bluuebunny/crossref_metadata_2025_split huggingface dataset:
Variation principle for the time dependence of density operators and its relation to linear decoupling procedures for propagators
Authors: Dalgaard Esper 
Published: December 2004

AbstractA modified form of Frenkel's time‐dependent variation principle, suggested by McLachlan for state vectors, is employed to discuss the optimal time evolution of a density operator ρ(t). An ansatz is made for this operator such that i(dρ/dt) = [S, ρ], where S(t) is a linear combination of operators belonging to a particular manifold of “basis operators.” The expansion coefficients are required to minimize the error Tr{[S – H, ρ]† [S – H, ρ]}. Linear response functions corresponding to the variationally determined density operator are compared to those derived by means of linear decoupling procedures for propagators based on the same operator manifold. The two approximation schemes are not equivalent, in general, and several consistency requirements must be fulfilled before it can be ascertained that a given linear decoupling procedure corresponds to an optimal time development of the density operator in the sense of McLachlan. Finally, the general applicability of the suggested variation principle is discussed.
Equations de Hartree–Fock dependant du temps
Authors: Smet F. , Tillieu J. , Van Groenendael Et A. 
Published: December 2004

AbstractThe time‐dependent Hartree–Fock (TDHF) equations are derived up to the second order when the system is perturbed by a monochromatic plane wave. The solutions of the equations are subjected to the orthonormalization conditions satisfied by the orbitals. In the equations, these conditions are expressed by the appearing of coefficients λ playing the part of Lagrangian multipliers. Relations between the coefficients λ are established. These relations are equivalent to the above‐mentioned orthonormalization conditions. This equivalence enables us to substitute for the solution of an integrodifferential equation system subject to constraint conditions, that of a free system. The TDHF equations obtained determine the first‐ and second‐order orbital perturbations, which no doubt verify the orthonormalization conditions. These orbitals can be used in the calculation, up to second order, of different nonlinear optical effects.
Effect of side‐chain disorder on the electronic structure of proteins
Authors: Suhai Sándor , Kaspar Johannes , Ladik János 
Published: December 2004

AbstractThe influence of side‐chain disorder on the electronic structure of proteins has been investigated in the case of polypeptides containing two or three different amino acid residues. It has been found that due to the different potentials of different side‐chain groups, the original valence and conduction bands of the homopolypeptides are split into narrow bands. The comparison of the densities of electronic states in simple homopolypeptides and in composite polymers shows that new forbidden regions in the energy spectrum of proteins may develop. The consequences of these effects for the semiconductive properties of proteins are discussed.
Criteria of accuracy of resonance eigenvalues
Authors: Moiseyev Nimrod , Weinhold Frank 
Published: December 2004

AbstractVarious criteria that have been suggested for assessing the accuracy of complex‐coordinate eigenvalues are critically tested in an application to the lowest helium 1S resonance, and found to be relatively ineffective. An alternative criterion is described, based on a generalized stabilization procedure, which seems better able to select the “best” resonance eigenvalue and to guide the construction of variational trial functions.
Effect of symmetry lowering on the band structure of polyacetylene
Authors: Morton‐Blake D. A. 
Published: December 2004

AbstractIn an attempt to apply band‐structure calculations to polymers with “reduced” symmetries, and simultaneously to examine some of the causes of band gaps in semiconducting polymers, two types of symmetry reduction are applied on the all‐trans polyacetylene chain. Using a neighboring‐segment approximation the consequent perturbations were used to calculate changes to the band structure of the undistorted chain and the energy gaps compared with those obtained from a direct calculation on the distorted polymer.
Hydrogenic orbitals as minimum‐basis functions for SCF calculations
Authors: Boerth Donald W. , Van‐Catledge F. A. 
Published: December 2004

AbstractThe orbital approximation that polyelectronic atoms can be qualitatively described by using hydrogenlike orbitals was tested quantitatively. Hydrogenic atomic orbitals (HTO's) were investigated as best‐atom minimum‐basis set functions and were compared to best‐atom Slater orbitals (STO's). The two sets of wave functions were compared in terms of the ground‐state energies, the electron densities at the nuclei, charge distribution in space, electron populations, and atomic “radii.” In the atomic calculations the HTO's give somewhat poorer atom descriptions that STO's. SCF calculations on diatomic molecules (N2, CO, and HF) indicate that while HTO's give higher molecular energies, they closely mimic STO's in terms of electronic reorganization. The principal difference between HTO and STO results was attributed to the compact nature of the HTO 2s orbital.
Toward a very simple picture of disorder effects in one‐dimensional organic conductors
Authors: Dugay M. , Roustan J. 
Published: December 2004

AbstractWe present a simple model for the calculation of some properties of disordered one‐dimensional organic conductors. We focus our attention on two points: the decrease of the insulator‐to‐metal transition temperature and the increase of the highest electrical conductivity temperature that both occur when the amount of disorder is increased. We give an explanation that brings a new insight to the still little understood zone around the insulator‐to‐metal transition. The model enters the general framework of the alternant molecular orbital method and seems to be a priori restricted to systems such as NMP‐TCNQ. We discuss its possible application to more complicated systems such as TTF‐TCNQ.
Degenerate symmetry‐adapted perturbation theory. Convergence properties of perturbation expansions for excited states of H2+ ion
Authors: Chałasińaski Grezegorz , Szalewicz Krzysztof 
Published: December 2004

AbstractThe degenerate Rayleigh‐Schrödinger (RS), Murrell–Shaw–Musher–Amos (MSMA), and Eisenschitz–London—Hirschfelder–van der Avoird (EL–HAV) perturbation theories are formulated and applied through high order to the interaction of a hydrogen atom with a proton for the excited 2sσg, 3pσu, 3dσg, and 4fσu states of the system. In contrast to the 1sσg and 2pσu states all the formalisms fail to provide a convergent series for any of the above states. However, numerical evidence is given that for sufficiently large internuclear separations the RS series is able to reproduce the Coulomb energy with an accuracy better than an arbitrary fraction of the exchange energy. Similarly the exchange energy can be reproduced by the sum of a few exchange corrections of the MSMA method. It has also been shown that both the MSMA and EL–HAV series can be continued analytically beyond the convergence radius by means of Padé approximants.
Concept of charge transfer: Investigation of charge transfer in the glyoxal–formamide, water dimer, and water–lithium cation systems
Authors: Otto P. , Ladik J. 
Published: December 2004

AbstractIn this paper an attempt is made to define charge transfer trying also other ways than Mulliken's population analysis. To illustrate this the amount of transferred charge (q) in the complexes HCONH2 + OHC—CHO in two relative geometrical positions, H2O + H2O and H2O + Li+ has been calculated with the help of Mulliken's population analysis using the wave function of the complex on the one hand and in the framework of different methods to subdivide the total space of the complex into subspaces for the acceptor and donor molecule, respectively, on the other hand. The results obtained show that Mulliken's population analysis gives a qualitatively correct description of the charge transfer process but to obtain the finer details most probably the use of a single number (q) is not sufficient. Further, the change in the total energy with respect to an effective charge transfer of a water dimer and of a solvent–cation complex has been investigated using the new mutually consistent field method. In this way the amount of transferred charge as well as the charge transfer interaction energy have been obtained in one step. These results have been compared with calculations applying second‐order perturbation theory including overlap and the supermolecule method.
Molecular‐orbital treatment of complex cations of magnesium and beryllium with acetonitrile
Authors: Morton‐Blake D. A. , Russell N. R. 
Published: December 2004

AbstractSemiempirical MO calculations of the self‐consistent charge and configuration (SCCC) method are reported for the acetonitrile–metal solvated species (CH3CN)xMn+, where M = Be2+ and Mg2+. Comparison of the delocalization energies for various chemical structures x = 1, 2, 3, 4, 6 leads to an expectation of a tetrahedral structure for the Be2+ species and an octahedral structure for the Mg2+ species. The electronic nature of the donor–acceptor interaction is also discussed.
Variation of calculated single bond lengths in saturated hydrocarbon and silicon hydride molecules
Authors: Verwoerd W. S. 
Published: December 2004

AbstractA series of saturated XnHm (X being either C or Si) molecules is studied using the MINDO/3 and MNDO quantum‐chemistry procedures. We find that the proximity of H atoms tends to shorten XX bonds, and that this effect is (especially for MINDO/3) much larger than a similar trend in experimental values. The connection between this and hybridization values calculated from localized orbitals is investigated. It is found that the hybridization model works well for XH bonds, whereas for XX bonds non‐σ‐bond effects also play a significant role. It is shown that the changes in electronic structure may be directly connected to differences in X and H atomic levels, so that similar bond length differences may be expected in other molecular orbital calculations.
Model for the lipid monolayer‐phase transitions
Authors: Maekawa Hiroyuki , Hosokawa Masahiko 
Published: December 2004

AbstractA theoretical order–disorder model for the main phase transition of lipid monolayers is presented here. The model contains approximate treatments of hard‐core repulsions and attractive forces between molecules. The lipid molecules are assumed as the square plates and then the entropy of the configurations is obtained using the placement method in a two‐dimensional lattice. The approximation used to compute the thermodynamic behavior of the model turns out to be a kind of molecular‐field approximation. The resultant pressure–density isotherms for the three‐state model show the characteristic van der Waals loops.
Electrical and thermal properties of the partially oxidized salt of the magnus green salt with the formula Pt6 (NH3)10Cl10(HSO4)4
Authors: Kubota R. , Kobayashi H. , Tsujikawa I. , Enoki T. 
Published: December 2004

AbstractThe salt Pt6(NH3)10Cl10(HSO4)4(MGSPOS) with the average valence of Pt 2.33 was prepared in powder form by means of partial oxidization of [Pt(NH3)4][PtCl4](MGS) substituting HSO4− for 1/6 of both ligands NH3 and Cl−. The electrical conductivity σ has a maximum at about 230 K and a minimum at about 280 K. Therefore, the conductivity σ is semiconductive with the gap energy 0.04 eV below 230 K, and metallic in the sense of dσ/dT < 0 between 230 and 280 K, and again semiconductive above 280 K. The thermal analysis by DSC shows only on heating, three anomalies, G at ∼150 K, A at ∼ 200 K, and B at ∼250 K. The exothermic anomaly A succeeding to the weak anomaly G indicates that a glass transition takes place, and endothermic anomaly B corresponds to a structural transition. These two transitions may correspond to the two metal–semiconductor transitions found in the σ measurement. The ac calorimetry shows a specific heat anomaly also only on heating.
Use of noncanonical orbitals in many‐body perturbation theory
Authors: McDowell Keith 
Published: December 2004

AbstractA formalism for dealing with noncanonical Hartree–Fock spin orbitals is presented within the framework of many‐body perturbation theory. A test calculation on the planar methyl radical is carried out which displays certain features of open‐shell calculations and which demonstrates the viability of using noncanonical spin orbitals. Results are found to be in good agreement with a previous configuration interaction calculation.
Static dipole and quadrupole polarizabilities of alkaline earth atoms
Authors: Sen K. D. , Schmidt P. C. 
Published: December 2004

AbstractStatic dipole and quadrupole polarizability (αd and αq, respectively) values of coupled Hartree–Fock quality have been calculated for the alkaline earth atoms Be(2s), Mg(3s), Ca(4s), and Sr(5s) using a method based on the many‐body perturbation theory. The present values of αd and αq for Be and αd for Mg are in excellent agreement with the other available results of similar accuracy. The CHF calculations of αd for Sr and αq for Mg, Ca, and Sr are being reported for the first time.
Binding: A unifying notion or a pseudoconcept?
Authors: Del Re Giuseppe 
Published: December 2004

AbstractCommunication between chemists and physicists working in different domains of physical chemistry may sound like a dream, but actually it must be established in order that the whole field may develop in a more efficient way. Binding is a unifying concept whose discussion may make it possible for specialists of different extractions to profit from each other's work. When different specialists speak of binding, they have some intuitive picture in mind. Does it correspond to a unique well‐defined concept, or is it just a cover for ignorance? A tentative definition is proposed as a starting point for discussion, but it is also emphasized that only from an open‐minded and thorough analysis of the various interpretations it should be possible to make real progress. An example showing very clearly the relationship between the various topics is that of biopolymers, whose understanding depends on the specification of the role of a variety of binding effects, from chemisorption to charge transfer to (one‐dimensional) metallic binding.
Experimental evidence on the chemisorption bond with the chemisorption on metals
Authors: Wedler G. 
Published: December 2004

AbstractWhich information on the properties of the adsorption complex can be obtained from the application of the standard methods in experimental surface science is discussed. None of the methods gives all the information necessary for a complete description of the chemisorption bond so that the results delivered by as many methods as possible should be taken into account.
Metal clusters and surfaces
Authors: Borello E. 
Published: December 2004

AbstractThe point of view of a physical chemist interested in metal clusters because of their connection with heterogeneous catalysis is illustrated. The analogies and differences between clusters and surfaces are given special attention. After a review of experimental techniques and of types of clusters studied, very important questions of theory are briefly considered, with special reference to Lauher's rules for predicting the stoichiometry of clusters. The relevance of clusters to surface studies is discussed also with reference to fluxionality and reactivity.
Correlations between structure and properties of materials. X. Anomalous optical properties and Raman spectra of glassy ternary B2O3–BaTiO3–Na2O materials
Authors: Bertoluzza A. , Marinangeli A. , Morelli M. A. , Simoni R. 
Published: December 2004

AbstractThe anomalous behavior of refractive index and density in the glassy ternary B2O3–BaTiO3–Na2O materials is presented and discussed in terms of Raman spectroscopy results. The slope change in the optical properties as a function of composition takes place within the same range of composition in which the Raman spectra suggest a transition from a bridging type of attack on the glassy matrix by the oxygen present in the basic oxide, to a nonbridging type. The structural modifications of the materials at different compositions are inferred from a detailed analysis of the Raman spectra.
Binding of leucomycin and its derivatives to E. Coli ribosomes
Authors: Jain D. V. S. , Gombar Vijay 
Published: December 2004

AbstractStructure‐activity relationships have been discussed for inhibition of [14C] erythromycin binding to Escherichia coli ribosomes by leucomycin and its acyl derivatives in the light of the Fujita‐Ban de novo model. The group contributions of various substituents show that acylation at position 4″ of the mycarose moiety of leucomycin increases affinity for binding of leucomycins to ribosomes. It is also indicated that unlike at position 2' in mycaminose region a free hydroxyl at position 9 of the lactone ring is not important for binding to ribosomes.
Cytochrome c: Ascorbate reduction site and possible electron‐transfer path
Authors: Myer Yash P. , Pande A. , Pande J. , Thallam K. K. , Saturno A. F. , Verma Balbir C. 
Published: December 2004

AbstractThe ascorbate reduction reaction of the native and urea‐perturbed forms, 0–8M urea, of horse heart ferricytochrome c is found to be a three‐step process: a urea‐dependent equilibrium step between a reducible and an irreducible form with a midconcentration of urea of 7.4M, a binding step with a binding constant of 5.9M–1, and a reduction step with a urea‐independent rate constant of 2.9 ± 0.3 s–1 [J. Biol. Chem. 255, 9666 (1980)]. The effect of adding urea, in addition to the generation of an irreducible form, is a slight lowering of the ascorbate‐protein binding constant, 5.9 to 2.7M–1, which is limited to the 0–5.5M concentration range. The thermodynamics of the ureadenaturation process also yields a three‐step mechanism, NX1X2D, with midconcentrations of urea of 2.5–3M, 6.2M, and 7.5M, respectively, where N, D, and the Xs are the native, the 9‐M‐urea, and the intermediate forms. The three processes are described as the loosening of the heme crevice opening, the solvent exposure of the polypeptide backbone, and the disruption of the tryptophan–porphyrin interactions, respectively [Biochemistry 19, 199 (1980)]. The reaction of the protein with 2,3‐butanedione, a group‐specific reagent for the guanidinium groups and an electron donor for this protein, is inhibited in the presence of ascorbate, but only one of the two functional groups is involved [J. Biol. Chem. 255, 11094 (1980)]. A correlation of kinetic and thermodynamic observations led to the conclusion that the ascorbate reduction of the protein is independent of the state of the heme crevice opening and of the polypeptide organized structures; instead, it is determined by the integrity of the tryptophan indole–porphyrin interactions. This information, when taken in conjunction with the selective inhibition of the reaction of the arginine side chains by ascorbate, establishes the binding site of ascrobate as one of the two arginyl side chains, and not the opening of the crevice or its vicinity. From the three‐dimensional structure of the protein, and taking into consideration the variability of the protein sequence, it is suggested that Arg‐38 is the ascorbate binding site, and that the electronic interaction between the indole of Trp‐59 and the porphyrin moiety must constitute, at least in part, the electron‐transfer path to heme iron.
Ordered products of exponential operators by similarity transformations
Authors: Witschel W. 
Published: December 2004

AbstractThe well‐known Hausdorff similarity transformation is used to derive a number of identities for exponential operators. The method does not give new results, but is shown to be a simple alternative to Feynman disentangling, parameter differentiation–differential equation, coherent states, and Lie–algebraic‐matrix techniques.
Large‐Order perturbation theory
Authors: Wu Tai Tsun 
Published: December 2004

AbstractThe original motivation for studying the asymptotic behavior of the coefficients of perturbation series came from quantum field theory. An overview is given of some of the attempts to understand quantum field theory beyond finite‐order perturbation series. At least in the case of the Thirring model and probably in general, the full content of a relativistic quantum field theory cannot be recovered from its perturbation series. This difficulty, however, does not occur in quantum mechanics, and the anharmonic oscillator is used to illustrate the methods used in large‐order perturbation theory. Two completely different methods are discussed, the first one using the WKB approximation, and a second one involving the statistical analysis of Feynman diagrams. The first one is well developed and gives detailed information about the desired asymptotic behavior, while the second one is still in its infancy and gives instead information about the distribution of vertices of the Feynman diagrams.
Converging SCF calculations on excited states
Authors: De Mello P. Corrêa , Hehenberger M. , Zernert M. C. 
Published: December 2004

AbstractA practical scheme for the calculation of excited states of the same symmetry as a given reference state is outlined in the context of the Hartree‐Fock method. In order to prevent the excited state from “collapsing” into a lower‐lying state, the prediagonalized Fock matrix is diagonalized in a restricted subspace, deleting the component associated with the orbital which participates in the excitation. Computationally, the deletion is accomplished by means of a “big shift” of the associated diagonal element of the prediagonalized Fock matrix. The resulting wave function will not be fully relaxed, but can be shown to be orthogonal to the reference state. The method has been implemented in a molecular UHF program. Applications to the 4σ hole state of CO and to an excited state of the CuCl ion are reported.
Symmetry groups of chemical graphs
Authors: Balasubramanian K. 
Published: December 2004

AbstractThe symmetry groups of all trees are shown to be expressible as generalized wreath products by a tree pruning algorithm. The symmetry groups of certain cyclic graphs which can be expresssed as generalized compositions are also shown to be generalized wreath products. The symmetry groups of complete multipartite graphs can be obtained in a similar manner. Character tables of symmetry groups of certain chemical graphs are also obtained.
Ab initio calculations of the electronic structures of Dimethylnitrosamine (DMN) and its Metabolite Hydroxydimethylnitrosamine (HDMN): A possible intermediate in carcinogenesis by DMN
Authors: Skancke Anne , Thomson Colin 
Published: December 2004

AbstractThe structure of N,N'‐dimethylnitrosamine (DMN) and its α‐hydroxy derivative have been computed using the ab initio force method with a 4‐21 basis set. The results are compared with experiment for DMN.