bluuebunny/crossref_metadata_2025 huggingface dataset:
Many‐Electron, multicenter integrals in superposition of correlated configurations method. I. one‐ and two‐electron integrals
Authors: Preiskorn Aleksandra , Żurawski Bronislaw
Published: December 2004

The calculations by means of the superposition of correlated configurations method (Hylleraas‐CI), that is, the combination of configuration interaction with the Hylleraas‐type correlation factors, needs the effective evaluation of some nontrivial integrals. This series of papers gives the formulas for all types of integrals needed for molecular calculations when Gaussian lobe functions are used as a basis set. The formulas for two‐electron integrals are given in the present paper. The preliminary results for two‐electron systems are presented.
The antisymmetrized geminal power approximation to the excitation propagator
Authors: Weiner Brian , Kurtz Henry A.
Published: December 2004

A consistent propagator approximation, denoted as the excitation propagator, is introduced. This propagator describes excitations between N‐particle states and its approximation has properties required of consistent random phase approximation schemes. Several properties of this propagator are explored when based on a generalized antisymmetrized geminal power wavefunction. How singularities in the metric occur and how to remove them is discussed in detail. The excitation propagator is also contrasted with the principal (polarization) propagator.
Can nondegenerate many‐body perturbation theory Be applied to quasidegenerate electronic states?
Authors: Kaldor Uzi
Published: December 2004

Wilson, Jankowski, and Paldus have recently applied nondegenerate many‐body perturbation theory (MBPT) to simple models, in which the degree of quasidegeneracy could be varied continuously, and concluded that the nondegenerate theory was applicable even near degeneracy. The error in their results changes, however, considerably with geometry, leading to an incorrect potential surface. An extension of their calculations shows convergence even at exact degeneracy (square planar H4). It is shown here that the apparently good convergence is due to the suppression of the large (infinite at exact degeneracy) component of the perturbation energy in low order by the way the Hamiltonian is partitioned. This component will, however, resurface at higher orders, leading to slow convergence or even divergence. The low‐order sum of the perturbation series is not very meaningful, depends strongly on details of the zero‐order Hamiltonian, and yields, in general, incorrect potential surfaces. Multireference MBPT eliminates these problems.
A proposed definition of resonant states
Authors: Cisneros Arturo , McIntosh Harold V.
Published: December 2004

The widely cited definition of quantization in terms of square‐integrable wave functions does not apply to continuum wave functions, to such phenomena as metastable states, or many‐body resonances. A better philosophical foundation for quantum mechanics separates the probabilistic aspects based on square integrable Hilbert space functions from the dynamical aspects based upon the solutions of Shroedinger's (or Dirac's) equation. A Hilbert space may have a non‐Hilbert space basis, which may be described by Stieltjes integrals and a spectrum measure. This viewpoint is expounded by reference to a very detailed analysis of a simple model, through which a precise definition of a Bohr–Feshbach resonance can be given. We propose a definition of a “metastable state,” showing that it is consistent with accepted usage, and that it overcomes a series of objections which have been catalogued by Simon. Its rate of decay is given by the Fourier–Stieltjes transform of the spectral density function; it is moreover the longest‐lived initially localized state which can be formed from a small span of energy eigenfunctions near its mean energy.
The relationship between electron densities and the pKa values in a series of methylpyrazines
Authors: Pilarski Bogusław , OśMiałowski Krzysztof
Published: December 2004

Based on the literature values, the pK1 and pK2 for pyrazine and its six methyl derivatives, the correlation with electron density on the nitrogen atoms N1 and N4 was found. The effect of methyl group was proved to be additive. The total electron density, π‐electron density, and dipole moments for compounds under study were determined by the CNDO/2‐MO method.
The general analytic expression for S4‐symmetry‐invariant potential functions of tetra‐atomic homonuclear molecules
Authors: Schmelzer A. , Murrell J. N.
Published: December 2004

The potential energy of a homonuclear X4 molecule is an invariant of the atom permutation group S4 acting on the internal coordinates of X4. It is shown by means of invariant theory that six algebraically independent invariants and five additional invariants are required to express the general invariant function for this group. Explicit expressions for these 11 invariants are given.
Theoretical investigation of the Na+ + H2 system. I. Ab initio calculations including electron correlations of potential energy hypersurface
Authors: Jeziorek Danuta , Żurawski Bronisław
Published: December 2004

An ab initio investigation of the potential energy hypersurfaces of the Na+/H2 system is presented. The calculations have been performed on SCF level and with the inclusion of correlation effects by means of the CEPA‐PNO, IEPA‐PNO, and PNO‐CI methods. The basis set consists of gaussian lobes (12s, 8p, d) on Na and (8s, p) on H. The distance of the Na nucleus to the midpoint of H2 was varied between 0.0 and 10 bohrs, whereas the region of HH distances considered was 0.5 to 10 bohrs. The angular dependence of the hypersurface was investigated as well. The system Na+/H2 has an absolute energy minimum for C2v geometry at R equal approximately 5.0 bohrs and rHH = 1.40 bohrs.
Molecular integrals arising in the use of the Hiller–Sucher–Feinberg–Harriman identity for describing the Fermi‐contact interaction
Authors: Ishida Kazuhiro
Published: December 2004

General formulas have been derived for molecular integrals arising in the use of the identity derived by Harriman for describing the Fermi‐contact interaction in a molecule. Computational aspects are discussed and numerical examples are shown for several sets of the usual Cartesian Gaussian‐type orbitals.
Collisional transfer of rovibrational energy from quantum calculations. II. The case of LiH with He
Authors: Gianturco F. A. , Palma A. , Lamanna V. T. , Petrella G.
Published: December 2004

The collisional exchange of energy between He atoms and a strongly polar diatomic, the LiH target, has been studied theoretically over a range of relative energies that had been previously sampled by experiments with molecular beams. The relative importance of the rotational and/or vibrational channels is examined by studying in detail the effect of the interaction via a model potential coupling parameter. The different behavior exhibited by differential cross sections (total and partial inelastic) is also analyzed in terms of the strength and shape of the interaction anisotropy.
Ab initio calculations on the barrier to pseudorotation of model 2′‐deoxyfuranose and 2′‐deoxy‐2′‐fluorofuranose rings
Authors: Lesyng Bogdan , Marck Christian , Guschlbauer Wilhelm
Published: December 2004

Model ring systems 2′‐deoxy‐2′‐fluororibofuranose and deoxyribofuranose have been investigated using ab initio calculations with the 3–21G basis set. The energy barrier to pseudorotation between the N and S states has been evaluated for the three preferred orientations of the (3′)‐OH group. Positions of the energy minima and the transition state have been optimized with respect to the (3′)‐OH orientation. The barrier to pseudorotation of 2′‐deoxy‐2′‐fluorofuranose is high and asymmetrical (ΔEN→S ≈ 20, ΔEN←S ≈ 8 kJ/mol), whereas the barrier of 2′‐deoxyfuranose is lower and almost symmetrical (ΔE ≈ 11–12 kJ/mol). The results obtained show that the preferred configuration of the 2′‐deoxy‐2′‐fluororibo‐furanose (N state) is stabilized by an internal O(3′)‐H…︁F interaction in accord with the crystallo‐graphic data.
Hypervirial theorem and ladder operators. Recurrence relations for harmonic oscillator integrals
Authors: Morales J. , Palma A. , Sandoval L.
Published: December 2004

A second‐quantization formalism combined with a hypervirial theorem is used to derive new recurrence relations for one‐dimensional harmonic oscillator matrix elements. The most general case of 〈m|f(â, â+)|n〉 is considered, and the recurrence relations forf(â, â) = Xk, exp(−βX), and exp(−X2) are given as examples. The relations obtained are considerably simpler than those derived by using only the hypervirial theorem; comparatively, the recurrence relations presented here have the advantage of avoiding the use of the quantum mechanical sum‐rules when determining initial matrix elements. The proposed procedure can be used to determine the recurrence relations for other potentials as well as to evaluate the two‐center integrals.
A graphical evaluation of characteristic polynomials of huckel trees
Authors: Kiang(Y.Jiang) Yuan‐Sun , Tang Au‐Chin
Published: December 2004

Characteristic polynomials of acyclic carbon chains (Huckel trees) are treated in a systematic way. Formulas of coefficients (ak) of the polynomial are obtained in terms of connectivities that were introduced for dealing with moments in a previous paper. Based on the meaning of ak, a graph‐theoretical analysis is given such that ak can be expressed as a linear combination of binomial factors specified by a set of graphs containing ½k edges. The numerical relationship is disclosed between each binomial factor and its specified graph. This stimulates the proposal of a novel approach for evaluating ak by simply collecting the graph set of defnite edges. The approach is equally applicable for the evaluation of matching polynomials of cyclic systems and extendable to the investigation of general trees.
Hierarchy equations for reduced density matrices in different representations
Authors: Cohen Leon
Published: December 2004

We derive exact relationships for the reduced density matrices in representations where the transformation matrix is a product of one‐body transformation matrices. We specialize to the momentum and onebody energy representations. By decoupling the equations we are able to write the Hartree‐Fock equation in terms of the first‐order density matrix in an arbitrary representation. Applications to reduced local energy and the correlation problem are discussed.
On the excitonic nature of the first UV absorption peak in polyene
Authors: Suhai S.
Published: December 2004

The first absorption peak in the UV spectrum of polyene is interpreted in terms of charge transfer excitons. The exciton spectrum has been calculated from first principles using the Green's function formalism of charge transfer exciton theory. Electron correlation effects on the polyene band structure have been included with the help of second order Møller‐Plesset perturbation theory and of the electron polaron model of Toyozawa. The spectrum of bound singlet excitons starts at EK=0 = 1.86 eV above the top of the valence band. A deeper lying triplet level is observed at 0.72 eV. Further correlation effects on the band gap and dielectric screening of the electron‐hole ineraction are discussed.
Lcao‐Xα calculations of rotational energy barriers—prototypes of chemical reactions
Authors: Dunlap Brett I. , Cook Michael
Published: December 2004

The LCAO‐X α method is used to compute the X α rotational barrier for ethane both in the rigid‐rotor approximation and for optimized staggered and eclipsed geometries. The problem of how to pick points and weights in three dimensions for fitting the nonanalytic exchange and correlation functionals of local density‐functional theory is examined; a variational weighting procedure is developed which greatly reduces the size of errors due to the 3‐D grid as compared to simple volume‐element weighting. The accuracy of the X α results for the optimized geometries and the barrier height of ethane is found to be comparable to that of large‐basis Hartree–Fock calculations.
Interpretation of the vibrational spectra of matrix‐isolated uracil from scaled ab initio quantum mechanical force fields
Authors: Harsányi L. , Császár P. , Császár A. , Boggs J. E.
Published: December 2004

The vibrational spectrum of uracil trapped in an argon matrix has been interpreted based on ab initio Hartree–Fock SCF calculations with a split‐valence 4‒21 basis set. The directly computed theoretical general valence force field was scaled with empirical scale factors in order to correct for the systematic errors originating in the limitation of the theoretical model. Scale factors transferred from related molecules provided a priori prediction of fundamental frequencies and intensities, permitting several corrections to be proposed for earlier assignments. Using the observed spectrum with the few altered assignments, a new set of scale factors was optimized to give the best force field available from combined consideration of the experimental and the theoretical data. For unknown reasons, the out‐of‐plane force field predicted a spectrum agreeing slightly less well with experiment than did the in‐plane force field. However, the overall agreement between theory and experiment provided additional support for the assumptions involved in the method. The computed force fields were compared with others available from previous work. The comparison demonstrated the importance of expanding the energy surface around the true energy minimum and of using a proper scaling procedure. Previous scaled CNDO/2 calculations were found to be surprisingly good despite the large corrections required and the fact that they were made at an incorrect geometry.
A chemically useful definition of electron difference densities
Authors: Schwarz W. H. E. , Mensching L. , Valtaznos P. , Von Niessen W.
Published: December 2004

Molecular difference densities (DD) are conventionally constructed using spherically averaged atomic densities at the appropriate positions. For atoms in degenerate ground states, this is an unphysical choice, and artifacts dominate the DD. We suggest the extraction of both the position and the orientation of an atom with an open valence shell from x‐ray scattering or molecular density data. Subtracting the oriented atoms yields a uniquely defined, as well as chemically meaningful, DD. Covalent bonds to electronegative atoms such as O are no longer exceptional but show bond charges of normal magnitude. Lone pairs are characterized by a dipolar density shift from the bond to the back side of the atomic core.
The concept of the topological atom within the MO‐LCAO approach. I. On the effective charge of an atom‐in‐molecule
Authors: Proinov E. , Neshev N. , Andreev A.
Published: December 2004

A new definition of the MO‐LCAO effective atomic charge is proposed, based on the concept of the topo‐logical atom A general equation for the MO‐LCAO electron occupancy of a topological atom is derived. It is shown that the intuitively introduced Mulliken gross atomic population is in fact an approximation to the electron occupancy of the corresponding topological atom. An approximate semiempirical procedure for evaluating the effective charge and effective radius of the topological atom is discussed.
The electronic structure of low‐lying excited states of two model systems of retinal
Authors: Shoda Takayuki , Noro Takeshi , Nomura Tsutomu , Ohno Kimio
Published: December 2004

2,4‐pentadienal and 2,4,6,8‐nonatetraenal were studied as simple model systems of retinal. Four kinds of CI were performed on low‐lying excited states of 2,4‐pentadienal by using a split valence basis set. The results show that MR SD π CI is not adequate for the π–π* state and the single excitation σπ CI is a good compromise between cost and effectiveness as far as singly excited states are concerned. This CI was applied to the bigger model system. All‐trans and 11‐cis forms of aldehyde, Schiff base, and protonated Schiff base of the model system were studied. A fairly large energy lowering of about 1 eV of the first allowed excited state (π → π*) occurs when the Schiff base is protonated for both all‐trans and 11‐cis forms.
Inclusion of relativistic effects into ZDO methods. V. Effect of core and polarization functions in bonding of first‐group elements
Authors: Boča Roman
Published: December 2004

Electronic structure and geometry of XY diatomics (X, Y = H, Na, K, Rb, Cs, Cu, Ag, and Au) have been studied by nonempirical versions of ZDO methods which account for major relativistic effects. Influence of the basis set size is investigated for these systems exhibiting the bonding controlled by the [(n − 1)d10]ns1[np0] ⃛ [(n − 1)d10]ns1[np0] interactions; [] are optical.
On the relation between core electron binding energies of ions in solution and their solvation energies
Authors: Arbman M. , Moberg R. , Holmberg S. , Wahlgren U. I. , Siegbahn H. O. G.
Published: December 2004

Experimental and computational results from the study of positive and negative ions in solution are presented. The importance of short‐range interactions between ion and solvent is studied with regard to core ionization of the ion. Exchange repulsion is found to be a significant factor in the interpretation of data for both cations and anions. Experimental results are presented for the core ionization of the OH− ion in solution. The data show a strong similarity with corresponding data for the F− ion, resulting in a large negative solvation energy for the final core hole state. The Be2+ ion shows large solvation energies for both ground‐ and core‐ionized states, which is interpreted as due to charge transfer effects between solvent and ion.
Electrostatic water models
Authors: Hall G. G. , Smith C. M.
Published: December 2004

The possiblity of creating a useful model for the electrostatic potential around the water molecule is examined from two different starting points. It is suggested that the simplest and most accurate model will have three point charges on the nuclei together with a diffuse Gaussian charge centered near the nuclear center of the charge. This will represent the lone‐pair cloud, include the penetration effects, and improve the representation of the electron density. The alternative use of atomic dipoles is explored but seen to be less effective.
What does zeolitic water look like?: Modelization by molecular dynamics simulations
Authors: Leherte Laurence , Andre Jean‐Marie , Derouane Eric G. , Vercauteren Daniel P.
Published: December 2004

In this paper, we present Monte Carlo and molecular dynamics simulations of water molecules inside a ferrierite‐type framework. Detailed analyses of the energetic, structural, and dynamical properties are carried out and compared with liquid water results in order to study the influence of the framework on the physisorbed water molecules.
The electron–nucleus coupling: A breakthrough in the investigation of paramagnetic metalloproteins
Authors: Banci Lucia , Bertini Ivano , Capozzi Francesco , Luchinat Claudio
Published: December 2004

Water proton relaxation measurements as a function of magnetic field on solutions containing paramagnetic metalloproteins contain information on (i) number and distance of metal‐coordinated water molecules, (ii) dynamics of water exchange, (iii) electron relaxation rates, and (iv) dynamics of the metal coordination sphere influencing electron relaxation. The use of appropriate theoretical tools permits us in many cases to learn about some or all of the above properties.
Stochastic dynamic simulation of a protein
Authors: Bhattacharya D. K. , Clementi E. , Xue W.
Published: December 2004

The internal motions of a small protein, the bovine pancreatic trypsin inhibitor (BPTI) in solution, are investigated in the framework of the Langevin equation. In this approach, the effects of the solvent molecules are incorporated by suitably defining the friction and random forces. The friction coefficients are determined from a molecular dynamics simulation. The details of the rapid fluctuations of protein atoms obtained by stochastic and molecular dynamics simulation techniques are compared by calculating the generalized density of states obtained via an incoherent neutron scattering. Presently, our stochastic dynamics simulation is one order of magnitude faster than the molecular dynamics simulation with the explicit inclusion of the water molecules. Generalizations of the present stochastic dynamics approach for studying the large‐scale motion in proteins are briefly outlined and the probability of a further speedup by an additional order of magnitude is discussed.
Computation of auxiliary functions in STO molecular integrals up to arbitrary accuracy. I. evaluation of incomplete gamma function En(x) by forward recursion
Authors: Li Yanxin , Dong Xialan , Pan Shoufu
Published: December 2004

A new inequality and an approximate relation of incomplete Gamma function En(x) are derived in this paper. Applying both to simplify the error analysis on the recursive algorithm of En(x), we analytically derived a criterion about relative error. The criterion decides whether the forward recursive relation can be used to compute {En(x), n = 0, 1, ⃛, N} with an acceptable loss of significant figures. Further analytical exploration leads to another criterion that decides whether the group of En(x) can be evaluated with an acceptable absolute error by the same methods. Both criteria are not only superior to those given in previous work by other authors, but also strongly supported by our numerical experiments. A strategy is illustrated to explain how to apply both criteria to evaluate molecular integral over Slater‐type orbitals. © 1993 John Wiley & Sons, Inc.
A comprehensive approach to molecular charge density models: From distributed multipoles to fitted atomic charges
Authors: Ángyán János G. , Chipot Christophe
Published: December 2004

Some alternative methodologies for the construction of reliable charge density models, involving distributed multipole schemes, potential fitted atomic charges, and a recent approach proposed by Ferenczy, are compared and analyzed critically. Ferenczy's method, which combines the distributed multipole description and the classical least‐squares fit, allows one to get an insight into the well‐known shortcomings of standard potential‐ or field‐derived atomic charge models. It is now demonstrated that the conformational dependence of fitted point charges is not exclusively due to through‐space inductive effects, but that it can be partly rationalized by the reorientation of the atomic multipoles. © 1994 John Wiley & Sons, Inc.
Ab initio calculations of isotropic hyperfine coupling constants in β‐ketoenolyl radicals
Authors: Fortunelli Alessandro
Published: December 2004

Ab initio unrestricted Hartree–Fock (UHF), unrestricted second‐order Møller–Plesset (UMP2) perturbation, unrestricted coupled cluster (UCCD), and unrestricted quadratic configuration interaction (UQCISD) calculations have been performed on the organic radicals CH3, CH3CH2, CH2CHCH2, CH3CHCOO−, HCOCHCOH, CH3COCHCOH, CH3COCHCOCH3, and CH3COC(CH3)COCH3, using double‐zeta and split‐valence‐plus‐polarization basis sets. These radicals are derived from common organic ligands and have been observed in recent experimental work on tris(β‐ketoenolato)cobalt(III) complexes. Their geometry has been optimized at the UHF level using the two mentioned basis sets. From these calcuations, values for the isotropic hyperfine coupling constants at the hydrogen atoms are predicted and compared with the experimental results. The usefulness of semiempirical extrapolations based on limited basis sets and treatment of electron correlation effects is carefully analyzed in the examples considered. © 1994 John Wiley & Sons, Inc.
Extrapolation of real‐space quantum chemical calculations from finite‐size super cells to the ideal infinite system. I. Theory
Authors: Nolting V. , Verwoerd W. S.
Published: December 2004

A method is proposed how to calculate the correct density matrix of an infinite polymeric chain from that of a standard finite supercell calculation. The density matrix of the finite supercell is transformed into k‐space for all k‐values allowed by the periodic boundary conditions. The k‐dependent matrices are then unitarily transformed, with each unitary matrix being represented by a set of complex rotation matrices. It is shown that the corresponding angles can be interpolated and extrapolated toward the zone boundaries in a straghtforward manner and that this extrapolation can be done from any finite supercell with reasonable accuracy. This gives rise to an infinite system density matrix for which all fundamental properties are guaranteed by construction. This infinite system density matrix may be used to construct a corrected density matrix for the finite supercell calculation. © 1994 John Wiley & Sons, Inc.
Applicability of quasi‐degenerate many‐body perturbation theory to the ground state of the F2 molecule
Authors: Mášik Jozef , Hubač Ivan , Mach Pavel
Published: December 2004

The ground‐state potential energy curve of the F2 molecule as well as spectroscopic constants were calculated by means of the second‐order quasi‐degenerate many‐body perturbation theory within a full (eight) valence orbital space using a DZP basis set. The problem encountered with a large number of valence electrons is avoided by a proper redefinition of the Fermi vacuum. A comparison with other related multireference techniques is also provided. © 1995 John Wiley & Sons, Inc.
Ab initio study of the structures of polythionylphosphazene molecular mimics with H, Cl, and CH3 side groups: 3‐21G* and 6‐31G* Basis sets comparison
Authors: Lagowski J. B. , Jaeger R.
Published: December 2004

The stable structures of substituted polythionylphosphazene single chains have been modeled with small molecular compounds consisting of one repeat unit of the polymer. The geometrical parameters of the nonplanar “trans‐cis” conformations of these molecular models are obtained using the ab initio molecular orbital theory. The substituents studies include hydrogen and chlorine atoms and methyl groups. Two basis sets, 3‐21G* and 6‐31G*, were used in the computations. We have found a very good agreement between the molecular geometries obtained from the two basis sets computations for the methyl‐substituted model compounds. The agreement is not as good for the hydrogenated and especially for the chlorinated model compounds. The comparison seems to indicate that the 6‐31G* is an essential basis set for the chlorinated compounds. The magnitude of the total dipole moments for these compounds ranges between 3.7 and 7.9 Debye. © 1995 John Wiley & Sons, Inc.
Second‐order intermediate Hamiltonian method: Pilot applications to vertical excitations in π‐electron systems
Authors: Zaitsevskii Andrei , Malrieu Jean‐Paul
Published: December 2004

Second‐order intermediate Hamiltonian quasi‐degenerate perturbation theory is applied to calculate π‐electron vertical excitation energies of ethylene and trans‐butadiene. The choice of model spaces follows the physical intuition, including the excitations within the π‐shell and leaving the σ‐orbitals doubly occupied. Model spaces are optimized using the state‐average CASSCF procedure. The agreement between calculated and experimental transition energies is better than 0.1 eV. The importance of a decontracted treatment of the model‐space projections of wave functions is demonstrated. A simple model study of deviations from size‐consistency is reported and implications for calculations on larger molecules are briefly discussed. © 1995 John Wiley & Sons, Inc.
On the connections between Brueckner–coupled‐cluster, density‐dependent Hartree–Fock, and density functional theory
Authors: Scuseria Gustavo E.
Published: December 2004

The existence of an effective one‐particle Hamiltonian in the Brueckner coupled cluster model naturally leads to the definition of an effective interaction G, which is a function of the T2 amplitudes. Two types of approximations to G are proposed: One is purely phenomenological, while the other is based on approximations to the Brueckner T2 equation. In both cases, the resulting effective interaction may be viewed as electron‐density‐dependent. Generalizing Hartree–Fock theory to accommodate density‐dependent interactions (DDHF), a method is obtained that is capable of accounting for correlation effects in an independent particle framework. The heuristic Skyrme force, successfully used in nuclear physics to model nucleon–nucleon interactions, is presented here as an example of an effective electron–electron correlation interaction. Due to the δ‐function character of the Skyrme force, it is possible to express the energy in this model by an integral over an energy density, thus formally providing a connection between DDHF and density functional theory for this particular case. An approximation to the Brueckner T2 equation is also proposed in the coordinate representation. In this model, the density‐matrix dependence of T2 is reduced to a nonlocal electron density dependence by means of an expansion which introduces terms that depend on the gradient of the density. The first term in this expansion amounts to a “local density approximation” to Brueckner coupled cluster theory. © 1995 John Wiley & Sons, Inc.
Nonadditive interactions and the relative stability of neutral and anionic silver clusters
Authors: Kaplan I. G. , Santamaría R. , Novaro O.
Published: December 2004

We discuss the physical nature of nonadditivity in many‐particle systems and the methods of calculations of nonadditive contributions to the interaction energy. For neutral clusters, a closed recurrence formula which expresses the energy of m‐body interactions through the energies of 2‐, 3‐, and (m – 1)‐body ones is obtained. The general approach for calculation of the nonadditive contribution in the interaction energy of charged systems is developed. The comparative calculation of anionic and neutral silver clusters shows that the geometry of the most stable anionic clusters is established mainly by the additive forces. The stability of neutral silver clusters is determined by the competition of attractive additive forces and repulsive nonadditive ones. © 1995 John Wiley & Sons, Inc.
Electron correlation effects in target molecule in low‐energy e− + N2 scattering
Authors: Pal Sourav , Ghose Keya B. , Meyer Hans‐Dieter
Published: December 2004

In this article, we discuss how to incorporate correlation aspects in the interaction of the electron with a “frozen” correlated target. We discuss the important aspects of correlation for e− + N2 scattering using configuration interaction, coupled cluster, and perturbative methods to describe the correlation in the target molecule. © 1995 John Wiley & Sons, Inc.
The Principle of maximum physical hardness
Authors: Pearson Ralph G.
Published: December 2004

The proof of Parr and Chattaraj for the Principle of Maximum Chemical Hardness is very general and can be applied to many more observables. The case of the physical, or mechanical, hardness of a solid is taken as an example. It is shown that this also should be a maximum in an equilibrium system. Assuming the validity of the argument leads to new information about the compressibility of solids. A simple expression is also given for the Gruneisen constant. These maximum principles are necessary consequences of the equilibrium conditions of quantum mechanics and thermodynamics. © 1995 John Wiley & Sons, Inc.
Density functional theory for systems of very many atoms
Authors: Kohn W.
Published: December 2004

The standard Kohn‐Sham formulation of density functional theory (DFT) is limited, for practical reasons, to systems of less than about 50‐100 atoms. The computational effort scales as N, where Nat is the number of atoms and 2 < α > 3. (By comparison, conventional configuration interaction methods are limited to 5‐10 atom systems.) This article deals with the prospect of practical methods that scale linearly in Nat and may thus allow calculations for systems of 103‐104 atoms. The physical reason (“near‐sightedness”) for linear scaling is presented. Implementations of linear scaling DFT by the use of generalized Wannier functions or the one‐particle density matrix are discussed. © 1995 John Wiley & Sons, Inc.
Density functionals for the Yukawa electron‐electron interaction
Authors: Savin Andreas , Flad Heinz‐Jürgen
Published: December 2004

Short‐range nonclassical electron‐electron interaction is described by a density functional in a scheme that allows multideterminant wave functions. The parameter that determines the coupling with the configuration‐interaction‐type calculations can be chosen in a controlled manner. Results are presented for the He and the Be series using a Yukawa‐type interaction. © 1995 John Wiley & Sons, Inc.
Some efforts beyond the local density approximation
Authors: Sham L. J.
Published: December 2004

The exchange‐correlation density functional can be expressed as a many‐body perturbation series in terms of the Coulomb interaction using the exact Kohn‐Sham orbitals as the basis. A self‐consistent equation is derived for the exact exchangecorrelation potential. This perturbation approach forms a basis for going beyond the local density approximation (LDA). The discontinuity in the exchange‐correlation potential for semiconductors calculated by the perturbative approach gives a good account of the discrepancy of the band gap calculated in LDA. The discontinuity also plays an important role in the interface band diagrams. A theory to account for the interaction effects of localized d or f orbitals is reviewed and the physics of the applications to a model test, to some 3d transition metals, and to heavy fermions is discussed. The perturbative approach to improvement beyond LDA tends to be computation‐intensive and to be system‐specific. © 1995 John Wiley & Sons, Inc.
Exact theorems concerning noninteracting kinetic energy density functional in D dimensions and their implications for gradient expansions
Authors: Holas A. , March N. H.
Published: December 2004

The focal point of the present work is the single‐particle kinetic energy density tensor in D dimensions. This quantity enters both differential and various integral forms of the virial theorems, which are again set up in D dimensions. Major new results lie in (i) demonstrating that, by one‐dimensional quadrature, it is possible to construct the Pauli potential directly from the kinetic energy tensor, without the need for functional differentiation and (ii) generating the gradient expansion for the kinetic energy tensor, in D dimensions. © 1995 John Wiley & Sons, Inc.
New exact relations for improving the exchange and correlation potentials
Authors: Levy Mel , Görling Andreas
Published: December 2004

For the purpose of improving present approximations to the exchange and correlation potentials, newly derived properties of the exact exchange and correlation potentials are summarized. Present approximations are not expected to generally satisfy these properties. The summarized properties include relations at the Fermi level, low‐density requirements, and a new density functional formula for computing ionization energies. © 1995 John Wiley & Sons, Inc.
Average local electrostatic potential and the core‐valency separation in atoms
Authors: Sen K. D. , Gayatri T. V. , Krishnaveni R. , Kakkar M. , Toufar Helge , Janssens Geert O. A. , Baekelandt Bart G. , Schoonheydt Robert A. , Mortier Wilfried J.
Published: December 2004

The average local electrostatic potential function, V(r)/ ρ(r), is calculated for 87 atoms, Li‐Ac, in the ground state using the nonrelativistic average‐over‐configuration numerical Hartree‐Fock density. It is found empirically that in a given atom the shell boundaries are expressed as the successively increasing maxima in V(r)/ ρ(r) and the outermost maximum presents good approximate estimates of the core‐valence separation in atoms. The likeness in behavior of V(r)/ ρ(r) at each shell boundary with the maximum hardness principle is discussed. The single‐exponent‐fit parameters for the electron density in the valency region are provided for all atoms. © 1995 John Wiley & Sons, Inc.
The implementation of analytical energy gradients based on a quasi‐relativistic density functional method: The application to metal carbonyls
Authors: Schreckenbach Georg , Ziegler Tom , Li Jian
Published: December 2004

The capabilities of the quasi‐relativistic scheme due to J. G. Snijders at al. [Mol. Phys, 38, 1969 (1979) Ibid., J. Phys. Chem. 93, 3050 (1989)] has been extended by deriving expressions for the energy gradients with respect to the total energy EQR and implementing them into the ADF program system [B. to Velde and E. J. Baerends, Int. J. Quantum Chem. 33, 87 (1988)]. This implementation enables automated geometry optimization at the relativistic level. The new scheme has been applied together with a self‐consistent nonlocal density functional scheme, NL–SCF + QR, to the calculation of MCO bond lengths and the first bond dissociation energy (FBDE) in the binary transition metal carbonyls M(CO)5 (M = Fe, Ru, Os) and M(CO)6 (M = Cr, Mo, W). The calculated MCO bond lengths are in good agreement with available experimental data with an error typically smaller than 0.01 Å. The calculated FBDES are 45.7 (Fe), 33.0 (Ru), 34.7 (Os), 46.2 (Cr), 39.7 (Mo), and 43.7 (W) kcal/mol, respectively. These values compare well with the available experimental estimates of 42 (Fe), 28 (Ru), 31 (Os), 37 (Cr), 41 (Mo), and 46 (W), respectively. The relativistic effects are found to contract MCO bonds by between 0.07 and 0.16 A and strengthen the FBDEs by 5‐11 kcal/mol for third‐row compounds. The relativistic stabilization of the FBDES among the 5d elements makes, in general, the MCO bond of the 4d element weakest within a triad. © 1995 John Wiley & Sons, Inc.
Quantitative correlation of XPS linewidth with dislocation density in shock‐loaded ammonium perchlorate
Authors: Beard B. C. , Sandusky H. W. , Glancy B. C. , Elban W. L.
Published: December 2004

The linewidths of x‐ray photoelectron spectra have been correlated with dislocation densities in a shock‐damaged crystal of ammonium perchlorate (AP). A centimeter‐size AP crystal was non‐uniformly damaged by a rapidly decaying shock (peak pressure of 24.4 kbar at the entry surface), recovered intact and cleaved. The cleaved planes permitted interior analysis of the crystal by x‐ray photoelectron spectroscopy (XPS) over a pattern of 1 mm × 1 mm areas. The linewidth of the Cl 2p3/2 peak ranged from 1.70 eV for the region of greatest damage to 1.22 eV for the region demonstrating no visible damage, the same linewidth as that for unshocked AP (control). The correlation between dislocation density and XPS linewidth was quantitatively established by chemically etching and counting dislocations on cleaved planes from the shocked crystal. By this technique, a 100 × increase in dislocation density was determined for the region of greatest shock damage relative to undamaged crystal. Molecular orbital energy variation caused by distortion of the lattice in the vicinity of a dislocation is the physical mechanism responsible for the broadening of the photoelectron lines. Ab initio calculations of the Cl 2p energy level from various distorted perchlorate anion geometries have predicted orbital energy shifts sufficient to produce the observed linewidth broadening.
XPS reference procedure for the accurate intensity calibration of electron spectrometers— results of a BCR intercomparison co‐sponsored by the VAMAS SCA TWA
Authors: Seah M. P.
Published: December 2004

A calibration procedure has been developed for XPS instrument intensity scales. The calibration is achieved by measuring the spectrum for reference foils of Ag, Au and Cu for which the true spectrum has been measured usng the metrology spectrometer. The ratio of the measured and reference spectra at 1 eV intervals from 200 eV kinetic energy to 1600 eV provides the calibration known as Q(E). The calibrations from all reference foils for a given instrument and setting should be the same and may be averaged to enhance accuracy. An interlaboratory study involving 58 different instruments, covering 25 different models from 9 manufacturers, has been completed with over 1200 individual spectra measured. The results show that the calibrations are excellent but that care is required or systematic errors may arise from instrumental problems such as x‐ray anode contamination, sample contamination, contributions from the sample holder and internal scattering in the spectrometer. However, these problems may be readily diagnosed. The Cu data often show carbon contamination and so those data are omitted from the assessments of Q(E).

The instruments provide unmonochromated x‐rays from Al and Mg anodes as well as monochromated Al sources. The Q(E) curves for the unmonochromated sources always agree with each other whereas that for the monochromated source may agree with those for the unmonochromated sources in some instruments but not in others. The reason for the differences may lie in stray magnetic fields or lens magnification effects. The different reference materials give the same Q(E) on any given instrument for a given set of operating conditions with a typical scatter of 3% but actually ranging from 0.99% in the best case to 19.49% where problems, as noted above, occurred. Changing the slits or pass energy can change the relative intensities at low and high energy by more than a factor of 2. However, even larger differences may occur between instruments of the same model operated under identical settings in different laboratories. It is thus shown that each individual instrument needs a separate and regular calibration and that such a procedure would reduce the present intensity variations of a factor of 7 over the energy range 200–1400 eV to an average scatter of 3% or less.

The individual calibrations for Q(E) for all of the 58 instruments are presented and it is shown that the methodology and reference data work effectively and consistently in all known situations where the reference procedure is adequately conducted.
Surface analytical study of the cause of mass gain of the kilogram prototype
Authors: Hashiguchi Y. , Mizuno K. , Nagoshi M. , Kasamura H. , Shiozawa K. , Fujita D. , Yoshihara K.
Published: December 2004

The International Prototype of the Kilogram, which is the only artificial material defining one of the fundamental constants, has been reported to gain mass as time elapses after it is cleaned. The surface of a Pt‐Ir alloy that has been used for the newer Kilogram prototypes was analysed by XPS and thermal desorption spectroscopy (TDS) to investigate the cause of mass gain, its time dependence and the effect of the cleaning method. The main cause of the mass gain was found to be adsorption of carbon (in the form of hydrocarbons) and oxygen (as water) from the atmosphere. The amounts of the two contaminant elements were almost saturated 6 months after sample cleaning. Cleaning methods do not make a large difference to adsorption behaviour. Although surface iridium is slightly oxidized, this effect is negligibly small.
Quantification of oxygen and carbon in high Tc superconducting films by (α,α) elastic resonance technique
Authors: Vizkelethy G. , Revsez P. , Mayer J. W. , Li Jian
Published: December 2004

The quantification of oxygen and carbon in high‐temperature (Tc) superconducting oxide thin films was made by employing elastic resonance in He backscattering analysis. A method combining the oxygen resonance technique and channeling was presented for measuring the nature of the oxygen disorder near the surface and the interface in a YBCO superconducting film grown on an MgO substrate. The oxygen resonance technique was used to quantify the oxygen profiling in the metal/YBCO contacts, showing that Zr and Nb act as sinks to oxygen from YBCO films and are oxidized in the forms Zr/ZrO2/YBCO/MgO and Nb0.2/YBCO/MgO after annealing in a vacuum at 350°C. We combined the carbon and oxygen resonances to determine the carbon contamination and oxygen concentration changes on the YBCO surface after coating and baking the photoresist. Residual carbon on the surface and a thin layer of oxygen depletion near the YBCO surface have been observed. The residual carbon in Bi2Sr2CaCu2O8 films made by the decomposition of metallo‐organic precursors was quantified using carbon resonance.
Universal energy dependence of the inelastic mean free path
Authors: Jablonski A.
Published: December 2004

The accuracy of quantitative XPS analyses using relative sensitivity factors is increased if matrix effects are accounted for in calculations of surface composition. This requires knowledge of the energy dependence of the inelastic mean free path (IMFP) for a given sample. It is sufficient for routine analyses to use the IMFP energy dependence averaged over a number of solids. A universal energy dependence is found to be better expressed by the Bethe equation than an equation of the form λ = aEp, although in both cases the energy dependence is controlled by one parameter. Values for these parameters are shown to depend on the class of solid: elements, inorganic compounds and organic compounds. The deviations of the assumed universal IMFP energy dependence from the true IMFP energy dependences for different solids is largest, exceeding 20%, in the energy range 200–500 eV. This deviation decreases to 3% at energies 500–1500 eV. Photoelectrons with kinetic energies within the latter energy range are recommended for quantitative XPS analysis using calculated sensitivity factors.
XPS study of the a‐C : H/Al2O3 interface
Authors: Ernst K.‐H. , Patscheider J. , Hauert R. , Tobler M.
Published: December 2004

The interface between diamond‐like amorphous hydrogenated carbon (a‐C : H) and Al2O3 was analysed by sputter depth profile analysis via x‐ray photoelectron spectroscopy. X‐ray photoelectron spectra were also recorded during direct ion beam deposition (CH4, E = 400 eV) monitoring the initial build‐up of the interface. No stoichiometric interface compound was detected, although Al2O3 is reduced to Al2O3‐x (x ∼ 0.5–1) enabling an interaction with the carbon atoms. This results in an excellent adhesion of a‐C : H to the chemically inert Al2O3.
X‐ray photoelectron spectroscopic study of ion etching and electrical biasing of silicon nitride on a carbon fiber
Authors: Rooke Michael A. , Sherwood Peter M. A.
Published: December 2004

Carbon fibers with a chemical vapor‐deposited silicon nitride surface film have been investigated with core and valence band x‐ray photoelectron spectroscopy (XPS). The ability of the films to protect the fibers from oxidation was evaluated by heating, by argon and oxygen ion etching and by using an electrical biasing potential. Oxygen ion etching appears to oxidize the Si3N4 to SiO2, whereas argon etching removes the oxide and any hydrocarbon surface layer. The valence band data for Si3N4, Si2N2O, and SiO2 are interpreted by multiple scattered wave Xα calculations. The films crack when an electrical potential is applied, and the cracking is reproducible and irreversible, occurring particularly with a negative potential bias. We find chemical shifts on the core levels consistent with differential charging of the silicon nitride coating, an observation that is supported by sample biasing experiments. The work suggests that the cracking of the silicon nitride coating will significantly limit its effectiveness in protecting carbon fibers and carbon–carbon composites from oxidation.