bluuebunny/crossref_metadata_2025 huggingface dataset:
Low energy cluster ion–atom collision: Collisional energy transfer and complex formation of Ar+n with 36Ar
Authors: Ichihashi Masahiko , Nonose Shinji , Nagata Takashi , Kondow Tamotsu
Published: July 2002

The collision-induced reaction of Ar+n (n=2–23) with 36Ar was investigated at the collision energies of 0.2 and 2.0 eV. The product ions were Ar+n′ and 36ArAr+n′−1 (n′<n). The absolute cross sections for the product formation were obtained as a function of the size of the parent cluster ion. The cross section for the formation of Ar+n′ (evaporation) increased rapidly with n in the n≤9 range, and slowly above it; the cross section at n=23 was ≊180 Å2. On the other hand, the cross section for the 36ArAr+n′−1 formation (fusion) started to rise at n=7 and increased slowly as n increases. In order to elucidate the reaction scheme, the molecular dynamics (MD) simulation based on the diatomics-in-molecules (DIM) method was performed. The reaction (evaporation and fusion) cross sections estimated from the simulation reproduced the observed ones within an accuracy of 15% at n=6, 13, and 19 at the collision energy of 0.2 eV. The following reaction scheme based on this simulation was found to explain the experimental results: collisional excitation of the parent cluster ion Ar+n and subsequent unimolecular dissociation (evaporation), and formation of a collision complex 36ArAr+n and subsequent unimolecular dissociation (fusion). In addition, it was confirmed by the MD calculation that the fusion proceeded via perfectly inelastic collision and following evaporation in the 0.2 eV collision of Ar+10 with 36Ar.
A deterministic global optimization approach for molecular structure determination
Authors: Maranas Costas D. , Floudas Christodoulos A.
Published: July 2002

A deterministic global optimization algorithm is introduced for locating global minimum potential energy molecular conformations. The proposed branch and bound type algorithm attains finite ε-convergence to the global minimum through the successive refinement of converging lower and upper bounds on the solution. These bounds are obtained through a novel convex lowering bounding of the total potential function and the subsequent solution of a series of nonlinear convex optimization problems. The minimization of the total potential energy function is performed on an independent set of internal coordinates involving only dihedral angles. A number of example problems illustrate the proposed approach.
Quantum three-dimensional calculation of endohedral vibrational levels of atoms inside strongly nonspherical fullerenes: Ne@C70
Authors: Mandziuk Margaret , Bačić Zlatko
Published: July 2002

Detailed results of the quantum 3D calculation of the intermolecular vibrational levels of the endohedral fullerene complex Ne@C70, for the total angular momentum J=0, are presented. They elucidate, for the first time, the nature of the endohedral vibrations of an atom inside a strongly nonspherical fullerene. The calculations were performed using our recently developed method for accurate highly excited 3D intermolecular vibrational states of atom–large molecule complexes [J. Chem. Phys. 98, 7165 (1993)]. The treatment of the coupled endohedral vibrations of Ne@C70 involves no dynamical approximations, apart from taking the fullerene to be rigid, producing eigenstates which are essentially exact for the intermolecular potential energy surface (PES) employed. The 3D endohedral PES for Ne@C70, modeled as a sum of atom–atom Lennard-Jones pair potentials, is anharmonic, especially in the direction of the long (z) axis of C70. The endohedral vibrational energy level structure of Ne@ C70 which emerged from our calculations is surprisingly simple. The coupling between the vibrational mode parallel to the long (z) axis of C70 and the modes perpendicular to it, is weak. The two endohedral modes perpendicular to the (z) axis represent an exemplary case of a 2D isotropic, anharmonic oscillator. The fundamental frequencies for the parallel and perpendicular endohedral mode of Ne@ C70, calculated using the present PES, are 9.83 and 54.49 cm−1, respectively. The wave functions of all endohedral vibrational states of Ne@ C70 up to at least 240 cm−1 above the ground state, considered in this work, have exceptionally regular nodal patterns, allowing complete and unambiguous quantum number assignment.
Adsorption, desorption, and interparticle motion of hydrogen on silica-supported ruthenium: A study by in situ nuclear magnetic resonance
Authors: Engelke Frank , Vincent Robert , King Terry S. , Pruski Marek
Published: July 2002

1H NMR line shapes of hydrogen adsorbed on silica-supported ruthenium at pressures of 10−6–10 Torr were studied by using selective excitation via DANTE sequences. A transition from inhomogeneous to homogeneous line broadening was observed at hydrogen coverage of ∼0.5. The spectra were simulated by using generalized Bloch equations that included N-site exchange processes. The homogeneous line shape originates from increased hydrogen mobility, whereas proton–proton dipolar couplings are negligibly small. A rate parameter k obtained from this model quantifies the average mobility of hydrogen in the exchange process. This parameter increases by more than three orders of magnitude when the hydrogen coverage changes from 0.4 to 0.8. The simulations of line shapes obtained at variable temperatures showed that k exhibits Arrhenius behavior with an activation energy of 52 (±5) kJ/mol and preexponential factor k0=4×1010 s−1. It is implied that the motion of hydrogen must involve desorption, interparticle diffusion, and readsorption.
Evaluation of 〈S2〉 in restricted, unrestricted Hartree–Fock, and density functional based theories
Authors: Wang Jiahu , Becke Axel D. , Smith Vedene H.
Published: July 2002

A simple formalism for the evaluation of 〈S2〉 in terms of the two-particle density matrix is presented. The implementation of the formalism in the restricted open-shell Hartree–Fock (ROHF), unrestricted HF (UHF) and density functional (DFT) based theories is discussed. Rules governing the nonzero S2 matrix elements in the UHF based methods are presented. Further examples are given of 〈S2〉 in several atomic and radical systems from very simple density functional models.
A study of some organic reactions using density functional theory
Authors: Baker Jon , Muir Max , Andzelm Jan
Published: July 2002

Twelve organic reactions (six closed shell and six radical) were studied using semiempirical, traditional ab initio and density functional methodologies. Full geometry optimizations of all species, both minima and transition states, were performed, and calculated geometries and barrier heights compared with experimental data. Our results demonstrate that although currently available density functionals tend to underestimate barrier heights, especially for radical reactions—in some cases reactions with low barriers are predicted to be essentially barrier free—they provide a significant improvement over standard methods. The adiabatic connection method recently proposed by Becke [J. Chem. Phys. 98, 5648 (1993)], in which a portion of the exact Hartree–Fock exchange is mixed in to the density functional, looks very promising.
Ab initio studies of cyclic water clusters (H2O)n, n=1–6. III. Comparison of density functional with MP2 results
Authors: Xantheas Sotiris S.
Published: July 2002

The optimal structures and harmonic vibrational frequencies of ring water clusters (H2O)n, n=1–6 are computed using density functional theory (DFT). The exchange functionals of Slater (S), Becke (B), the correlation functionals of Lee–Yang–Parr (LYP), Vosko–Wilk–Nusair (VWN), Perdew’s local (PL), and gradient-corrected (P86) as well as their combinations are used to perform benchmark calculations on the water monomer and dimer. We use the augmented correlation-consistent polarized valence orbital basis set of double (aug-cc-pVDZ) and triple zeta quality (aug-cc-pVTZ) in order to compare the DFT with earlier MP2 results [J. Chem. Phys. 99, 8774 (1993); 100, 7523 (1994)]. Better overall agreement with the MP2 and experimental results for the water monomer and dimer is observed for the B–LYP and B–P86 functional combinations. The optimal structures, harmonic vibrational frequencies, and energetics of the clusters trimer through hexamer are computed at the B–LYP/aug-cc-pVDZ level of theory. This functional combination yields structures, energetics, and trends with cluster size that are in good agreement with the corresponding MP2 results.
The permanent electric dipole moment of PtO, PtS, PtN, and PtC
Authors: Steimle T. C. , Jung K. Y. , Li B.-Z.
Published: July 2002

The permanent electric dipole moments of the ground, and the low-lying excited electronic states of platinum monocarbide, PtC, platinum monoxide, PtO, and platinum monosulfide, PtS, were measured using a molecular beam optical Stark spectroscopic scheme. The determined values were (in Debye): PtO(X 3Σ−) 2.77(2); PtO(A 1Σ+) 1.15(4); PtS[X(Ω=0)] 1.78(2); PtS[B(Ω=0)] 0.54(6); PtC(X 1Σ+) 0.99(5); and PtC(A 1Π) 2.454(3). These results, along with the previous results for PtN(X 2Π1/2) 1.977(9); PtN(d 4Π1/2) 1.05(9) [J. Chem. Phys. 102, 643 (1995)], are used as a basis for a discussion of the nature of the electronic states.
Pseudospectral localized Mo/ller–Plesset methods: Theory and calculation of conformational energies
Authors: Murphy Robert B. , Beachy Michael D. , Friesner Richard A. , Ringnalda Murco N.
Published: July 2002

We have developed an algorithm based upon pseudospectral ab initio electronic structure methods for evaluating correlation energies via the localized Mo/ller–Plesset methodology of Pulay and Saebo. Even for small molecules (∼20 atoms) CPU times are diminished by a factor of ∼10 compared to canonical MP2 timings for Gaussian 92 and the scaling is reduced from N4−N5 in conventional methods to ∼N3. We have tested the accuracy of the method by calculating conformational energy differences for 36 small molecules for which experimental data exists, using the Dunning cc-pVTZ correlation consistent basis set. After removing 6 test cases on the grounds of unreliability of the experimental data, an average deviation with experiment of 0.18 kcal/mol between theory and experiment is obtained, with a maximum deviation of ∼0.55 kcal/mol. This performance is significantly better than that obtained previously with a smaller basis set via canonical MP2; it is also superior to the results of gradient corrected density functional theory.
On the theory of multiphonon relaxation rates in solids
Authors: Egorov S. A. , Skinner J. L.
Published: July 2002

A theory of multiphonon relaxation for electronically or vibrationally excited impurities in crystals is developed. Two alternative approaches within the static-coupling scheme are presented and their relative importance is discussed. For each approach a closed-form analytical expression for the relaxation rate is given as a function of energy gap and temperature. We consider relaxation either by optical or acoustic phonons. From the analytical expressions we derive approximate energy gap laws for the zero-temperature rates, and discuss the origin and applicability of a popular phenomenological assumption for the rate’s temperature dependence. The predictions of the theory are compared with energy gap- and temperature-dependent experimental data for various electronic transitions of rare-earth impurities in YAlO3.
Electronic spectrum of Tl2
Authors: Stangassinger A. , Bondybey V. E.
Published: July 2002

Several UV electronic transitions observed by laser induced fluorescence of products of pulsed dc discharge sputtering of thallium electrodes are assigned to T12 molecules produced in the supersonic expansion.
Structure and equation of state of a long chain amphiphile monolayer adsorbed on ice Ih: A molecular dynamics study
Authors: Bell Kyle-Pierre , Rice Stuart A.
Published: July 2002

We report the results of molecular dynamics simulations of the structure of the rotator phase of a monolayer of a long chain alkyl carboxylic acid, H(CH2)15COOH, supported on the 0001 face of ice Ih, as a function of area per molecule. The potential field generated by this surface has an ordered component arising from the lattice of oxygens and an irregular component arising from the disordered distribution of hydrogens. Consequently, the many possible head group pinning sites of the ice Ih 0001 surface are irregularly distributed and not commensurate with the oxygen lattice. Our results, which are derived from simulations that use a pseudoatom model of the amphiphile molecule and a rigid lattice model of the ice surface, show that the constraint imposed by the potential field of the rigid ice Ih 0001 surface generates properties of the ice supported monolayer that are different from those of the corresponding liquid water supported monolayer. As expected, we find that the head groups are pinned and that as the area per molecule is increased the monolayer develops a collective tilt; the collective tilt increases as the area per molecule increases. More striking, we find that the projection of the unit cell of the monolayer in the plane of the surface remains, on average, hexagonal for all collective tilt angles and all surface coverages. In contrast, the introduction of a collective tilt of the molecules in the L2 (rotator) phase of a liquid water supported monolayer is always accompanied by a distortion of the projection of the unit cell in the plane of the surface, typically from hexagonal at zero tilt to centered rectangular for all nonzero tilt.

Despite this difference in packing structure, and the pinning of the head groups, the surface pressure as a function of area per molecule, and the collective tilt as a function of area per molecule of a monolayer supported on ice Ih closely resemble those for the L2 phase of a monolayer supported at the air/water interface. The only structural data available for monolayers of long chain molecules on ice Ih refer to a low-temperature (∼100 K) and high surface density (∼18 Å2 per molecule), under which conditions the amphiphile molecules pack in a herringbone structure with a rectangular unit cell in the interface plane. The difference between the properties of this low-temperature monolayer on ice Ih, which is not in a rotator phase, and the properties of the rotator phase we have simulated is briefly discussed.
Theory and modeling of ion–molecule radiative association kinetics
Authors: Klippenstein Stephen J. , Yang Yu-Chuan , Ryzhov Victor , Dunbar Robert C.
Published: July 2002

Various aspects of the theory and modeling of ion–molecule radiative association are discussed. A general formalism for evaluating the effective rate constant for radiative and collisional association is reviewed. The implementation of variable reaction coordinate transition state theory estimates within this formalism is described. A detailed discussion is given of the limiting cases of high and low stabilization efficiency. The basic validity of the algorithm is illustrated through sample calculations for the high efficiency limit. The low efficiency limit allows for the determination of binding energies which are independent of any transition state model. The relation between the predicted and observed temperature dependence in the low efficiency limit is explored. Sample calculations employing the general formalism illustrate the usefulness of this modeling in estimating the binding energy of the complex. Modest levels of quantum chemistry (e.g., MP2/6-31G*) are found to provide satisfactory estimates of the vibrational frequencies and intensities required in the modeling. Overall, the modeling provides estimated binding energies for the protonated acetone dimer, NO+...3-pentanone, and Al+...C6H6 complexes which agree with the available literature values to within 2 kcal/mol.
Predissociation of HONO upon excitation into the S1 state: An ab initio and dynamics study
Authors: Cotting René , Huber J. Robert
Published: July 2002

We have investigated the vibrational predissociation of nitrous acid in the first excited singlet state, HONO (S1) → OH (X̃ 2Π) + NO (X̃ 2Π), by calculating the potential energy surface as a function of the pertinent coordinates. Of the ab initio methods tested in this context, the multiconfigurational second-order perturbation theory (CASPT2) was found to deliver the best trade-off between accuracy and expense of computer time. We solved the nuclear dynamics for a three dimensional model system treating the OH group as a pseudo atom and obtained the absorption spectrum, the lifetimes and the partial cross sections of the NO fragment state distributions. The OH rotational state distribution was assessed with a model treating the NO group as a pseudo atom and keeping the OH bond frozen. Special attention was given to the mechanistic features of the decay such as the coupling effects among the degrees of freedom and the state specific lifetimes, and the assignment of the S0→S1 absorption spectrum of anti- and syn-HONO.
Geometric phase effects in H+O2 scattering. II. Recombination resonances and state-to-state transition probabilities at thermal energies
Authors: Kendrick Brian , Pack Russell T
Published: July 2002

The general vector potential (gauge theory) approach for including geometric phase effects in accurate 3D quantum scattering calculations in hyperspherical coordinates is applied to low-energy (thermal) H+O2 collisions. The hybrid DVR/FBR (discrete variable representation/finite basis representation) numerical technique is used to obtain accurate surface function solutions which include geometric phase effects due to the C2v conical intersection in HO2. The relevant potential coupling and overlap matrices are constructed and a log-derivative matrix of solutions to the coupled-channel radial equations is propagated and transformed to obtain the scattering matrix S. The results for zero total angular momentum (J=0) show significant shifts in the resonance energies and lifetimes. Significant changes in the state-to-state transition probabilities are also observed. The results indicate that geometric phase effects must be included for H+O2 scattering even at low energies.
Statistical mechanics of the extensible and shearable elastic rod and of DNA
Authors: Shi Yaoming , He Siqian , Hearst John E.
Published: July 2002

We have developed a new statistical mechanical theory for wormlike chains and elastic rods which will account for deformations of bending, twisting, shear, and axial extension/stretching. We have derived a Fokker–Planck equation for Green’s function. We have also obtained an exact expression for the mean square end-to-end distance. Our new theoretical model is the most general statistical mechanical model for wormlike chain polymers available to date. The Kratky–Porod wormlike chain and the Yamakawa–Fujii helical wormlike chain models are found to be special cases of this new model. This new theory may provide deeper understanding of recent experimental data regarding overstretching single DNA molecules.
Electron correlation, extended geminal models, and intermolecular interactions: Theory
Authors: Ro/eggen I. , Wind P. A.
Published: July 2002

Within this framework of extended geminal models, new and improved approximations are introduced for the calculation of electron triple pair correlation terms. There are three levels of increasing accuracy for these terms based on the coupled-cluster models: a model of single and double excitations, a model correct up to fourth order, and a model correct up to fifth order CCSD(TQ). A test calculation on the neon atom demonstrates that by adopting the CCSD(TQ) model for the triple pair correlation terms, the extended geminal model recovers 99.88% of the full CI valence shell correlation energy. To reduce the computational work involved in calculating double pair correlation terms and triple pair correlation terms, a modified set of natural orbital (NOs) is introduced. On the basis of these NOs a truncated virtual orbital space is defined. Test calculations on the neon atom, the helium dimer, and the helium trimer demonstrate that the dimension of this truncated space can be chosen to be considerably smaller than the dimension of the full virtual space at a small sacrifice in accuracy. The additive structure of the correlation terms implies that the models are appropriate for implementation on parallel processor computers. The extended geminal models have properties which make them ideally suited for describing intermolecular interactions: the models have a conceptual structure which facilitates interpretation, they can be applied for any intersystem distances, the models are size-extensive, they generate no basis set artifacts (basis set superposition errors), and the models are reliable and accurate.
Resonant vibration–vibration energy transfer between highly vibrationally excited O2(X 3Σ−g,v=15–26) and CO2, N2O, N2, and O3
Authors: Mack J. A. , Mikulecky K. , Wodtke A. M.
Published: July 2002

Vibrational-state-specific total-removal relaxation rate constants, kv(M), for O2(X 3Σ−g, v=15 to 26) by M=CO2, N2O, and N2 have been obtained using the stimulated emission pumping (SEP) method in a pump–dump and probe configuration. Relaxation by O3 was studied using the chemical activation method, where the reaction: O(3P)+O3→O2(v)+O2, was employed to produce highly vibrationally excited O2 in an excess of ozone. Efficient (1%–2% of the gas kinetic limit) near-resonant 2–1 and/or 1–1 vibration-to-vibration (V–V) energy exchange was observed whenever the energy resonant condition was fulfilled and the transition in the quench partner would have been an allowed infrared transition in the isolated molecule. For M=CO2 and N2O, the temperature dependence of the 2–1 near-resonant energy transfer rate constants was found to be inverted. In contrast, the temperature dependence of the V–R, T relaxation rate constants for M=O2 was normal. For M=N2, a weak but positive temperature dependence was found. By extrapolating the temperature dependence to mesospheric temperatures (200 K) the effect of highly vibrationally excited O2 on the thermal budget can be discussed. The rate constant for the reaction of O(3P)+O3 was determined for an elevated collision energy of ∼10 kcal/mol and was found to be 5000 times larger than the room temperature rate constant.
Unified semiclassical theory for the two-state system: Analytical solutions for scattering matrices
Authors: Zhu Chaoyuan
Published: July 2002

Unified semiclassical theory is established for general two-state system by employing an exactly analytical quantum solution [C. Zhu, J. Phys. A29, 1293 (1996)] for the Nikitin exponential–potential model which contains the two-state curve crossing and noncrossing cases as a whole. Analytical solutions for scattering matrices are found for both three- and two-channel cases within the time-independent treatment. This is made possible by introducing a very important parameter d(R0)=√)/[V22(R0)−V11(R0)]2 (V11(R), V22(R) and V12(R) are diabatic potentials and coupling, R0 is real part of complex crossing point between two adiabatic potentials) which represents a type of nonadiabatic transition for the two-state system. For instance, d=∞ represents the Landau–Zener type and d=√ represents Rosen–Zener type. Since d(R0) runs from unity to infinity, this parameter provides a quantitative description of nonadiabatic transition. The idea used here is the parameter comparison method which makes a unique link between the model and general potential system at the complex crossing point. This method is testified not only by numerical examples, but also by agreement of the present semiclassical formulas with all existing semiclassical formulas.
Laser-induced photochemistry of methane on Pt(111): Excitation mechanism and dissociation dynamics
Authors: Matsumoto Yoshiyasu , Gruzdkov Yuri A. , Watanabe Kazuo , Sawabe Kyoichi
Published: July 2002

Adsorption states and photochemistry of methane and deuterated methane on a Pt(111) surface have been investigated by measuring temperature-programmed desorption spectra, x-ray photoelectron spectra, work function changes, and angle-resolved time-of-flight distributions of desorbed species. Methane weakly adsorbed on the Pt(111) surface at 40 K is dissociated to methyl and hydrogen fragments with laser irradiation at 193 nm. This is remarkably different from the photochemistry of methane in the gas phase where photodissociation takes place only at λ<145 nm. While the photofragments mostly remain on the surface, some fraction of methyl desorbs with average translational energy of 0.27 eV. Photodesorption of methane is a minor channel. Desorbed methane is sharply collimated along the surface normal and shows two hyperthermal velocity components. Among the two, the faster component is attributed to associative recombination between a methyl adsorbate and a hydrogen atom produced by the photodissociation of adsorbed methane. The photochemical processes are substantially suppressed when the surface is covered with methyl adsorbate of 0.14 ML achieved by an extensive irradiation of 193-nm photons. In contrast, no photochemical reactions result from the 193-nm irradiation of methane adsorbed on a Xe/Pt(111) overlayer or from the 248-nm irradiation of methane on the bare Pt surface. These results indicate that the photochemical processes occur only for methane in close contact with substrate atoms under the irradiation of 193-nm photons. The incident angle dependence of cross sections of the photochemistry obtained with linearly polarized light indicates that direct electronic excitation of methane adsorbate plays an important role in the photochemistry of methane. The photochemistry of methane on Pt(111) at the wavelength substantially longer than that in the gas phase implies that the electronic excited state of methane is significantly mixed with substrate electronic states.
Accurate quantum-chemical calculations: The use of Gaussian-type geminal functions in the treatment of electron correlation
Authors: Persson B. Joakim , Taylor Peter R.
Published: July 2002

We investigate augmenting conventional Gaussian-type one-electron orbital basis sets with two-electron functions that have a Gaussian dependence on the interelectronic distance. We observe substantial improvements in calculated correlation energies for helium and neon atoms and for the water molecule. A feature of our approach is that there is no nonlinear optimization of the two-electron basis function parameters at all.
First-principles molecular dynamics on multiple electronic states: A case study of NaI
Authors: Martinez Todd J. , Levine R. D.
Published: July 2002

We present the first application of first-principles molecular dynamics to a chemical process occurring on more than one electronic state. The example is the collisional chemi-ionization of NaI using a novel ab initio technique for the electronic states and a previously described full multiple spawning (FMS) classically motivated quantal method to describe the nuclear dynamics. The results for the dynamics are compared with fully exact quantal propagation. The FMS method which generates quantal amplitudes and inherently conserves normalization is shown to perform remarkably well for this heavy particle problem. The ab initio generated potentials and interstate couplings are compared with empirical potentials for NaI. Particular attention is given to the localized molecular orbital/generalized valence bond (LMO/GVB) method used for the electronic problem and to its interface with the equations of motion for the nuclei. The ability to incorporate atomic input (such as the ionization potential or the electron affinity) into the LMO/GVB method is emphasized.
An improved theory of multiphonon relaxation in solids
Authors: Egorov S. A. , Skinner J. L.
Published: July 2002

We derive a general formula, using Fermi’s golden rule, for the multiphonon relaxation rate for a two-level system coupled off-diagonally to a bath of phonons. Our result is valid for any scalar form of the system-bath interaction potential. With our result we analyze experimental data involving the energy gap and temperature dependence of electronic relaxation rates for rare-earth ions in YAlO3.
Millimeter-wave spectroscopy of FeF (X 6Δi): Rotational analysis and bonding study
Authors: Allen M. D. , Ziurys L. M.
Published: July 2002

The pure rotational spectrum of the FeF radical in its 6Δi ground electronic state has been recorded using millimeter/submillimeter direct absorption techniques. Transitions arising from all six spin-orbit components have been observed in the v=0, 1, and 2 vibrational levels of 56FeF, the main isotopic species, and also in the less abundant 54Fe isotopomer. Hyperfine splittings, arising from the 19F nuclear spin of I=1/2, were resolved in the majority of transitions recorded, and lambda-doubling interactions were observed in the Ω=3/2, 1/2, and −1/2 spin-orbit ladders. The complete data set has been analyzed using a 6Δ Hamiltonian, and rotational, spin-orbit, spin–spin, lambda-doubling, and hyperfine constants determined. This study has conclusively demonstrated that the ground electronic state of FeF is 6Δi. It also suggests that FeF has more covalent character to its bonding than alkaline earth or alkali metal counterparts.
From clusters to bulk: A relativistic density functional investigation on a series of gold clusters Aun, n=6,…,147
Authors: Häberlen Oliver D. , Chung Sai-Cheong , Stener Mauro , Rösch Notker
Published: July 2002

A series of gold clusters spanning the size range from Au6 through Au147 (with diameters from 0.7 to 1.7 nm) in icosahedral, octahedral, and cuboctahedral structure has been theoretically investigated by means of a scalar relativistic all-electron density functional method. One of the main objectives of this work was to analyze the convergence of cluster properties toward the corresponding bulk metal values and to compare the results obtained for the local density approximation (LDA) to those for a generalized gradient approximation (GGA) to the exchange-correlation functional. The average gold–gold distance in the clusters increases with their nuclearity and correlates essentially linearly with the average coordination number in the clusters. An extrapolation to the bulk coordination of 12 yields a gold–gold distance of 289 pm in LDA, very close to the experimental bulk value of 288 pm, while the extrapolated GGA gold–gold distance is 297 pm. The cluster cohesive energy varies linearly with the inverse of the calculated cluster radius, indicating that the surface-to-volume ratio is the primary determinant of the convergence of this quantity toward bulk. The extrapolated LDA binding energy per atom, 4.7 eV, overestimates the experimental bulk value of 3.8 eV, while the GGA value, 3.2 eV, underestimates the experiment by almost the same amount. The calculated ionization potentials and electron affinities of the clusters may be related to the metallic droplet model, although deviations due to the electronic shell structure are noticeable. The GGA extrapolation to bulk values yields 4.8 and 4.9 eV for the ionization potential and the electron affinity, respectively, remarkably close to the experimental polycrystalline work function of bulk gold, 5.1 eV. Gold 4f core level binding energies were calculated for sites with bulk coordination and for different surface sites. The core level shifts for the surface sites are all positive and distinguish among the corner, edge, and face-centered sites; sites in the first subsurface layer show still small positive shifts.
Pseudospectral localized generalized Mo/ller–Plesset methods with a generalized valence bond reference wave function: Theory and calculation of conformational energies
Authors: Murphy Robert B. , Pollard W. Thomas , Friesner Richard A.
Published: July 2002

We describe a new multireference perturbation algorithm for ab initio electronic structure calculations, based on a generalized valence bond (GVB) reference system, a local version of second-order Mo/ller–Plesset perturbation theory (LMP2), and pseudospectral (PS) numerical methods. This PS-GVB-LMP2 algorithm is shown to have a computational scaling of approximately N3 with basis set size N, and is readily applicable to medium to large size molecules using workstations with relatively modest memory and disk storage. Furthermore, the PS-GVB-LMP2 method is applicable to an arbitrary molecule in an automated fashion (although specific protocols for resonance interactions must be incorporated) and hence constitutes a well-defined model chemistry, in contrast to some alternative multireference methodologies. A calculation on the alanine dipeptide using the cc-pVTZ(−f) basis set (338 basis functions total) is presented as an example. We then apply the method to the calculation of 36 conformational energy differences assembled by Halgren and co-workers [J. Comput. Chem. 16, 1483 (1995)], where we obtain uniformly good agreement (better than 0.4 kcal/mole) between theory and experiment for all test cases but one, for which it appears as though the experimental measurement is less accurate than the theory. In contrast, quadratic configuration interaction QCISD(T) calculations are, surprisingly, shown to fail badly on one test case, methyl vinyl ether, for which the calculated energy difference is 2.5 kcal/mole and the experimental value is 1.15 kcal/mole. We hypothesize that single reference methods sometimes have difficulties describing multireference character due to low lying excited states in carbon–carbon pi bonds.
The low-lying electronic states of CrF and CrCl: Analysis of the A 6Σ+→X 6Σ+ system of CrCl
Authors: Bencheikh M. , Koivisto R. , Launila O. , Flament J. P.
Published: July 2002

A high-resolution near infrared spectrum of the CrCl radical has been recorded in thermal emission with Fourier transform techniques in the interval from 6 900 cm−1 to 11 500 cm−1. Two dominating band systems have been found in this region; (1) The A 6Σ+→X 6Σ+ system, analyzed in the present work, consisting of the (0,0), (0,1), (1,0), (2,0), and (3,0) bands, the (0,0) band being located at around 9450 cm−1; (2) the B 6Π→X 6Σ+ system, with a band interpreted as (0,0), located between 8700 cm−1 and 9000 cm−1, overlapping the (0,1) band of the A 6Σ→X 6Σ+ system. A rotational analysis of the A 6Σ+→X 6Σ+ system has been carried out, and the following principal parameters (cm−1) have been derived: X 6Σ+: ωe=396.6621, Be=0.167 587 3, De=1.1835⋅10−7; A 6Σ+: ωe=379.39, Be=0.156 51. Local perturbations in the A 6Σ+ (v=0,1) levels have been attributed to interactions with the B 6Π (v=1,2) levels. In the present work, we have constrained the investigations on the B 6Π→X 6Σ+ system to the verification that the lower state is indeed X 6Σ+. This verification has been achieved through successful matchings of branches in that system to known combination differences of X 6Σ+. The assignment of the upper state as B 6Π is based on the characteristic appearance of the band. Ligand field and density functional calculations have been performed on CrF and CrCl. The results from these calculations indicate that the approximate structure of the B 6Π and 1 6Δ excited states of CrF and CrCl is Cr+[3d44s(6D)]X−, while for the X 6Σ+ ground state in both molecules it is Cr+[3d44s(6D)∼3d5(6S)]X−. A combined DFT/LFT treatment has been carried out on the Cr+ 3d5and 3d44s configurations of both molecules in order to calculate the location of the A 6Σ+ state and of a number of low-lying states in the quartet manifold.
Absorption, resonance Raman, and Raman excitation spectra of hafnium trimers
Authors: Wang Huaiming , Hu Zhendong , Haouari Hanae , Craig Robert , Liu Yifei , Lombardi John R. , Lindsay D. M.
Published: July 2002

We report on the optical, resonance Raman, and excitation profile spectra of mass selected hafnium trimers in argon matrices at 14 K. The absorption spectrum consists of four overlapping transitions in the range 605–620 nm. The Raman spectrum is too complex to be attributed to a single ground state. We may explain the observed spectrum by assuming five low-lying excited states A, B, C, D, and E at 319.0, 413.4, 609.6, 642.8 (weak), and 785.4 cm−1, respectively. The ground (X) state shows complex structure which may be interpreted as the result of a strong, but linear Jahn–Teller effect. Evidence is obtained for two pseudo-rotational progressions having states of vibronic angular momentum of j=±1/2,…,±7/2 based upon a1′ normal frequencies of 142.8 and 278.1 cm−1. This indicates a fluxional ground state with E symmetry in the D3h limit. No such effects are apparent in the low-lying excited states, although a1′ modes of 143–152 cm−1 are observed and in some cases geometrical information may be inferred. The A state (319.0 cm−1) is geometrically an equilateral triangle (D3h symmetry). The B state (413.4 cm−1) shows an a1′ vibration, but without an observed nontotally symmetric mode no geometrical information can be obtained. The C state (609.6 cm−1) shows only a nontotally symmetric mode at 116.7 cm−1, while the D state (642.8 cm−1) and the E state (785.4 cm−1) both appear to have D3h symmetry. Raman excitation profiles appear as several distinct types and correlate well with the four absorption bands at 606, 610, 615, and 619 nm.
Pump–probe spectroscopy of dissipative energy transfer dynamics in photosynthetic antenna complexes: A density matrix approach
Authors: Kühn Oliver , Sundström Villy
Published: July 2002

The photoinduced ultrafast dynamics of singlet excitons in light-harvesting antennae is investigated using multilevel Redfield theory. Formulating the equations of motion for the reduced exciton density operator in terms of one- and two-exciton eigenstates we focus attention on the influence of dynamic exciton-vibrational coupling and static diagonal disorder on transient absorption spectra of peripheral antennae in photosynthetic purple bacteria. The simulations are discussed in view of recent experimental results obtained for the B850 absorption band of Rhodobacter sphaeroides. Further, we suggest a new way of estimating the size of the exciton coherence domain in these systems which puts emphasis on the dynamic character of exciton localization. For the B850 pigment pool we find that at room temperature the pump–pulse initially prepares a coherent superposition of one-exciton eigenstates which can be delocalized over the whole aggregate. With increasing delay time the exciton coherence domain shrinks to cover about four pigments in the asymptotic limit.
Dynamics of polymer “isotope” mixtures: Molecular dynamics simulation and Rouse model analysis
Authors: Kopf A. , Dünweg B. , Paul W.
Published: July 2002

We report results of a molecular dynamics simulation of an “isotope” mixture of polymer chains, which are represented by a standard bead–spring model, and whose two species differ only by their monomer masses. Detailed analysis of the Rouse modes shows that for sufficiently short (non-entangled) chains this system can be well described by the Rouse model. Each species is described by its individual monomeric friction coefficient, whose dependence on both mass ratio as well as mixing ratio is studied. The main effect of mixing is an acceleration of the slower chains and a slowdown of the faster ones, while both species remain dynamically different. Some microscopic insight into the mechanism is obtained by studying the short-time behavior of the monomeric velocity autocorrelation function. Studies in the slightly entangled regime (chain length up to N=150, where the typical entanglement chain length is Ne≈35) seem to further corroborate the hypothesis that the “tube diameter” of the reptation model is a quantity which results mainly from the static configurations, i.e., is an equilibrium thermal average. The usefulness of recently suggested analysis methods in this regime is briefly discussed.
A hybrid classical/quantum approach to cluster fragmentation dynamics: Application to the vibrational predissociation of He2Cl2
Authors: Hernández M. I. , Garcı́a-Vela A. , Garcı́a-Rizo C. , Halberstadt N. , Villarreal P. , Delgado-Barrio G.
Published: July 2002

A new hybrid classical/quantum method is proposed and applied to investigate the vibrational predissociation (VP) dynamics of the He2Cl2 complex. The full dimensionality of the system (assuming zero total angular momentum) is included in the method. The VP process of He2Cl2 is dominated by a sequential mechanism of dissociation of the two van der Waals bonds. The hybrid approach describes the first weak bond fragmentation classically, and the second one quantum mechanically. The rotational distribution of the Cl2 fragment is calculated both with the hybrid method and with a fully classical trajectory simulation, and compared with the experimental distribution. The hybrid distribution is found to agree very well with the experimental one, and to involve a substantial improvement with respect to the classical result.
Impulsive stimulated scattering in ice VI and ice VII
Authors: Baer Bruce J. , Brown J. M. , Zaug Joseph M. , Schiferl David , Chronister Eric L.
Published: July 2002

We demonstrate the use of forced thermal Brillouin scattering to obtain acoustic velocities for oriented high-pressure crystals of ice VI and of ice VII. The elastic constants of ice VI and ice VII determined from these measurements compare favorably with classical Brillouin scattering results.
Light emission during the agglomeration of silver clusters in noble gas matrices
Authors: Rabin I. , Schulze W. , Ertl G.
Published: July 2002

The agglomeration of small silver clusters in noble gas matrices to form larger ones may be accompanied by the emission of light. Spectral analysis reveals that part of radiation intensity can be attributed to fluorescence from excited metal atoms, dimers and trimers the formation of which results from cluster/cluster agglomeration as a consequence of the gain in binding energy. The remaining spectral features must be assigned to excited clusters Agn, with n⩾4.
Validation of molecular dynamics simulation
Authors: van Gunsteren Wilfred F. , Mark Alan E.
Published: July 2002

How the results obtained by computer simulation of molecular systems can be validated is considered. The overall quality of the simulated properties of a molecular system will depend on (i) the quality of the theory or model, (ii) the accuracy of the interatomic interaction function or force field, (iii) the degree of sampling, statistics and convergence reached in the simulation, (iv) the quality of the simulation software, and (v) how competently the simulation software is used. These five validation issues are discussed and illustrated with examples. Guidelines for different members of the scientific community are formulated which are aimed at enabling and improving the validation of simulation results in the literature.
Classical Heisenberg model of magnetic molecular ring clusters: Accurate approximants for correlation functions and susceptibility
Authors: Luscombe James H. , Luban Marshall , Borsa Ferdinando
Published: July 2002

We show that the measured magnetic susceptibility of molecular ring clusters can be accurately reproduced, for all but low temperatures T, by a classical Heisenberg model of N identical spins S on a ring that interact with isotropic nearest-neighbor interactions. While exact expressions for the two-spin correlation function, CN(n,T), and the zero-field magnetic susceptibility, χN(T), are known for the classical Heisenberg ring, their evaluation involves summing infinite series of modified spherical Bessel functions. By contrast, the formula CN(n,T)=(un+uN−n)/(1+uN), where u(K)=coth K−K−1 is the Langevin function and K=JS(S+1)/(kBT) is the nearest-neighbor dimensionless coupling constant, provides an excellent approximation if N⩾6 for the regime |K|<3. This choice of approximant combines the expected exponential decay of correlations for increasing yet small values of n, with the cyclic boundary condition for a finite ring, CN(n,T)=CN(N−n,T). By way of illustration, we show that, for T>50 K, the associated approximant for the susceptibility derived from the approximate correlation function is virtually indistinguishable from both the exact theoretical susceptibility and the experimental data for the “ferric wheel” molecular cluster ([Fe(OCH3)2(O2CCH2Cl)]10), which contains N=10 interacting Fe3+ ions, each of spin S=5/2, that are symmetrically positioned in a nearly planar ring.
Exciplex emission from bilayers of poly(vinyl carbazole) and pyridine based conjugated copolymers
Authors: Gebler D. D. , Wang Y. Z. , Fu D.-K. , Swager T. M. , Epstein A. J.
Published: July 2002

We present photoluminescence and electroluminescence studies of bilayers and blends formed from poly(vinyl carbazole) (PVK) and poly(pyridyl vinylene phenylene vinylene) (PPyVPV) copolymer derivatives. Bilayers of PVK and the PPyVPV copolymers have a photoluminescence emission which cannot be assigned to either the photoluminescence of PVK or the PPyVPV layer. The blends of the two polymers show a similar new photoluminescence emission for a large range of concentrations. Absorption and photoluminescence excitation spectra confirm that the additional feature is an excited state species which results from an exciplex at the polymer/polymer interface. Bilayer light-emitting devices utilizing the PPyVPV copolymers show an electroluminescence spectrum consistent with emission from the exciplex. The efficiency of the bilayer devices as compared to single layer devices increases by over three orders of magnitude due to the exciplex formation and the elimination of exciton formation near the luminescence quenching electrodes.
Solvation forces in thin films confined between macroscopically curved substrates
Authors: Schoen Martin , Gruhn Thomas , Diestler Dennis J.
Published: July 2002

The microscopic structure of molecularly thin fluid films confined between solid substrates with macroscopically curved surfaces is investigated by means of grand canonical ensemble Monte Carlo (GCEMC) simulations, in which the thermodynamic state of the film is determined by its chemical potential μ and temperature T. This situation is akin to experiments involving the surface forces apparatus (SFA) in which the film is confined between two crossed cylinders of macroscopic radius R. The key quantity measured directly in SFA experiments is the “force per radius R,” F(h)/R, exerted by the film on the curved surfaces. This “solvation force” can be related to the local stress Tzz(h) normal to the locally planar surfaces, where h is the shortest distance between them. Because Tzz(h) and the microscopic structure of the confined film can be computed by GCEMC, the relation between Tzz and the macroscopically defined quantity F/R can be employed to interpret the dependence of the latter in terms of variations of the film’s local microscopic structure with h. For a liquid-crystalline film it is shown that reorientational effects are distinctly manifest in Tzz(h) but are reflected only weakly in F(h)/R and are therefore likely to be missed if the interpretation of SFA experiments is based solely upon the latter. Pseudo-experimental F(h)/R curves generated by GCEMC for a nematic liquid-crystalline film are in qualitative agreement with recent SFA data [M. Ruths, S. Steinberg, and J. N. Israelachvili, Langmuir 12, 6637 (1996)], which suggests that one may gain deeper insight into the microscopic structure of confined films through a decomposition of experimentally determined solvation forces F(h)/R into Tzz(h) by inverting the integral relation linking the two.