated by metal based occupied-virtual d→d couplings. In particular, the HOMO-1(a1′) and the HOMO (e2′) couple with the LUMO (e1″). It is argued that the Fe57 nucleus in ferrocene is less shielded than in the reference compound (iron pentacarbonyl) due to a smaller HOMO-LUMO gap, resulting in stronger interactions between occupied and virtual orbitals. The influence of the XC functional on the calculated molecular orbital (MO) energies of frontier orbitals is discussed. Different generalized gradient approximations (GGA) give similar results whereas hybrid functionals that incorporate part of the Hartree–Fock exchange stabilize occupied MOs strongly and destabilize virtual MOs. HOMO-LUMO gaps are nearly doubled as a result. The previously noted “dramatic influence” of different exchange-correlation (XC) functionals on the calculated chemical shifts is analyzed. The influence of the XC functional is realized through the paramagnetic part of the shielding; hybrid functionals increase it in absolute terms as compared to pure DFT (GGA). It is argued that three factors are responsible. These are (i) the increased occupied-virtual gaps, (ii) the more diffuse nature of virtual orbitals, and (iii) the coupling due to the Hartree–Fock exchange in hybrid functionals. The last two factors increase the paramagnetic part of the shielding, and this effect is only partly reversed by the increased occupied-virtual gaps that result in reduced interactions. It is suggested that new model XC functionals for the calculation of NMR shieldings should be developed aiming at both, accurate energetics and a correct representation of the XC potential. The wealth of precise experimental NMR data could be used as input for this purpose.
Lie algebraic approach to potential energy surface for symmetric triatomic molecules
Authors: Ding Shiliang , Zheng Yujun
Published: July 2002

Potential energy surfaces play an important role in studying theoretical chemistry. In this work, the expression of the potential energy surface containing information about the bending motion of triatomic molecules is derived by using the semiclassical limit of the algebraic Hamiltonian with the dynamical symmetry group U1(4)⊗U2(4). And, we also obtain the force constants. The method can be applied to a number of stable triatomic molecules, such as, H2O, H2S.
Electron diffraction studies of metal phthalocyanines, MPc, where M=Sn, Mg, and Zn (reinvestigation)
Authors: Ruan Chong-yu , Mastryukov Vladimir , Fink Manfred
Published: July 2002

The structure of two metal phthalocyanines, MPc where M=Sn and Mg, have been determined by gas-phase electron diffraction for the first time; furthermore, the structure of ZnPc established earlier by the same technique has been reanalyzed. The focus of these studies is the position of the metal ion relative to the 16-membered C8N8 ring. Tin(II)phthalocyanine we find to be nonplanar with the Sn(II) ion 1.0(1) Å above the molecular plane in accordance with its structure in crystal established earlier; the two other molecules were found to be planar. Slight nonplanarity of ZnPc found in the previous study is now removed by a new procedure of structural analysis; this is in accordance with the results of ab initio/DFT calculations for this molecule performed by Pulay. From comparisons presented here we can conclude that there is a close structural similarity of metal phthalocyanines in the gas phase and in crystal.
Ultrafast extended x-ray absorption fine structure (EXAFS)—theoretical considerations
Authors: Brown Frank L. H. , Wilson Kent R. , Cao Jianshu
Published: July 2002

Inspired by the recent experimental demonstration of ultrafast x-ray absorption spectroscopy, we present a framework for the calculation of extended x-ray absorption fine structure (EXAFS) spectra on the ultrafast (femtosecond to picosecond) time scale. Model calculations for gas phase I2, evolving under the influence of laser pumping, demonstrate that ultrafast EXAFS has the potential to serve as a direct probe of nuclear dynamics, including time-dependent interatomic separations and relative orientations. The feasibility of ultrafast EXAFS as a viable and useful experimental technique is discussed.
An integral representation of isolated binary collisions in vibrational relaxation
Authors: Cherayil Binny J.
Published: July 2002

Path integral techniques are used to show that the quantum mechanics of a harmonic oscillator driven by Campbell’s noise provides an exact integral representation for a model of condensed phase vibrational relaxation in which the rate of transitions from one excited state to another is governed by independent, pairwise collisions between the excited state species and the molecules of the surrounding medium. In the limit of infrequent collisions and delta correlated noise, this exact representation of the rate can be reduced to the product of the frequency of collisions and a quantity that is related to the likelihood of a vibrational transition in a given collision. In this form, the vibrational relaxation rate has the same structure as the rate expression that can be regarded as the defining relation of the isolated binary collision model.
Theoretical characterization of the electronic properties of extended thienylenevinylene oligomers
Authors: Krzeminski C. , Delerue C. , Allan G. , Haguet V. , Stiévenard D. , Frère P. , Levillain E. , Roncali J.
Published: July 2002

We present semiempirical tight binding calculations on thienylenevinylene oligomers up to the hexadecamer stage (n=16) and ab initio calculations based on the local density approximation up to n=8. The results correctly describe the experimental variations of the gap versus size, the optical spectra, and the electrochemical redox potentials. We propose a simple model to deduce from the band structure of the polymer chain the electronic states of the oligomers close to the gap. We analyze the evolution of the gap as a function of the torsion angle between consecutive cells: the modifications are found to be small up to a ∼30° angle. We show that these oligomers possess extensive π-electron delocalization along the molecular backbone which makes them interesting for future electronic applications such as molecular wires.
Description of spin interactions in model [Fe6S6]4+ supercluster
Authors: Matusiewicz M. , Czerwiński M. , Kasperczyk J. , Kityk I. V.
Published: July 2002

A theoretical study of Heisenberg exchange and double exchange interactions in iron–sulphur superclusters is presented. The clusters can play an important role in biological systems (proteins and enzymes) acting as so-called active centers. The cluster with valence 4+ can be modeled by four Fe(III) and two Fe(II) ions. An idealized structure of double cubane has been considered instead of a more realistic defected double cubane structure of lower symmetry. Possible spin ground states (S=0,1,2,3,4,5,6) have been predicted. The total spin S=6 is preferred within a partially ferromagnetic region if a Heisenberg exchange parameter [between the nearest Fe(III) ions] J2<0 although increase of a double exchange parameter b leads to narrowing of those region in favor of the states with lower spins. If J2>0, domination of the spin states with S=0 and S=4 is expected for larger values of the parameter b. The state of S=4 is predicted for J2>0 in agreement with experimental data.
Multireference perturbation configuration interaction methods for solvated systems described within the polarizable continuum model
Authors: Mennucci Benedetta , Toniolo Alessandro , Cappelli Chiara
Published: July 2002

We present a method to include solvent effects described within the polarizable continuum model into the CIPSI multireference perturbation algorithm. In the methodology we have formulated and implemented, solvent interactions are explicitly included in the configuration interaction scheme and in the following perturbative corrections, through proper operators. The nonlinear character induced by such operators leads to an iterative procedure in which solute and solvent can mutually equilibrate. Applications to the electronic excitation spectrum of formamide are considered. In these cases, effects due to an incomplete electrostatic response of the solvent (nonequilibrium model) as well as repulsion interactions between solute and solvent have been included in the quantum mechanical description.
Electron–phonon interactions in poly(para-phenylene) oligomers
Authors: Lavrentiev Mikhail Yu. , Barford William
Published: July 2002

The influence of molecular vibrations on the low-lying electronic levels of short oligomers of poly(para-phenylene) is studied. Within an interacting molecular orbital basis, the complete electron–phonon Hamiltonian is diagonalized numerically. Energy shifts and equilibrium phonon displacements are calculated for the totally symmetric vibrational modes. For the 1 1Ag+–1 1Bu− transition, Huang–Rhys and Franck–Condon factors, as well as the change of equilibrium geometry, are computed. The importance of vibrations with a frequency of about 0.2 eV is confirmed. The intrinsic Stokes shift is found to be zero, in agreement with experimental data.
Response to “Comment on ‘Calculation of ab initio dynamic hyperpolarizabilities of polymers’ ” [J. Chem. Phys. 112, 1616 (2000)]
Authors: Otto P. , Gu F. L. , Ladik J.
Published: July 2002

The probable explanation for the discrepancy between our calculations of the hyperpolarizability and those of Jacquemin and Champagne [J. Chem. Phys. 112, 1616 (2000)] is considered.
Nuclear dynamics in the vicinity of the crossing seam: Theory and application to vibrational spectrum of H3
Authors: Varandas A. J. C. , Xu Z. R.
Published: July 2002

Based on the equivalence of the gauge transformations for the nuclear and electronic wave functions, the physical meaning of the geometric phase effect has been disclosed for a two coupled-state system. It is found that the geometric phase A(R) is defined by the argument of the complex electronic vector state in the complex plane spanned by the two real-valued electronic components. Such an angle is identical (up to a constant) to the mixing angle γ(R). Novel generalized Born–Oppenheimer equations for the two coupled-state problem in the vicinity of the crossing seam have been derived, and numerical calculations of vibrational spectra done for H3. The results demonstrate significant differences in relation to those obtained from the assumption that A(R)=φ/2.
Optical limiting properties of mono- and multiple-functionalized fullerene derivatives
Authors: Riggs Jason E. , Sun Ya-Ping
Published: July 2002

Optical limiting properties of a series of mono- and multiple-functionalized fullerene derivatives were investigated systematically in room-temperature solutions at different concentrations. The results provide overwhelming experimental evidence for the strong concentration dependence of optical limiting in fullerene solutions. Possible explanations of the experimental results in terms of nonlinear scattering and nonlinear absorption mechanisms are considered. For the latter, the involvement of excited state bimolecular processes is proposed. Within the context of reverse saturable absorption, the strong concentration dependence of optical limiting in fullerene solutions is explained in terms of the formation of strongly absorptive transient species from the excited state bimolecular processes. The bimolecular processes are probably dependent not only on the solution concentration but also on the molecular diffusivity or medium viscosity, which thus accounts for the fact that the optical limiting responses in a highly dilute solution agrees well with those in a polymer matrix. The relationship of such a mechanism that involves excited state bimolecular processes with the classical five-level reverse saturable absorption model is discussed.
Cross polarization induced by temporary adsorption: NMR investigations on nanocapsule dispersions
Authors: Hoffmann D. , Mayer C.
Published: July 2002

Nanocapsule dispersions are investigated by 1H–13C cross polarization experiments. The capsule walls, made up from polybutylcyanoacrylate, are detectable merely as a solid state signal, whereas the triglyceride forming the liquid core and the surfactant on the outside of the capsules give rise to narrow resonance lines in the cp-spectrum indicating their mobility. In order to clarify the mechanism of cross polarization for the mobile components, the time-resolved polarization of the corresponding 13C-signals is investigated. Further, the spin-lattice relaxation times for the proton magnetization in the rotating frame are studied under direct excitation as well as under cross polarization conditions. Based on the results, we propose a special mechanism of cross polarization which is initiated by a temporary adsorption of mobile molecules to the more rigid capsule surface. During this short period of immobilization, the polarization is transferred from 1H to 13C nuclei. After desorption, the freshly generated 13C-signal is essentially conserved, now being motionally averaged by a high molecular mobility and detectable as a narrow line in the corresponding spectrum.
Adsorption of water on the NaCl(001) surface. II. An infrared study at ambient temperatures
Authors: Foster Michelle C. , Ewing George E.
Published: July 2002

Water adsorbed on the (001) face of NaCl under ambient conditions has been studied by infrared spectroscopy. From these measurements, combined with recent Monte Carlo calculations, we find evidence of two structures for the adsorbed water. At low coverages, the water molecules aggregate into islands on the surface. When a critical concentration is reached, multilayer growth becomes favorable, creating a thin film on the surface with properties similar to liquid water.
Exponentially correlated Gaussian functions in variational calculations: Quadrupole moment for the ground state of helium dimer
Authors: Komasa Jacek
Published: July 2002

Exponentially correlated Gaussian wave functions of 1200 terms were employed to compute expectation values of the quadrupole moment, Θ, for the ground state of helium dimer. The quadrupole moment was calculated in the frames of the Born–Oppenheimer approximation for a wide range of internuclear distances, 0.0⩽R/a0⩽15.0. At the equilibrium distance Θ=−6.2(1)×10−4 ea02 and the estimated vibrational average amounts to −5×10−7 ea02. For small R, the quadrupole shielding function, γ(R)=−Θe(R)/Θn(R), was constructed, which allowed the quadrupole shielding factor γ∞ of beryllium atom to be estimated as equal to +1.5(1). As the wave functions employed are the most accurate variational wave functions for He2, the quadrupole moment results presented in this paper may serve as benchmarks.