bluuebunny/crossref_metadata_2025_split huggingface dataset:
A theoretical study of tautomerism: 2‐ and 4‐oxopyrimidine and some of their derivatives
Authors: Leś A. , Ortega‐Blake I. 
Published: December 2004

AbstractHere we present a detailed study of the theoretical approach to the tautomerization processes considering 2‐ and 4‐oxo‐pyrimidine and several of their derivatives. We look into molecular relaxation, suitability of semiempirical methods, and the effect of basis sets size in pseudopotential ab initio calculations. We found that no semiempirical method is suited for studying the relative stability of the tautomers, even ab initio calculations with minimal basis sets. On the other hand MNDO appears to be very useful for molecular relaxation and can be used, as well as Ab initio calculations, for reasonable estimates of relative tautomerization. We also considered the quality of prediction of other parameters, in particular the ionization potentials. We considered the correlation of experimental and theoretical values as a means to adjust the theoretical results to obtain more reliable predictions.
N‐Representability conditions generated by a one‐particle grassmann factor of an antisymmetric function
Authors: Grudziński Hubert 
Published: December 2004

AbstractN‐representability conditions for a two‐particle density operator implied by positive‐semidefiniteness of the projection operator PN+1(ϕ1 Λ ΨN) are derived and discussed. The operator PN+1(ϕ1 Λ ΨN) projects onto an (N + 1)‐particle antisymmetric function ϕ1 Λ ΨN, the Grassmann product of a one‐particle factor ϕ1 and an N‐particle factor ΨN. The polar subcone 𝒫2N(g, q) to the set of N‐representable two‐particle density operators 𝒫2N which corresponds to these conditions is found. It is shown that its extreme rays belong to two orbits for the action of the unitary group of transformations in one‐particle Hilbert space. The facial structure of the convex set 𝒫2N exposed by elements of 𝒫2N(g, q) is analyzed. An example of the operator that changes the structure of its bottom eigenspace when the number of fermions N surpasses a certain value is noted. A new approach to the diagonal conditions for N‐representability is found. It consists of the decomposition of the N‐particle antisymmetric identity operator onto the mutually orthogonal projection operators.
A constant‐denominator perturbation theory using circulant orbitals
Authors: Chen Min‐Bo , Parr Robert G. 
Published: December 2004

AbstractA constant‐denominator perturbation formalism is developed employing a basis of circulant orbitals and a projected Møller–Plesset partitioning of the Hamiltonian operator. A formal justification for the classical Unsöld approximation is thereby provided. A calculation of correlation energy in the beryllium atom is carried out, and the results are compared with results obtained by the full configuration interaction method and conventional Møller–Plesset perturbation theory.
Correlated electron propagator theory and applications for open‐shell atoms and molecules
Authors: Born Gregory 
Published: December 2004

AbstractA unitary group reformulation of electron propagator theory is used to derive the second‐order and 2p – h Tamm–Dancoff self‐energy approximations for open‐shell applications. A highest weight representation of the reference state is chosen in order to facilitate matrix element evaluation, but two special cases precluded by this choice are also discussed. Detailed numerical calculations are described for the lithium atom, oxygen molecule, and the amidogen radical.
Supplement to spherical tensor theory of long‐range interactions between two molecules
Authors: Piecuch Piotr 
Published: December 2004

AbstractWe simplify a recently obtained approximate formula for the third‐order interaction energy between two molecules expressed in terms of irreducible spherical polarizabilities and multipole moments of interacting systems. Similarly, we can simplify analogous higher‐order expressions. Then we briefly examine the magnitudes of the various third‐order contributions in comparison with the second‐order categories of long‐range interactions for some specific systems.
Spectral polynomials of systems with general interactions
Authors: Balasubramanian K. , Randíc M. 
Published: December 2004

AbstractGeneral structures represented by graphs with unequal interactions (weighted edges) are considered and their characteristic polynomials (spectral polynomials) are obtained. It is shown that the procedure based on pruning terminal vertices previously developed by one of the present authors (K.B., Ref. 1) can be generalized to more common graphs in which nonuniform interactions (unequal couplings) occur. Special cases of the present approach are Möbius graphs, signed graphs, directed graphs, multigraphs, and pseudographs (i.e., graphs with multiple connections and graphs with loops, respectively). Weighted graphs (with general weights) are applicable to a variety of chemical problems such as problems of chemical kinetics, analysis of NMR spectra, the study of simple molecular orbitals, and molecular vibrations.
Length dependence of the optical transition energies of the exactly solvable Hubbard model for linear polyenes
Authors: Hashimoto Katsufumi 
Published: December 2004

AbstractSome accurate results on the length dependence of the excitation energies from the ground state to ionic excited states in the Hubbard model of linear polyenes are obtained based on the method of Lieb and Wu. To this end, it is first shown that singly ionic excited states with “plus” alternancy symmetry in the Hubbard model are described by the wave functions in which the two electron operator [∑(−)nCC] is acted on (N − 2)‐electron covalent eigenstates. Then by solving the Lieb‐Wu equations the exact excitation energies of the lowest ionic state, which corresponds to the E state in this model, are calculated for systems with up to 50 electrons. The result, together with a correction for the end effect, indicates that the excitation energies do not decrease as 1/N but converge to the limiting value more rapidly when the number of electrons N becomes large.
Energetics of proton transfer between carbon atoms (H3CH  CH3)−
Authors: Latajka Zdzisław , Scheiner Steve 
Published: December 2004

AbstractAb initio calculations were carried out to study the potential energy surface of (H3CHCH3)−. The 6–31G* basis set is supplemented by a set of diffuse p functions on both C and H (with a range of exponents for the latter). The binding energy of CH4 and CH3− to form the (H3CHCH3)− complex is about 2 kcal/mol, much smaller than for comparable ionic H‐bonded systems involving O or N atoms. Nearly half of this interaction energy is due to correlation effects, computed at second and third orders of Møller‐Plesset perturbation theory. Correlation is also responsible for substantial reductions in the energy barrier to proton transfer within the complex. This barrier is computed to be 13‒15 kcal/mol at the MP3 level, depending upon the exponent used for the H p functions.
Energy levels of paramagnetic ions: Algebra. V. Weak‐field models
Authors: Kibler Maurice , Grenet Geneviève 
Published: December 2004

AbstractWe explore a generalized weak‐field model for the description of the electronic properties of a partly filled shell(S) ion in a crystalline field. Such a model corresponds to the one developed, in parts III and IV of this series, for dN and fN ions in cubical symmetry except that the constraint relations are relaxed. This leads to a fourteen‐parameter weak‐field model for dN ions in octahedral symmetry and to a 33‐parameter weak‐field model for fN ions in octahedral symmetry. The latter two models are completely equivalent to the corresponding strong‐field models as developed by Griffith and by Tanabe, Sugano, and Kamimura. The constraint relations of parts III and IV are further discussed. In particular, the role they play in fitting procedures is examined. As a conclusion, the weak‐field model of III and IV appears as a phenomenological version of the generalized weak‐field model introduced in this paper.
The bonding properties of MoFes clusters
Authors: Chunwan Liu , Jianmin Hua , Zhida Chen , Zhenyang Lin , Jiaxi Lu 
Published: December 2004

AbstractIn this paper the electronic structure of a series of MoFeS cluster compounds (from dinuclear to nonanuclear) are calculated by using the MAD‐SCC and EH methods. The MO character, bonding property, as well as some other physicochemical properties are discussed. Some common regularities of these cluster compounds have been revealed. An extensive electron delocalization has been found in all these types of cluster compounds. There exists a certain degree of MM bonding in each type of the clusters. The Mülliken bond order of the MM bond are found to be as follows: MoMo > MoFe > FeFe, and for remaining bonds: MoL > MoSb > FeSb > FeL > MM. The chief contribution to the formation of stable MoFeS clusters is found to arise from the bonding interaction between the inorganic sulfur S* and metal atoms. The calculated results show that the magnified image rhomb is the basic structural unit in the MoFeS clusters. It is suggested that in the process of spontaneous assembly, the electron can be transported from the S* to the M atom, therefore the bridging effect of the S* plays an important role in this process.
Generalized 1/Z expansion for heteronuclear molecules
Authors: Pucci R. , March N. H. 
Published: December 2004

AbstractWe have first studied empirical regularities in various series of heteronuclear diatomic molecules between the energy E, the total number of electrons N, the equilibrium distance Re and Z̄ = (Z1Z2)1/2 where Z1e and Z2e are the nuclear charges in the diatom. In particular, for various alkali halide series, Re2|E|/N5/3 is shown to correlate rather simply with Z̄Re3. Some theoretical basis is afforded by generalizing the 1/Z expansion used early by the writers in work on homonuclear diatomics. Finally, when Z2/Z1 → ∞, a model is presented which predicts a finite asymptotic bond length and this prediction is confronted with available experimental data for both heteronuclear diatoms and for the polyatomic series CH4 to SnH4.
Construction of orbital angular momentum eigenfunctions for many‐particle systems by harmonic projection
Authors: Avery John , Larsen Peter Sommer 
Published: December 2004

AbstractFormulas are derived which allow the direct construction of total orbital angular momentum eigenfunctions for many‐particle systems without the use of Clebsch–Gordan coefficients. One of the equations is closely analogous to Dirac' identity for the total spin operator. This equation describes the action of L2 on a function of the particle coordinates in terms of a class operator of the symmetric group and a "contraction operator." A general projection operator for constructing symmetric eigenfunctions of L2 is presented.
The use of inner projections of the polarization propagator to study different contributions to the magnetic shielding tensor. I. Proton deshielding by steric compression
Authors: Ferraro M. B. , Natielo M. A. , Contreras R. H. 
Published: December 2004

AbstractA new method for studying different contributions to the magnetic shielding tensor is proposed. These contributions come from different molecular fragments, which are defined through a localization procedure. They are obtained by means of inner projections of the polarization propagator onto the subspace defined by the chosen molecular fragment. To illustrate the utility of this method, results are presented for the proton desheilding effect of steric crowding. Different degrees of approximation both in the polarization propagator formalism and in the ground state wave function can be used. Results presented in this paper were obtained at the INDO level, within the CHF scheme. Good agreement with experimental values is obtained.
On the stability of single‐ and double‐stranded DNA helices: The application of the PPT‐MCF method on large fragments of DNA
Authors: Otto P. 
Published: December 2004

AbstractThe pseudo‐polarization tensor mutually consistent field (PPT‐MCF) method recently introduced [1] has been applied to study the stacking interactions between the nucleotide bases in large periodic B‐DNA fragments. The effects on the global and local binding properties caused by replacing one base in the periodic sequence by another base are investigated.The increase in the stability for comparable fragments owing to this base substitution is further enforced in the case of periodic alternating helices. The most important results are that the stacking interaction between two bases is slowly converging with the interbase distance and that the average contribution per base to the binding energy is repulsive.Furthermore, the energetical properties of double helix models in B‐ and Z‐DNA configurations, respectively, consisting of up to five base pairs have been compared. It turns out that the G C G C sequence in Z‐DNA is significantly more stable than either in periodic or periodic alternating B‐DNA. In these cases the average energy contribution of a single Watson–Crick‐type base pair is predicted also to be positive. From the calculations it follows that the double helix is not stabilized owing to the hydrogen bonding between the bases belonging to both strands, in contradiction to most other investigations.
Application of the Green's function theory to the calculation of ionization potentials of model oligomeric systems
Authors: Deleuze Michaël , Delhalle Joseph , André Jean‐Marie 
Published: December 2004

AbstractThe vertical ionization potentials of model hydrogen chains have been calculated, in the framework of STO‐3G and 4‐31G bases, at the second‐order level of the many‐body Green's function theory. Compared to the second‐order many‐body perturbation theory, this approach provides a qualitative description of trends observed with orbital relaxation, pair removal, and pair relaxation effects when studying oligomeric systems of increasing size with varying bond length.
Finite‐size dimensional analysis of the superconducting vibronic interaction
Authors: Tachibana Akitomo 
Published: December 2004

AbstractEven in a finite‐size system, the vibronic interaction acts as the attractive force to bind a pair of electrons. For small‐size systems, the electron repulsion overwhelms the vibronic attraction. As the size of the system becomes large, the electronic repulsion diminishes to zero in proportion to the volume of the system, whereas the vibronic attraction (1) grows to infinity for a one‐dimensional system, (2) converges to a finite value for a two‐dimensional system, and (3) diminishes to zero for a three‐dimensional system. Even for a three‐dimensional system, the vibronic attraction diminishes much slower than does the electronic repulsion. This brings about a concept of the critical size for an any‐dimensional system, over which size the vibronic attraction overwhelms the electronic repulsion, thereby creating purely attractive interaction for a pair of electrons, which may lead to superconductivity.
On the quality of basis sets
Authors: Sordo T. L. , Sordo J. A. 
Published: December 2004

AbstractAn analysis of the quality of a given basis set is presented in terms of the three fundamental parts of total energy. This analysis clearly displays three types of error occasionally present in the components of total energy. As an illustration of the application of these concepts, several STO and Gaussian basis sets for the Ni atom are analyzed.
Quantum chemical calculation of the barrier for tunneling of hydrogen in hydrogen abstraction from methane by methyl
Authors: Chandra Asish K. , Malar E. J. Padma , Gupta Debasis Sen 
Published: December 2004

AbstractAn adiabatic reaction path for hydrogen abstraction from methane by methyl is computed by quantum chemical methods and then symmetrized by properly defining the reaction coordinate. The theoretical barriers are then fitted with the barriers defined by the parabolic and Eckart functions. Rate constants for the hydrogen and deuterium‐abstraction processes via tunneling at low temperatures are then computed.
Localization of electron group functions
Authors: Zapol Boris P. , Puchin Vladimir E. 
Published: December 2004

AbstractThe requirements of variational freedom of electron group functions and their spatial separation are basic statements of the group functions method as they are necessary for taking into account the intragroup electron correlation while neglecting the intergroup one. But these requirements seem to be inconsistent with one another. This contradiction can be removed using the notion of antisymmetrical annulment of many‐electron functions introduced in the present work. The transformation of group functions (GF) by means of functions antisymmetrically annulling (ASA) other GF's is proposed that does not affect the whole system's wave function but can be used for localizing GF's. The problem of construction of a function \documentclass{article}\pagestyle{empty}\begin{document}$ \tilde \Phi $\end{document} ASA a given Φ can be reduced to solving a system of linear algebraic equations. A sufficient condition of existing of nontrivial \documentclass{article}\pagestyle{empty}\begin{document}$ \tilde \Phi $\end{document} is obtained.
Integral representation of a fundamental functional for the study of the zero‐point vibrational energy of hydrocarbons and the total Pi‐electron energy of alternant hydrocarbons
Authors: Arimoto Shigeru , Hall George G. 
Published: December 2004

AbstractA further step is made in investigating the zero‐point vibrational energies En (or the total pielectron energies En for the case of alternant hydrocarbons) of lineraly extended system BAnB′ having n repeating identical moieties. By an approach using the aspects of form and general topology, an integral representation αint of a fundamental functional α, where α(φ) = limn→∞ (En(φ)/n), has been established, which enables one to treat these energies quantitatively in a unified manner.
Broken symmetry solutions of the extended Hubbard model
Authors: Ozaki Masa‐Aki 
Published: December 2004

AbstractThis paper describes a group theoretical classification of the unrestricted Hartree–Fock (UHF) solution of the extended Hubbard model including nearest‐neighbor repulsion and exchange interactions. We derived systematically all types of the broken symmetry solution with a single‐order parameter for the ordering vector Q = (π,π). It is shown that there are four nonmagnetic states: a charge density wave (CDW), a charge current wave (CCW), and two bond‐order waves (BOW), and five magnetic states: an ordinary antiferromagnetic state (AF), a spin current wave (SCW), two axial spin bond‐order waves (ASBOW), and an helical spin bond‐order wave (HSBOW). We showed that these phases can be derived naturally from the irreducible decomposition of the Fourier transformed interaction and the self‐consistent field (SCF) condition. Some numerical results are given for the half‐filled case.
Ab initio study of K adsorption on a Ag(100) surface
Authors: Gayen Sanjukta , Ermler Walter C. 
Published: December 2004

AbstractAb initio Hartree–Fock calculations are reported for the chemisorption of K on Ag using three different types of clusters to model the system. Geometry optimization is done in 4 degrees of freedom. It is found that since there is an absence of complete charge transfer between the adsorbate and substrate, the interaction can be interpreted as being predominately covalent in nature.
Molecular mechanics study of the β‐spiral conformations of the Phe4, Tyr4, and Trp4 analogs of elastomeric poly(Val1–Pro2–Gly3–Val4–Gly5)
Authors: Luan Chi‐Hao , Urry Dan W. 
Published: December 2004

AbstractThe elastin‐derived polypentapeptide, poly(VPGVG), and analogs are becoming model systems for protein as well as forming a new class of biomaterials. The β‐spiral of poly(VPGVG) represents a new family of molecular structures in fibrous proteins. In particular for our interests here, aromatic amino acid residue substitutions at position four of poly(VPGVG) give rise to new polypeptides with interesting properties of increased elastic modulus, pressure sensitivity, etc. Molecular mechanics computations were carried out to answer the question, can the polypeptides with Phe, Tyr, and Trp substitution at position four of poly(VPGVG) form a β‐spiral structure like poly(VPGVG)? Employed in this study was a combination of conformational search using ECEPP/2 and the build‐up strategy and of molecular dynamics calculations using CHARMm. The results indicate that the VPGX Type II β‐turn structure appears to be the most prominent secondary structural feature for the three aromatic residues containing polypentapeptides, although there is more evidence for VPGFG and VPGVG to assume the VPGVG‐type structure than for VPGWG to do so. The study also shows that the aromatic side chains do not interrupt the β‐spiral structure, even in the case of tryptophan.
A general algorithm for computing Voronoi volumes: Application to the hydrated crystal of myoglobin
Authors: Procacci Piero , Scateni Riccardo 
Published: December 2004

AbstractIn this study is presented a general algorithm for computing Voronoi volumes of atoms of group of atoms in condensed phases. The method is essentially an extension of the Medvedev procedure to allow vertice determination for any Voronoi polyhedron, primitive or with degenerate vertices. The algorithm has been employed for computing time‐averaged volumes in the hydrated crystal of met‐myoglobin, using the data of a molecular dynamics simulation. The results, compared to previous volume determination in myoglobin, emphasize the fundamental role of solvent structure close to the protein surface in relation to the packing density properties of the residues. Relative volumes fluctuations of myoglobin residues have been found to be correlated to the corresponding mean square displacements from X‐ray diffraction studies and to the theoretical hydration energies.
The relative stability of the tautomers of α‐Hydroxytetronic acid
Authors: Bäucker Thomas , Klessinger Martin , Eckert‐Maksić Mirjana , Maksić Zvonimir B. 
Published: December 2004

AbstractThe relative stability of the four tautomeric forms of α‐hydroxytetronic acid was calculated with full geometry optimization at the STO‐3G, 3–21G, 6–31G*, and 6–31G** SCF levels. Correlation effects were estimated using the MP2 method. Intramolecular hydrogen bonds are found to be of primary importance for the relative stability of the various tautomers, and the same tautomer that in the case of L‐ascorbic acid is experimentally observed in the crystal as well as in solution is found to be the most stable one, if polarization functions on the hydrogen atoms are taken into account. It is concluded that even in the gas phase this tautomer predominates in α‐hydroxytetronic acid as well as in L‐ascorbic acid. © 1994 John Wiley & Sons, Inc.
Theoretical investigation of the enantioselective hydrogenation of α‐ketoesters over pt/alumina modified with cinchonidine
Authors: Schwalm O. , Weber J. , Minder B. , Baiker A. 
Published: December 2004

AbstractEnantioselective hydrogenation of α‐ketoesters with cinchona‐modified platinum catalysts has been studied theoretically in order to rationalize the interaction between the chiral modifier and the substrate. The structure of the probable transition complex has been calculated for the system methyl pyruvate (substrate)–cinchonidine (modifier) using quantum chemistry techniques at both ab initio and semiempirical levels and molecular mechanics. The calculations indicate that crucial interaction occurs via hydrogen bonding of the quinuclidine nitrogen and the oxygen of the α‐carbonyl moiety of methyl pyruvate and confirm earlier experimental evidence that the quinuclidine nitrogen of the cinchonidine is involved in the interaction leading to enantiodifferentiation. The resulting complex, which resembles to a half‐hydrogenated state of the substrate, is shown to be a possible transition state for the enantioselective hydrogenation of α‐ketoesters. © 1994 John Wiley & Sons, Inc.