bluuebunny/crossref_metadata_2025 huggingface dataset:
Irreducible tensor operator methods in intermediate‐field coupling
Authors: König E. , Kremer S.
Published: December 2004

The algebra of irreducible tensor operators is developed in the intermediate‐field coupling case. The Wigner‐Eckart theorem is formulated for a simple irreducible tensor operator as well as for the Kronecker and scalar products of these operators. The expressions required for the calculation of Coulomb repulsion, crystal field splitting, spin‐orbit interaction, and Zeeman effect are given in detail. Recent applications to various problems in spectroscopy and magnetism of transition metal compounds are referred to.
A density functional representation of quantum chemistry. II. Local quantum field theories of molecular matter in terms of the charge density operator do not work
Authors: Primas H. , Schleicher M.
Published: December 2004

A comprehensive review of the attempts to rephrase molecular quantum mechanics in terms of the particle density operator and the current density or phase density operator is given. All pertinent investigations which have come to our attention suffer from severe mathematical inconsistencies and are not adequate to the few‐body problem of quantum chemistry. The origin of the failure of these attempts is investigated and it is shown that a realization of a local quantum field theory of molecular matter in terms of observables would presuppose the solution of many highly nontrivial mathematical problems.
Direct minimization of the energy by simultaneous variation of parameters in nonorthogonal basis functions. II. Real STOS for first row atoms
Authors: Weltin E. E
Published: December 2004

The parameter optimization method of Part I is applied to the exponents of real STOS of first row atoms. In addition to minimum basis ground states, some independently optimized excited states are discussed in the case of Be. Local minima on the energy versus parameter surface are found in 4‐configuration functions for the ground state of N. They are not present in either the simpler minimum basis function or in a more complete 8‐configuration function.
A non‐empirical molecular orbital study of valence tautomers of C2H3N
Authors: Hopkinson Alan C. , Lien Min H. , Yates Keith , Csizmadia Imre G.
Published: December 2004

The valence tautomers of C2H3N have been examined by non‐empirical molecular orbital calculations using two split‐valence shell basis sets. All geometries were fully optimized using the 4–31G basis set and these structures were then used in 6–31G basis set calculations. The order of stability of the three possible cyclic isomers is 1‐azirine > cyclic carbene > 2‐azirine. The profiles for conversion of vinylmethylene into cyclopropene, vinylnitrene into 1‐arizine, and iminomethylene into 2‐azirine have all been shown to have barriers.
Branching diagram and serber‐type spin functions. Algorithms for their construction and special properties
Authors: Pauncz Ruben
Published: December 2004

Branching diagram and Serber‐type spin functions are ordered according to their diagrams. The method of Graebenstetter et al. is slightly modified; it provides an effective algorithm for the construction of genealogical spin functions. A counterpart of the method is developed for Serber‐type spin functions. Special properties of branching diagram functions and Serber‐functions are established.
Comments on excited state polarizabilities of molecules
Authors: Barnett Gene
Published: December 2004

A comparison of experimental and theoretical values for Δα = αe ‐ αg, assuming no sigma‐electron contribution, shows a great distortion by π‐electron calculations. We analyze existing electrochromic data, using this assumption, and report Δα values of 44 Å3 for coronene and 43 Å3 for triphenylene. Some general properties of Δα are discussed in terms of potential energy curves which suggest interesting calculations to relate excited matrix elements with experimental measurements.
Theory of mutations. II. Physical model of genetic stability
Authors: Cooper W. Grant
Published: December 2004

A physical model for genetic stability of DNA is constructed in terms of the “microscopic” model for uninduced point mutations and the electromechanical thermoelastic properties of the macroscopic “global” DNA system. Theoretical considerations show how experimental data from one of the least complicated but useful genetic systems, T4 bacteriophage, support the proton‐code model of DNA discussed by Löwdin. The stability of an unperturbed DNA system is treated to illustrate the dynamical coupling between the “global” motion of DNA and the stability at local genetic sites. The method for analyzing a DNA system perturbed by “low‐energy” optical or acoustical frequencies is indicated. The model provides a physical explanation for “hot spots” (genetic sites highly susceptible to mutation) and suggests that quantum theory may be as useful in the biological sciences as it is in the physical sciences with respect to designing and interpreting certain experiments in genetics. The incidence of human cancer as a function of chronological age is discussed in terms of the model.
A direct CI method with a multiconfigurational reference state
Authors: Roos Björn O. , Siegbahn Per E. M.
Published: December 2004

A method for direct configuration–interaction (CI) calculations with a multiconfigurational reference function is described. The reference state can contain several closed‐shell electronic configurations and the CI expansion comprises all single and double replacements out of all these configurations. The resulting secular problem is solved using a variation‐perturbation method. A number of examples are given showing the efficiency of the method. The largest CI expansion used in calculations with this program so far contains 76,471 spin‐ and space‐symmetrized configurations.
Floating spherical Gaussian orbital model study of some hydrogen‐bridged systems: LiBeH3, LiBH4, LiCH5, and BeBH5
Authors: Bhargava Shobha , Ray N. K.
Published: December 2004

The floating spherical Gaussian orbital (FSGO) method has been used to study the equilibrium geometries and electronic structures of some hydrogen‐bridged systems containing lithium, beryllium, boron, and carbon. The predicted geometries are in good agreement with other theoretical estimates. The binding energies for these hydrogen‐bridged systems are estimated and discussed. The FSGO total energies (EFSGO), for all systems studied here, are found to be well correlated to nuclear–nuclear repulsion (Vnn) and nuclear–electron attraction (Vne) terms by a relation of the type EFSGO =\documentclass{article}\pagestyle{empty}\begin{document}$ {\textstyle{3 \over 7}} $\end{document} (2 Vnn + Vne).
New method for approximate Hartree–Fock calculations using density approximations and coulomb field corrections. I
Authors: Noack Wolf‐Eckart
Published: December 2004

A method is proposed that reduces the computational effort of HF calculations considerably by reducing the number of two‐electron integrals that have to be calculated. The following concepts are used: (i) approximation of the electron density by only few functions for the Coulomb part of the HF matrix; (ii) modification of this approximate density, to improve its Coulomb field; (iii) in the exchange part, a basis function χ is replaced by a function \documentclass{article}\pagestyle{empty}\begin{document}$ \tilde \chi $\end{document} consisting of fewer Gaussian lobes; (iv) the error caused by this replacement is reduced by a modification of the densities \documentclass{article}\pagestyle{empty}\begin{document}$ \tilde \chi _i \tilde \chi _j $\end{document} in the exchange integrals. The computation time of the integral part is reduced by a factor 6 for molecules containing five first‐row atoms as, e.g., CF4, if one uses a 7S/3P basis set contracted to (5, 1, 1/3). The integral time increases roughly with n3, if n is the number of Gaussian lobes.
New method for approximate Hartree‐Fock calculations using density approximations and coulomb field corrections. II
Authors: Noack Wolf‐Eckart
Published: December 2004

The HF approximation method that was outlined in Paper I is tested with respect to several molecular properties. Three different levels of approximation a, b, and c are considered. Satisfactory results—compared to corresponding “exact” HF calculations—are obtained with the STO‐3G basis and the approximation level a. At this level the error in the binding energy is 0.001–0.025 a.u. for all considered molecules which contain up to six first‐row atoms as, e.g., cyclopentanone (C5OH8). The error in the reaction energies considered here is about 4 kcal/mol (the maximal error is 9 kcal/mol). Orbital energies, dipole moments, gross charges, equilibrium geometries, and barriers to internal rotation are well reproduced by the approximation method at all three levels.
Analyses of the electronic collective motions by means of a nonlinear dynamical equation
Authors: Toyoda Kaoru
Published: December 2004

A nonlinear dynamical equation for the electronic collective motion is presented by employing the traditional time‐dependent procedure. The equation is applied to the model π‐electron system of ethylene, and the effects of nonlinear interactions are investigated using its several states as reference states. It is shown that the nonlinear correlation plays an important role in describing the change of the lower excited states.
Ab initio molecular fragment calculations with pseudopotentials. Some fragments containing nitrogen and oxygen
Authors: Gáspár R. , Gáspár R.
Published: December 2004

Parameters for the OH (sp2) and NH3 (planar, sp2) pseudopotential–FSGO molecular fragments have been established in this paper. Calculations for the molecules of formic acid, formaldehyde, and formamide show good agreement with experiment.
Experimental evidence on the chemisorption bond of two‐ and one‐dimensional boundaries
Authors: Kern R.
Published: December 2004

This paper is intended to provide an experimentalist's contribution to a critical discussion of adsorption energies and their significance. The experimental requirements, with special reference to the limitations and the quality of the information provided by the various techniques are briefly reviewed. The significance of the measurements, and the background theoretical (phenomenological and model) schemes which experimentalists use for interpreting their results, are discussed in order to show that the interpretations of surface measurements must be carefully analyzed by theoreticians intending to contribute to an understanding of surface bonding.
State of ions near a cathode: Adions or adatoms?
Authors: Laforgue‐Kantzer D.
Published: December 2004

It is the intent of this article to illustrate certain aspects of the present status of electrochemistry as regards theoretical interpretations. First a brief critical review of classical electrochemical kinetics is given and the directions are indicated in which improvement in theoretical treatments are hardly needed. Next, experimental results on the cathodic discharge of tin in darkness and in light are presented, and a tentative explanation based on the assumption that adatoms (and not adions) diffuse on the electrode surface as a result of stabilization by light is presented.
Bond polarization in the FeCO system: Semiempirical MO–SCF (BMV) calculations
Authors: Barbier C. , Vincent C. , Del Re G.
Published: December 2004

Charge distributions in FeCO for different Fe–C distances and the Fe–C–O angle equal to 180° and 90° have been computed by the BMV method, a semiempirical SCF scheme including overlap and especially suited for transition‐metal atoms. A comparison with available EHT and ab initio calculations suggests that the BMV method is a useful complement to ab initio calculations. The information obtained on the dependence of the binding in FeCO on the Fe–C distance is also briefly discussed in connection with the views of experimentalists of the state of CO absorbed on iron.
Diamagnetism as a structure and bonding related property in semiconductors
Authors: Garbato L. , Manca P. , Spiga A.
Published: December 2004

The reformulation of the quantum mechanical expression of the diamagnetic susceptibility, in terms of bond and band related parameters, leads us to evaluate separately the Langevin and the Van Vleck components of the lattice magnetic susceptibility of semiconductors. Its validity, checked for a number of semiconductors covering seven different structures and almost the entire range of variability of Suchet's ionicity parameter, confirms that a definite link exists between the diamagnetism of semiconductors and dielectrics and the nature of their interatomic bonds.
Neutron elastic scattering studies of biological molecules and complexes
Authors: Chidambaram R.
Published: December 2004

Neutron diffraction can provide very precise stereochemical information about hydrogen bonds and other features involving hydrogen atoms in crystals. This is because, unlike for x rays, the hydrogen atom has a significant scattering amplitude for neutrons relative to other atoms. High‐precision studies of a class of molecules, like those on α‐amino acids carried out at Brookhaven and Trombay, can provide important information on systematics of the structure, conformation, and hydrogen bonding parameters. Some attempts have also been made to extend these single‐crystal studies to some proteins like myoglobin, lysozyme, and trypsin. The large anomalous scattering amplitudes for some stable isotopes like 113Cd and 149Sm and the easy variability of neutron wavelength also provides, in principle, a method for solving the phase problem without crystallizing too many heavy atom derivatives. The combination of neutron and x‐ray diffraction data, now being used to study charge density distributions in organic molecules and hydrogen bonds, may be extended in the future to larger molecules of biological importance. The small‐angle scattering of neutrons is also a powerful technique for studying large biological particles in solution at low resolution to obtain information about sizes, shapes and distribution of constituents.
Spin‐Free quantum chemistry. XXII. Multiconfiguration self‐consistent field theory
Authors: Matsen F. A. , Nelin C. J.
Published: December 2004

Second order MCSCF theory is presented in a unitary group formulation for any multiplicity without spin projection. Its reduction to lower order theory is discussed, as well as its extension through the use of effective Hamiltonians.
MSW–SCF–Xα studies of some silicon compounds. I. Iminosilicon (HNSi)
Authors: Scheire L. , Rotthier R. , Phariseau P.
Published: December 2004

MSW–SCF–Xα calculations have been carried out to obtain ionization potentials (via Slater's transition state concept) and charge distributions for HNSi. The results are interpreted and compared with previous ab initio and CNDO/2 studies.
Variants of Rayleigh–Schrödinger perturbation theory—a new look
Authors: Bhattacharyya Kamal
Published: December 2004

Several useful “redefined zero‐order Hamiltonian” variants of the Rayleigh–Schrödinger perturbation theory are derived by exploiting the freedom of choice for the orthogonality integral between zero‐ and first‐order wavefunctions. It is found that the more common Hamiltonian repartitioning schemes follow quite naturally, as a consequence of certain consistency requirements. The strategy adopted may as well be employed to derive various newer and profitable zero‐order Hamiltonians depending on the nature of the problems. This implication is strengthened by demonstrating the calculational success of one such derived scheme. Feenberg's “scale‐changed” Hamiltonian approach is also reinterpreted in the present context.
Meaning of the perturbation theory for a class of multiple‐well anharmonic oscillators
Authors: Graffi S.
Published: December 2004

It is proved that the Borel sum of the Rayleigh‐Schrödinger perturbation expansion eigenvalue of the triple well anharmonic oscillators p2 + x2 − 2g2nx2n+2 + g4nx4n+2, g > 0, n = 2.3,… is a complex eigenvalue of a different problem. Its relation with the eigenvalues of the original Hamiltonian is discussed.
Some remarks on the use of rational functions as variational functions
Authors: Jolly P. , Sane K. V. , Saxena R. P. , Srivastava P. K.
Published: December 2004

When a Padé‐type function is used as a trial function, it is necessary to investigate the location of singularities and the zeros of the optimized function. It is shown that an unacceptable wave function can sometimes yield numerically acceptable values because of the nature of the integration procedure. It is also shown that it is possible to remove the undesirable features of a lower order form by invoking the next approximation.
Relation of the charge and spin correlation functions to the structures of π electron wave functions in alternant hydrocarbons
Authors: Hashimoto Katsufumi
Published: December 2004

We analyze, in the Pariser–Parr–Pople (PPP) model of alternant hydrocarbons, how the charge and spin correlation functions (CF's) are related to the structure of a CI wave function and the MO's of the systems. The analysis is based on the fact that an uncorrelated electron present in an orbital does not contribute to the linked dynamically correlated parts of the CF's. By using the fact, simple rules are deduced predicting the covalent or ionic nature of the correlation structure in a low‐lying state with two correlated electrons. The rules predict that the singlet minus and triplet plus states are covalent, while singlet plus and triplet minus states are ionic, where the plus and minus mean the alternancy symmetry. The rules also give a prediction for the unknown charge and spin correlation structures between different sites.
Correlation hole for an exactly solvable problem
Authors: Cohen Leon , Frishberg C.
Published: December 2004

The probability distribution of the interparticle distance is obtained for the N‐body boson problem where the potential is an attractive delta function. This is done for both the exact wave function and the Hartree wave function allowing the calculation of the correlation hole.
Calculation of systems involving strong delocalized bonds within a modified PCILO framework. I. Valence‐bond PCILO method
Authors: Lochmann R. , Weller Th.
Published: December 2004

In describing mesomer structures by the original PCILO method several problems may occur because of the use of localized bonds. A so‐called VB‐PCILO including a superposition of several mesomer boundary structures in the CI perturbation treatment will be proposed.
Ab initio calculations of guanidinium–carboxylate interaction
Authors: Sapse A. M. , Russell C. S.
Published: December 2004

Ab initio calculations (self‐consistent‐field Hartree–Fock) using 6‐31G and STO‐4G basis sets are used to investigate the interaction between guanidinium and methylguanidinium ion with the carboxylate group of formate. Binding energies and optimum geometries are obtained and compared with reported results using a smaller basis set (STO‐3G). The importance of this interaction in proteinsubstrate binding is discussed.
Approximate symmetries in the singly excited states of helium
Authors: Flurry R. L.
Published: December 2004

Consideration of R(4) as the degeneracy group for the singly excited states of helium allows the assignment of R(4) irreducible representations (irreps) to these states. Induction of these irreps onto the irreps of SU(4) and U(4) provides a sufficient number of invariants to define the energy levels. Empirical equations allow all of the singlet states to be related to the first ionization energy and the Rydberg constant, to a good approximation. Empirical equations can also be derived for the triplet states, but their relation to the ionization energy and/or the Rydberg constant is not obvious.
On a unified treatment of kinetics and diffusion, and a connection to a nonlocal boltzmann equation
Authors: Ulmer W.
Published: December 2004

Fick's law of diffusion has been generalized to include kinetic processes, the transport term of the Boltzmann equation, and nonlocal interaction processes. It is shown that the collision interaction term can be obtained by the introduction of a quantum stochastic potential equation. Some approximations of a nonlocal Boltzmann equation can be solved exactly. The solutions can be applied to problems of molecular pattern in biology.
The INDO/2‐AHP (average hole potential) method for excited states: Comparison with the simple INDO/2‐HP (hole potential) method
Authors: Banerjee Manas , Bhattacharyya Sankar Prasad
Published: December 2004

The INDO/2 version of the average hole potential (AHP) model is analyzed. The model is applied to study the geometric features, molecular inversion barriers, singlet‐triplet splittings, etc., of a few small carbonyl molecules (H2CO, HFCO, F2CO) in the 1,3nπ* states with partial as well as complete optimization of all geometric parameters in the excited states. The results are compared with those obtained by a simple hole‐potential (HP) model.
On the relationship between the electron‐pair distribution function and the correlation energy of an atom
Authors: Ugalde J. M. , Boyd Russell J.
Published: December 2004

The pair distribution function h(r12;r1, γ) and the virial theorem are used to derive a general expression for the local contributions to the total correlation energy of an atom. A direct link between correlation effects and the correlation energy is obtained by use of G(r1, γ) and Γ(r1, y). The former is the probability associated with a given choice of r1 and γ, while the latter describes the local contribution to the correlation energy. Explicit calculations for the ground state of helium indicate that the angular dependence of the local contribution to the correlation energy is essentially independent of r1, whereas the local correlation energy shows a strong r1 dependence. The maximum contribution to the correlation energy occurs at intermediate values of γ where there is close agreement between the Hartree–Fock and exact densities.
The quantum mechanical many‐body problem in hyperspherical coordinates. Analysis of systems with coulomb interactions in terms of many‐dimensional hydrogen‐like wave functions
Authors: Avery John , Larsen Peter Sommer , Hengyi Shen
Published: December 2004

A method is described for using many‐dimensional hydrogen‐like wave functions as a starting point for constructing solutions to the Schrödinger equation of an N‐particle system. The solutions are built up from symmetry‐adapted hyperspherical harmonics, multiplied by functions of the hyperradius, r. Approximate asymptotic solutions for large values of r are discussed, as well as approximate solutions valid near r = 0. Properties of hyperspherical harmonics are discussed. General methods are presented for resolving arbitrary functions into hyperspherical harmonics and for constructing simultaneous eigenfunctions of generalized angular momentum and total orbital angular momentum.
Optical properties of large molecules and clusters in the frenkel exciton picture
Authors: Avery John , Hvidt Søren
Published: December 2004

Frenkel exciton theory is reviewed. This theory yields general formulas relating the absorption and circular dichroism spectra of large molecules or clusters to the optical properties of their subunits. It is shown that for weakly interacting systems, the circular dichroism associated with a band of exciton states is proportional to tr(FH), where F is an “optical matrix” constructed from the positions and transition dipole moments of the subunits, and H is an interaction‐energy matrix. It is shown that if a system expnds isotropically, then tr(FH) falls off as the inverse square of the linear dimensions. This result is compared with experimental measurements of the low‐temperature optical rotatory dispersion of proteins as a function of temperature. The optical properties of helical polymers are also discussed as a function of their geometrical parameters.
Toward the variational treatment of spin‐orbit and other relativistic effects for heavy atoms and molecules
Authors: Hess Bernd A. , Buenker Robert J. , Chandra Praphull
Published: December 2004

Observation of trends in computed spin‐orbit splittings for relatively light molecules leads to the conclusion that relativistic corrections to the electronic charge distribution are important when treating molecules containing heavy atoms (Z > 18). In order to preserve the nature of the successful computational techniques currently applied to light molecules in so far as possible, particularly to allow for the treatment of correlation effects in an efficient CI procedure on an equal footing with relativistic effects, emphasis is placed on the development of a two‐component formalism for this purpose. A first attempt in this direction consists of formulating a spin‐free quantum mechanical operator that reflects relativistic kinematics. The mass‐velocity term in the Breit‐Pauli Hamiltonian is not appropriate for a variational treatment, however, since it drastically alters the spectrum and gives results that are not bounded from below. To avoid this problem the relativistic free particle energy has been used directly for the representation of the kinetic energy, and in addition the Darwin term has been included as a correction to the potential energy. This approach can be justified with reference to the Foldy–Wouthuysen reduction of the Dirac equation, but the class of basis functions used in a variational procedure with this Hamiltonian must be restricted to avoid the formation of a node in the wavefunction at the nucleus; the same problem is circumvented in the Cowan–Griffin method by imposing Dirac boundary conditions on the wavefunction. With this method, accurate spin‐orbit splittings have been computed for Br, I, Xe+, CBr, and XeF, but the resulting total energies are found to be overly sensitive to the representation of the inner shells of these systems. Improved results for both valence and inner shells are then shown to follow from the use of the no‐pair equation, which provides a variationally tractable two‐component method employing a momentum dependent potential that gives a realistic description of relativistic effects for atoms and molecules over a suitably large range of Z.
Soliton defects in polyacetylene: Local‐density functional results
Authors: Kutzler F. W. , White C. T. , Mintmire J. W.
Published: December 2004

We report local‐density functional results for the electronic structure of neutral soliton defects in polyacetylene. The results were obtained using a modified version of the discrete variational method Xα molecular cluster model. For defect‐free chains the calculated X‐ray and ultraviolet photoelectron spectra are in good agreement with experiment. For chains with low concentrations of solitons, we find the forbidden gap broadens slightly, and two nearly dispersionless bands, split by several tenths of eV, are introduced into the gap. The predicted splitting is in accord with recent experimental results for this defect.
The omega function for the lithium atom
Authors: Smeyers Yves G. , Delgado‐Barrio G. , Doreste‐Suárez L. , Martin‐González J. M.
Published: December 2004

The omega model is reviewed in the case of a three‐electron function and the omega function determined for the lithium atom ground state by using a procedure based on Brillouin' theorem. The resulting function is analyzed in terms of natural configurations, and the correlation coefficient determined as a function of the distance to the nucleus. It is found that the omega function is essentially equivalent to two configurations built up with the occupied NO' and that the model introduces mainly correlation at large distance. These results contrast with those obtained for the beryllium atom using the same model.
Molecular valence calculations based on stepwise approximations for orthogonality and core‐valence interactions
Authors: Pakkanen Tapani A. , Lindblad Marina , Laitinen Risto S.
Published: December 2004

An approximate method for the calculation of the electronic structure of molecules has been developed. The principal approximations involve freezing of the atomic core orbitals, description of the core‐valence interactions by density matrix expansion techniques, and the enforcement of the core‐valence orthogonality by auxiliary bases and projection operator techniques. The general principle in the approximations is their stepwise execution. Each approximation can be separately relaxed and tested for reliability. The developed computational techniques have been described. A detailed analysis for the approximations is given for LiH. Other molecular test calculations include HCl, H2S, H2Se, and H2S2 molecules. The results demonstrate the suitability of the method for molecular studies.
Theory and mechanism of electron transfer at low temperatures
Authors: Zasukha V. A. , Volkov S. V.
Published: December 2004

A theory of electron transfer at low temperatures has been developed. The mechanism of electron transfer consists in the fact that donor ionization and electron capture by the acceptor take place in a tunnel manner.
Fluorescence lifetime distribution of folded and unfolded proteins
Authors: Gratton E. , Silva N. , Mei G. , Rosato N. , Savini I. , Finazzi‐Agro A.
Published: December 2004

Time‐resolved fluorescence of single tryptophan proteins have demonstrated the complexity of protein dynamic and protein structure. In particular, for some single tryptophan proteins, their fluorescence decay is best described by a distribution of fluorescence lifetimes rather than one or two lifetimes. Such results have provided further confirmation that the protein system is one which fluctuates between a hierarchy of many conformational substates. With this scenario as a theoretical framework, the correlations between protein dynamic and structure are investigated by studying the time‐resolved fluorescence and anisotropy decay of holo and apo human superoxide dismutase (HSOD) at different denaturant concentrations. As a function of guanidine hydrochloride (GdHCl), the width of the fluorescence lifetime distribution of HSOD displays a maximum which is not coincident with the fully denatured form of HSOD at 6.5M GdHCl. Furthermore, the width of the fluorescence lifetime distribution for the fully denatured forms of holo and apo HSOD is greater than that of the native forms.
A semiempirical study of the reaction of the hemimercaptal of methylglyoxal and glutathione at the active center of glyoxalase I
Authors: Ventura Oscar N. , Cubas María L.
Published: December 2004

An AM1 and MNDO–PM3 semiempirical study of the reaction mechanism of glyoxalase I is presented. According to some authors, this enzyme serves as a detoxifying agent, transforming ketoaldehydes to hydroxiacids. According to others, however, the enzyme serves as a growth‐control agent, destroying 2‐oxopropanal (MG), which might be involved in some, mostly unknown, biological pathways. Following a previous study on the conformation of the hemimercaptal of MG and glutathione (HSG) done using molecular mechanics, we study here the possible reaction path of this substrate at a model reaction site in the enzyme. First, the active center of the enzyme was modeled using a Zn2+ cation with several ligands. Second, its complex with the hemimercaptal of MG and HSG (SAG) was calculated using semiempirical methods. The reaction proceeds from the substrate to the enediol intermediate in two steps: In the first one, a proton from the activated carbon in SAG is captured by a base–which we model in this work by HO−, CH3COO−, and imidazole anion. Our calculations show that the likeliness of the reaction is enhanced if imidazole is used as the base instead of an acid conjugate base or a free hydroxide anion. Moreover, we found a stable intermediate carbanion–described for the first time as an intermediate in this mechanism–whose existence is supported by some of the available experimental evidence. In the second step of the mechanism, a proton is transferred from a Zn‐bound water to the intermediate carbanion, giving the enediol species, originally postulated as the intermediate in this mechanism, and leaving a hydroxide anion bound to the Zn2+ complex. Comparing this reaction with the same process when no metal complex is present–as if the reaction occurred in aqueous solution–it is shown that the metal ion activates water, facilitating the proton transfer. Our calculations point out the existence of two potential energy barriers to be overcome in the process: the need of the metal center to be coordinated to water as a prerequisite for the second proton transfer to take place, and the fine tuning caused in the first proton transfer by the very nature of the base that initially captures the proton. No significant differences in the qualitative characteristics of the reaction were found in using AM1 or PM3 as the method of calculation. © 1992 John Wiley & Sons, Inc.
Benchmark values for two‐center Coulomb integrals over slater‐type orbitals
Authors: Jones Herbert W.
Published: December 2004

The Löwdin α‐function method, in which displaced orbitals are expanded in an infinite series of spherical harmonics, is implemented for Slater‐type orbitals using a commercial computer algebra program, Mathematica, The program, which is included, generates a C matrix with integer elements that characterizes our approach to multicenter molecular integrals. The general two‐center, two‐electron Coulomb repulsion integral is produced analytically with a finite number of terms. Each Coulomb formula may be evaluated to arbitrary precision, since Mathematica works with integer arithmetic. Hence, cancellation errors can be overcome. © 1993 John Wiley & Sons, Inc.
A quantum monte carlo simulation of the two‐dimensional H2 molecule
Authors: Bianchi R. , Bressanini D. , Cremaschi P. , Mella M. , Morosi G.
Published: December 2004

The problems connected with the simulation by the diffusion quantum Monte Carlo method of atomic and molecular systems in the two‐dimensional (2D) space have been investigated on the 1 2S and 2 2S states of the hydrogen atom and on the 1 1S and 1 3S states of the helium atom, assuming r−1 Coulomb interactions between the particles. The potential surface of the 1Σg+ state of the 2D hydrogen molecule has been calculated in the range 0.1–1.5 bohr: A dissociation energy of 1.2703 Hartrees is found at the equilibrium distance of 0.3639 bohr. © 1994 John Wiley & Sons, Inc.
A vibrational Hamiltonian model for triatomic molecules based on the Kratzer and Poschl Teller potentials
Authors: Requena Alberto , Alacid Mercedes , Bastida Adolfo , Zúñiga Jośe
Published: December 2004

A Hamiltonian model to describe molecular vibrations of triatomic molecules is proposed. The Hamiltonian is based on the use of the Kratzer potential variable for the stretching motions and a perturbed Poschl Teller potential for the bending one. The perturbation and variational treatments to compute the vibrational energies of this Hamiltonian can be developed using a zero‐order system that includes part of the couplings between the stretching and bending motions. All the matrix elements involved in these calclations can be then evaluated in closed form. A numerical application to the HCN molecule is made. © 1994 John Wiley & Sons, Inc.
Fast and stable algorithm for the analytical computation of two‐center Coulomb and overlap integrals over Slater‐type orbitals
Authors: Hierse W. , Oppeneer P. M.
Published: December 2004

Proceeding from analytical expressions for two‐center kernel functions that we derived recently, we present new analytical formulas for the two‐center Coulomb and overlap integrals over Slater‐type orbitals. These formulas are of an exceptionally simple analytical structure and high numerical efficiency. An especially important point is that for the most frequently needed ranges of discrete quantum numbers, the formulas are completely stable in the cases of nearly equal scaling parameters or vanishing interatomic distances, except for one particular case of the Coulomb integral. No special asymptotic formulas are needed any more to compute the two‐center integrals over Slater‐type orbitals in these case. Furthermore, a largely recursive formulation makes the integral evaluation very economical and fast. In particular, we assess the numerical performance of a new kind of angular momentum recurrences that we have proposed in a previous article [W. Hierse and P.M. Oppeneer, J. Chem. Phys. 99, 1278 (1993)]. © 1994 John Wiley & Sons, Inc.
Charge‐exchange in the nonadiabatic Na/W gas‐surface reaction using a diabatic representation
Authors: Kristyan Sandor , Olson John A.
Published: December 2004

A theoretical description of near‐resonant charge exchange is presented for gas‐surface collisions, based on the eikonal formalism, short wavelength approximation, and diabatic representation. The dynamics of the charge transfer is analyzed for the Na/W system. Accurate model diabatic potential surfaces are used to compute the charge exchange during scattering and desorption. The Pade method is applied to approximate the charge exchange probability. The derivations, developments, and comparisons of approximations of the charge‐exchange probability are discussed. These approximations are useful in experimental surface analysis to estimate the integral value of diabatic coupling between the atomic and ionic states. The frequency of oscillation during the charge‐exchange process is also discussed. Model diabatic potentials in analytic forms for the Na/W system are reported. © 1995 John Wiley & Sons, Inc.
Properties of the atomic form factor: Applications and shell structure
Authors: Gálvez F. J. , Porras I.
Published: December 2004

A relationship between the atomic form factor, F(k), and its first derivative, both at the origin, is presented. A new function f(k), related in a simple way to F(k), has been studied and some applications have been performed. They led us to find lower bounds to F(k) for all k and to other quantities such as the charge density at the origin, ρ(0), and radial expectation values. Finally, interesting effects on the Laplacian of f(k) due to the atomic shell structure were found. © 1995 John Wiley & Sons, Inc.
Decay of charged stabilized jellium clusters
Authors: Vieira Armando , Brajczewska Marta , Fiolhais Carlos
Published: December 2004

The stabilized jellium model is a simple modification of the jellium model, which more realistically describes simple metals of high density, such as Al, Ga, Pb, etc. We analyzed the fragmentation processes of charged spherical Al clusters in the framework of the stabilized jellium model. Kohn‐Sham calculations of the parents and daughters, using the local density approximation, have been made. We evaluated the dissociation energies of Al, Al, and Al, with N=1‐30 atoms, in all possible decay channels. We discuss the most favorable decay channels, which are ruled by the shell structure (magic numbers of valence electrons in the parents and the daughters) oscillations around an average trend given by a liquid drop model. We compare our calculations with others and with the available experimental data. © 1995 John Wiley & Sons, Inc.
Density functional theory of born couplings: Consequences for electron flow in Jahn–Teller molecules and superconductors
Authors: Sukumar N.
Published: December 2004

The dynamics of Jahn–Teller systems has recently been discussed in terms of generalized electronic charge and current densities in nuclear‐coordinate space. The introduction of the electronic phase as a function of both electronic and nuclear coordinates, in addition to the electronic density, was a crucial component of this formulation. Here, a densitybased treatment of Born couplings is derived from first‐principles quantum mechanics beyond the Born–Oppenheimer approximation. Because of the degenerate electronic configuration of a Jahn–Teller molecule, there are an infinite number of ways in which the charge distribution can be oriented for the same energy, leading to a vanishing bond hardness for the molecule in the symmetric nuclear configuration. Further, the moving nuclear framework serves as the perturbation necessary to define the orientation of the charge density, leading to unhindered rotation of the charge cloud. This leads to the dynamical Jahn–Teller problem, namely, the coupling of electronic and nuclear motions through the Born coupling terms. Applications to superconductivity theory are discussed. © 1995 John Wiley & Sons, Inc.
A density functional theory formulation of the reaction field model of solvent effects
Authors: Cemtreras Renato , Pérez Patricia , Aizman Arie
Published: December 2004

It is possible to reformulate the reaction field (RF) model of continuum solvent effects, by considering an approximate expression describing the energy changes from one ground state to another, in the frame of density functional theory (DFT). The energy functional for an arbitrary electronic system coupled to a spin‐independent electrostatic external perturbation is used to derive the well‐known Born expression giving the electrostatic component of the solvation energy of an atomic ion. The approximate RF–DFT model is illustrated for a series of representative singly positive and negatively charged atomic ions. A Kohn–Sham (KS)‐like formalism is then proposed to compute solvation energies within a self‐consistent field scheme. The extension of the RF‐DFT model to molecular systems is also outlined. © 1995 John Wiley & Sons, Inc.
Stationary point structure and energetics: Density functional study including solvent effects on the tautomerization of formamide and 2‐pyridone
Authors: Adamo Carlo , Lelj Francesco
Published: December 2004

Solvent effects on the thermodynamics and kinetics of some tautomerization reactions, chosen as the most representative of this wide class of reactions, are analyzed. Solvent effects have been modeled by the self‐consistent reaction field approach that we have implemented in the ADF package. We investigated the structure and energetics of the saddle points for the formamide/formamidinic acid and 2‐pyridone/2‐hydroxypyridine tautomerizations. Our results, carried out both at local and gradient corrected levels, are in good agreement with previous post‐Hartree–Fock computations in predicting the structure and the energy of the investigated saddle points. On the other hand, our computations show that solvent effects play a relevant role on the energetics of the reactions. In particular, the interconversion of 2‐hydroxypyridine into 2‐pyridone is sensibly favored in aqueous solution, being that the latter tautomers are stabilized by solvent interactions. This latter result is in agreement with experimental findings. © 1995 John Wiley & Sons, Inc.
Selected area X‐ray photoelectron spectroscopy
Authors: Keast D. J. , Downing K. S.
Published: December 2004

A technique is described which enables selected areas of a specimen to be located and examined using XPS. Results obtained using the technique are given which indicte that areas less than 0.5 mm square can be analysed.
Corrosion behaviour of stainless steel implants in biological media
Authors: Hofmann J. , Michel R. , Holm R. , Zilkens J.
Published: December 2004

The corrosion behaviour of orthopaedic metal implants was studied in an animal experiment in which 30 rabbits had stainless steel plates implanted after a varization osteotomy of the tibiae of their hind legs for one year. ESCA was used for the investigation of the surfaces of the inplants. The removed plates were covered by an easily destroyable 3–5 nm thick oxidic layer which contained Cr(III), Fe(II), Fe(III), Mo(VI) and Mo(IV), but no Ni. This passive layer (and possibly also the nearest metal layer below it) is strongly enriched in Cr. Additionally, corrosion experiments with radioactivated implants in Ringer's solution demonstrated that the passivation of the stainless steel is accompanied by marked selective solution of the steel constituents. The observed solubilities (Co > Ni > Fe > Mo > Cr) are inversely correlated with the abundance of the respective element in the passive layer. The trace element composition of the tissues was measured by instrumental neutron activation analysis. It turned out that the actual burdening of the tissues surrounding the implants by corrosion products is exhibited by strong enrichments of the elements Cr, Fe, Co, Ni and Mo relative to their normal concentrations. While Cr, Co, Ni and Mo are abundant in fairly constant ratios in the contact tissues, the excess of Fe is influenced by biological regulation mechanisms. The extent to which the distribution patterns of steel‐specific elements in animal (and human) tissues can be explained by the dissolution of the steel during the passivation process is discussed.
X‐ray photoelectron spectroscopic studies on ZnO/Bi2O3 varistors
Authors: Sainkar S. R. , Badrinarayanan S. , Sinha A. P. B. , Date S. K.
Published: December 2004

The intergranular layer and its microstructure, proposed earlier to behave non‐linearly in the ZnO/Bi2O3 varistor ceramic system, has been studied by XPS. Our results clearly indicate the formation of an intergranular layer rich in Bi2O3 separating the semiconducting ZnO grains. With increase in sintering temperature, XPS results show the enrichment of the intergranular layer (Bi2O3 phase) without an additional phase being formed. The non‐linear coefficient (α) in the current–voltage characteristic increases smoothly up to 1300°C. Afterwards, there is a loss of Bi ions from the Bi2O3 enriched intergranular phase resulting in the decrease of α, and in the characteristic XPS peak intensity.
A study of oxygen‐implanted Si0.5Ge0.5 alloy by XPS and thermodynamic analysis
Authors: Castle J. E. , Liu H. D. , Saunders N.
Published: December 2004

A thermodynamic study of oxygen‐implanted Si0.5Ge0.5 alloy based on an XPS analysis is reported for the first time. The alloy was grown by molecular beam epitaxy (MBE) and was implanted with high‐dose oxygen ions (up to 1.8 × 1018 cm−2) at an energy of 200 keV. The changes of Si, Ge and O activities with the concentration of oxygen were calculated using the THERMO‐CALC program, and chemical potentials of these elements throughout the oxygen‐buried region were plotted out according to the chemical state information obtained from XPS depth profiling results.

Transport during implantation and in subsequent annealing follows the gradient of chemical potential: elemental Si migrates into the oxygen‐rich layer to form SiO2 by the replacement of Ge in Ge oxides. This leads to a decrease of the total free energy for the system of Si, Ge and O in the sample. The description of the system in terms of activities and chemical potential enables true diffusion profiles to be used in place of phenomenological descriptions such as ‘snowplough’ or ‘pile‐up’.
Surface structural studies of polyethylene, polypropylene and their copolymers with ToF SIMS
Authors: Lianos L. , Quet C. , Duc Tran Minh
Published: December 2004

Static secondary ion mass spectrometry (SIMS) was applied for probing structural differences such as unsaturation and branching in the case of simple aliphatic hydrocarbon polymers (polyethylene, polypropylene and their copolymers). The parameter introduced for quoting surface amounts of branching is the percentage of C6 to C8 cluster emission relative to the sum of all C2 to C8 clusters. The ratio of the total emission of hydrogen‐deficient fragments to the sum of all C2 to C8 clusters is found to be a good pointer for unsaturation, especially for probing vinyl bonds. While applied successfully to polymers of the same family, these pointers cannot provide structural information on polyethylene/polypropylene copolymer surfaces in a straightforward way. The chemical surface composition of these copolymers is correlated to the percentage of the m/z 69 fragment.