bluuebunny/crossref_metadata_2025_split huggingface dataset:
Nonadditivity of interaction in water trimers
Authors: Clementi E. , Kołos W. , Lie G. C. , Ranghino G. 
Published: December 2004

AbstractThe energy of (H2O)3 has been calculated for 29 geometrical configurations of the trimer using the SCF LCAO MO method and extended as well as minimal basis sets of Gaussian functions. For two configurations two intermediate basis sets have also been tested. The results show the nonadditive component of the interaction energy to be small. They also indicate that fairly reliable results for the trimer can be obtained using minimal basis sets and the counterpoise method to eliminate the basis set superposition error. The nonadditive contribution to the interaction energy is shown to be mainly due to the long‐range induction interaction.
Exact‐exchange Hartree–Fock calculations for periodic systems. II. Results for graphite and hexagonal boron nitride
Authors: Dovesi R. , Pisani C. , Roetti C. 
Published: December 2004

AbstractThe ab‐initio LCAO‐HF method that was presented in a previous work is applied here to the study of graphite and hexagonal boron nitride monolayers. The dependence of total energy, band structure and density matrix on the computational parameters that control truncations in infinite sums over the translation vectors g of the direct lattice is first considered. For these systems it comes out that very good results can be obtained by neglecting all but the first few terms in the sums. For instance, for exchange contributions it is sufficient to consider integrals associated with the first 13 g vectors, suitably grouped, to obtain an error in total energy below 0.0001 a.u. The calculations were performed using an STO‐3G basis set; results concerning conformational minimum, bond energy, symmetric force constant, band structure, density of states, and population analysis are presented and discussed.
Resonance energy of very large benzenoid hydrocarbons
Authors: Randić Milan 
Published: December 2004

AbstractThe resonance energy of conjugated benzenoid systems is expressed as contributions arising from independent conjugated circuits. The scheme has been applied to numerous very large conjugated systems. In many cases, it was possible to find regularities in the increments for the resonance energy within a family of benzenoid systems as the number of benzene rings is increased.
Quantitative structure–activity study of some enzyme‐inhibitory quinazolines
Authors: Abdul‐Ahad P. G. , Blair T. , Webb G. A. 
Published: December 2004

AbstractRelationships are formulated to explain the variation in dihydrofolate reductase inhibitory potency for a series of 25 substituted quinazolines, with an antineoplastic potential. Highly significant correlations are obtained using CNDO/2–3R calculated indices and/or the empirically estimated molecular polarizability as independent variables. The MO calculated indices employed are the atomic polarizability, as defined herein, and bond energy. The molecular polarizability is represented by a simple sum of environment independent partial atomic polarizabilities. The partial polarizabilities, reported here for H, C, N, O, F, S, Cl, and Br, are obtained from a multiple regression forced through the origin.
Biological effects of magnetic fields and cooperative phenomena
Authors: Ducla‐Soares Eduardo 
Published: December 2004

AbstractCooperative phenomena may be involved in the biological effects of magnetic fields (H), since its effects on isolated atoms or ions, at room temperature, are very weak. It is suggested that interactions at the social level can considerably increase the efficiency of H in orienting the displacements of animals. This effect is discussed with a simple mathematical model inspired on superparamagnetism. In case of very weak interactions of H with isolated animals, it suggests that orientation could only be made visible through cooperativity at the social level. These results are generalized to other physical stimuli and effects, leading to the concept of cooperative sensory response, i.e., a group response to an external physical agent even when its effect on isolated members of the group is undetectable.
Pair density analysis of semiempirical wave functions: Study of water molecule including environmental effects
Authors: Constanciel R. , Gritli H. 
Published: December 2004

AbstractThe pair‐density‐analysis method is applied to the study of CNDO/2 wave functions of the water molecule in various configurations. Particular attention is devoted to the modifications introduced by the consideration of environmental effects. The validity of the separated pair approximation is discussed and a detailed analysis of group functions is given.
Self‐consistent calculation of 1s2s1S state wave function of helium
Authors: Datta Dilip Kumar , Sengupta Sankar 
Published: December 2004

AbstractA simple self consistent variation perturbation method in the coupled Hartree–Fock scheme has been proposed to calculate 1s2s 1S state of the He atom. The present paper deals with an 1s2s 1S wave function in which all the relevant orthogonality conditions are imposed in successive stages. The resulting wave functions together with some interesting features are discussed.
Applicability of coupled‐pair theories to quasidegenerate electronic states: A model study
Authors: Jankowski K. , Paldus J. 
Published: December 2004

AbstractThe coupled‐pair many‐electron theory, its linear approximation, and various other approximate coupled‐pair approaches based on the orthogonally spin‐adapted coupled‐pair theory are applied to simple model systems in which the degree of quasidegeneracy can be continuously varied over a wide range. The four‐electron models studied are three ab initio minimum basis set models involving four hydrogen atoms in various spatial arrangements as well as the semiempirical Pariser–Parr–Pople π‐electron model of cis‐butadiene. The results are compared with the exact full configuration interaction results as well as with those obtained with other approximate limited configuration interaction approaches and Davidson's formula estimate, and the effect of quasidegeneracy on various approaches is discussed. We also examine the difference between the correlation energies obtained with the coupled‐pair many‐electron theory and its linear version and discuss the source of these differences and of the breakdown of the latter approximation in the case of very strong quasidegeneracies.
Definition of invariant molecular tensors in an origin change. III. Comparison of the methods suitable for the obtainment of invariant tensors for refraction and the Faraday effect
Authors: Duprez‐Quesnoit C. 
Published: December 2004

AbstractIn a previous paper, denoted I [C. Duprez and J. Tillieu, Int. J. Quantum Chem. 17, 155 (1980)], a global tensor, invariant in an origin change, has been introduced for all optical phenomena. The results obtained have been applied to two particular cases: refraction and the Faraday effect, in Paper II [C. Duprez, Int. J. Quantum Chem. 17, 169 (1980)]. In this paper, we study the various methods suitable for the calculation of these tensors. Then, we compare the results obtained in II with those previously obtained by other authors who, when studying the two particular cases mentioned above, omitted some terms necessary to obtain the invariance.
Self‐consistent calculation of 1s2s3S state of helium and heliumlike ions
Authors: Dutta Dilip K. , Sengupta Sankar 
Published: December 2004

AbstractIn this paper we present a simple method within the coupled Hartree–Fock framework to calculate the 1s2s 3S state of helium and heliumlike ions. The results are in very good agreement with those obtained by the use of multiterm correlated wave functions. Some interesting observations associated with the wave function are presented.
Molecular orbital treatment of phenylfurans and bifurans
Authors: Abu‐Eittah Rafie , Hilal Rifaat , Hamed Maher M. 
Published: December 2004

AbstractThe molecule‐in‐molecule method of computation has been applied in a SCF study of the electronic absorption spectra of 2‐ and 3‐phenylfurans and 2,2′‐ and 3,3′‐bifurans. The computed values for the transition energies and band intensities differ for the different isomers and help one to assign the preferred conformer of the studied molecules. Computations were performed on planar as well as nonplanar conformers. The calculated parameters as well as the interaction energy suggest all‐planar configuration for the studied molecules. A model potential for the sigma framework for the ortho‐hydrogens is considered. The calculated steric repulsion energy is negligible compared to the π delocalization. The height of the rotational barrier was larger for 2‐phenylfuran than for 3‐phenylfuran, and larger for 2,2′‐bifuran than for 3,3′‐bifuran. This result is in accord with predictions based on the extent of “π” conjugation in these molecules.
Improved lower bound on the indirect Coulomb energy
Authors: Lieb Elliott H. , Oxford Stephen 
Published: December 2004

AbstractFor a Coulomb system of particles of charge e, it has previously been shown that the indirect part of the repulsive Coulomb energy (exchange plus correlation energy) has a lower bound of the form –Ce2/3∫ρ(x)4/3 dx, where ρ is the single particle charge density. Here we lower the constant C from the 8.52 previously given to 1.68. We also show that the best possible C is greater than 1.23.
Addendum to “variational principle for obtaining approximate analytical solutions of the temperature‐perturbed Thomas–Fermi equation for compressed atoms”
Authors: Csavinszky P. , Tarr C. E. 
Published: December 2004

AbstractIn the present work the total energy of a Ne atom at T = 0 K is calculated as a function of a spherical container radius. The calculation is based on the Thomas–Fermi (TF) equation, which is solved approximately by an equivalent variational principle. The effect of an approximate exchange correction on the variational TF energy values is investigated.
Internal dynamics of nonrigid molecules. I. Application to acetone
Authors: Smeyers Yves G. , Bellido M. Nieves 
Published: December 2004

AbstractThe group theory for nonrigid molecules is used for studying the internal dynamics of the two equivalent C3v rotor “bent” molecules. Special emphasis is given to the deduction of the symmetry basis vectors which represent in box form the Hamiltonian operator. It is shown that these basis vectors may be advantageously employed in order to simplify the resolution of the two‐rotor equation. The procedure is applied to the acetone molecule. It is found that the lowest solutions are clustered into groups of four. The four lowest levels are related to vibrational states, the upper 64 to vibro–rotational states, in which the rotors are rotating in a restricted manner. Only few states show some cogwheel effect. Internal rotation contributions to the principal thermodynamic parameters of acetone are also computed.
Comments on the status of the chemical bond in methods of quantum chemistry
Authors: Berthier G. 
Published: December 2004

AbstractChemists do not take the view that chemistry can be derived from quantum mechanics by computations too literally; the difficulties are illustrated by the double bond concept. Quantum mechanics is very useful, but recent calculations have been mainly concerned with physical properties rather than chemical processes. In general, the chemical formula plays a central role in all interpretation of quantum‐chemical results, thus suggesting that the chemical bond is a preexistent notion. Moreover, the Born–Oppenheimer approximation seems to be necessary for deriving the existence of bonds between atoms previously assigned to suitable positions. Recent analyses by Claverie and Diner, Woolley, Primas, and others are briefly recalled, concluding with an expression of hope that use of more general Hamiltonians can lead to progress in obtaining a fully independent quantum theory of chemistry.
Physical approach to chemisorption through Green's function method
Authors: Iadonisi G. , Ramaglia V. Marigliano , Preziosi B. 
Published: December 2004

AbstractFor the purpose of illustrating methods for treating the chemisorption problem, which take into account bulk properties of the crystal, it is shown how the single‐particle Green's function may be used to solve the self‐consistent Schrödinger equations for an adatom–substrate system. The absorption problem is then reduced to the solution of a set of integral equations in terms of the infinite crystal Green's function. The method is illustrated using the interaction between an impurity level and surface states in a semiconductor gap and using sp hybridization of an alkali atom on a metal.
Conformation of nucleoside antibiotics
Authors: Saran Anil 
Published: December 2004

AbstractMinor modifications or substitutions in the sugar or in the base part of pyrimidine and purine nucleosides have a profound effect on their biological activity. These modified nucleosides usually become antiviral, antibacterial, or cancerostatic agents and they are collectively called nucleoside antibiotics. The conformational properties of some of these nucleoside antibiotics have been studied by the PCILO method. The results obtained from such study indicate that the conformational preferences of these nucleoside antibiotics are very similar to those of their parent nucleosides and especially so in the situations that occur in aqueous solutions. The important biological significance of these results is that these nucleoside antibiotics can easily get incorporated into growing chains of DNA and RNA by mimicking their parent nucleosides and can interfere with the protein synthesis of RNA or DNA synthesis.
Superoxide and lens of the eye: A new theory of cataractogenesis
Authors: Varma Shambhu D. 
Published: December 2004

AbstractBased on in vitro culture studies, it has been proposed that the photochemical generation of superoxide in the intraocular chambers provides a significant cataractous insult. Rat lens when maintained in medium generating excessive O is damaged as indexed by deterioration of the cation pump. If O levels are low (ambient levels), peroxidative degradation of lipids has been observed. These reactions are intuitively deleterious to tissue physiology and may contribute to the manner through which light may be toxic to lens in the long run, giving rise to senile cataracts. It was interesting to note that these deleterious reactions could be attenuated greatly by physiological levels of ascorbic acid. The significance of these in vitro studies to the in vitro situatiion as offered by the diurnal habitat of mankind remains yet to be known, but studies simulating such situation in experimental animal models are in progress.
Application of the laser microprobe mass analyzer (LAMMA) to qualitative and quantitative single cell analysis
Authors: Seydel U. , Lindner B. 
Published: December 2004

AbstractThe laser microprobe mass analyzer (LAMMA) is a new type of analytical instrument. It is a combination of a laser microscope with a time‐of‐flight (TOF) mass spectrometer. By focusing an UV pulse laser beam onto the specimen positive and negative atomic and molecular ions are formed which are detected by the TOF mass analyzer. The detection limits reach 10–20 g for some elements in an excited sample volume of 10–13 cm3. With the aim of finding out whether the LAMMA instrument is capable of quantitative measurements on very small biological samples the 23Na+ and 39K+ contents of single mycobacterial cells (M. tuberculosis H 37 Ra) were measured and the results were compared with those obtained from “integral” methods (AAS, NAA, tracer). Furthermore, the influence of different temperatures applied during the washing procedure of the bacteria on the distribution of the Na+/K+ ratio within the cell population was investigated. First attempts to trace isonicotinic acid hydrazide (INH) in bacteria were made by comparison of the LAMMA spectra of INH treated and untreated cells.
Bender–Wu formulas and classical trajectories: Higher dimensions and degeneracies
Authors: Avron J. E. 
Published: December 2004

AbstractA method for the asymptotics of the perturbation series (Bender‐Wu formulas) which is not restricted to one dimension is outlined. The method has a classical mechanic flavor and involves computing classical trajectories using classical perturbation methods. The method applies also when the unperturbed eigenvalues are degenerate. It is illustrated for the Hydrogen Zeeman Hamiltonian which is not separable and has degeneracies.
Representation theory of so(4,2) for the perturbation treatment of hydrogenic‐type hamiltonians by algebraic methods
Authors: Adams B. G. , Čížek J. , Paldus J. 
Published: December 2004

AbstractThe representations of so(4,2) which are applicable to the perturbation treatment of one‐electron Hamiltonians of the form H = H0 + λV are discussed, where H0 is a hydrogenic Hamiltonian. A unified construction of the representations of so(2,1) and so(3) is outlined and the representations of so(4) [and also so(3,1)] are then obtained using both the vector operator method and angular momentum recoupling techniques. The merging of so(2,1) and so(4) then leads in a natural way to so(4,2). An outline of perturbation theory applications such as the Stark and Zeeman effects is also given.
Double wells
Authors: Damburg R. , Propin R. 
Published: December 2004

AbstractAsymptotic analysis of the energy levels of double‐well potentials is discussed. Based on a previously developed method, the asymptotic behavior of the perturbation theory coefficients an for the 1/R expansion of the ground state of the hydrogen molecular ion H+2 is presented in analytic form. Nine terms in the expansion of an in powers of 1/n are obtained, and the first four agree with the four obtained by numerical fitting by Čížek et al. The one‐dimensional oscillator double well and more complicated multiple wells are also discussed.
Small‐Angle electron scattering and electron density in carbon dioxide
Authors: Sasaki Yorihiko , Konaka Shigehiro , Iijima Takao , Kimura Masao 
Published: December 2004

AbstractThe total (elastic and inelastic) intensity of electrons scattered by CO2 was measured in the s range of 1 to 12 Å−1 and compared with the theoretical intensity calculated from the Hartree‐Fock molecular wave function and those calculated for the independent‐atom‐model (IAM) molecule. In the range of s ≲ 4 Å−1 the electron correlation effect on the total scattered intensity was found to be represented by that for the IAM molecule.
Symmetry properties of chemical graphs. IV. Rearrangement of tetragonal‐pyramidal complexes
Authors: Randić Milan , Katović Vladimir 
Published: December 2004

AbstractUsing a canonical numbering of vertices for the graph corresponding to a particular rearrangement of tetragonal–pyramidal complexes, all 120 permutations defining the symmetry for the rearrangement are derived. An examination of the permutations points to the symmetric groupS5, which has previously been found for isomerizations of trigonal–bipyramidal complexes and in the rearrangement of homotetrahedryl cations.
Novel graph‐theoretical approach to estimating the relative importance of individual Kekulé valence structures. I. Simple catacondensed systems
Authors: El‐Basil Sherif 
Published: December 2004

AbstractA novel graph‐theoretical approach for ordering Kekulé valence structures of benzenoid hydrocarbons is presented. The approach involves the transformation of the Kekulé structures into the subspaces of their individual double bonds. The submolecules generated in this way [H. Joela, Theor. Chim. Acta 39, 241 (1975)] are ordered according to suitable connectivity indices. The resulting orders parallel those predicted from the so called Kekulé indices [A. Graocvac, I. Gutman, M. Randić, and N. Trinajstć, J. Am. Chem. Soc. 95, 6267 (1973)]. A relation is thus illustrated between VB and MO theories. The method is new and allows the prediction of the relative stabilities of structures from purely combinatorial vent without resort to computer.
PCILO study of the conformation of 20‐oxopregnane hormones
Authors: Viana Luci Martins , Takahata Yuji 
Published: December 2004

AbstractPCILO calculations on conformations of the C‐17 side chains of three 20‐oxopregnane hormones, progesterone, corticosterone, and cortisol, reveal that the calculated conformations are generally in close agreement with those observed crystallographically. Average deviation of the calculated rotation angle around the C(17)—C(20) bond from the experiment is 9°. The PCILO calculations also predict the intramolecular hydrogen bond between the 20‐carbonyl group and the 21‐hydroxyl for the three hormones. This is in accordance with experiment. It was concluded that the PCILO method is reliable in predicting conformations of the side chains of hormones whereas the extended Hückel theory (EHT) predicted wrong conformations. Based upon the PCILO calculations, biological activities of the hormones are discussed.
Invariance properties of the multipole expansion
Authors: Piecuch Piotr 
Published: December 2004

AbstractThe calculations of long‐range interaction energy are often based on multipole expansion. The truncated multipole expansion and interaction energy calculated with it are noninvariant with respect to an arbitrary choice of local coordinate systems. In this paper we show that truncated multipole expansion of form Σ CkR−k is “numerically” independent on a choice of local coordinate systems, if convergence conditions are satisfied.
Longuet‐Higgins groups of XeF6
Authors: Brocas J. , Rusu C. 
Published: December 2004

AbstractA complete permutational analysis of the XeF6 dynamics is presented. The Longuet‐Higgins group for each mode of rearrangement is obtained. On the basis of the splitting scheme of rigid molecule levels and statistical weights and selection rules for the non rigid molecule levels, it appears that transverse digonal twists and turnstile mechanisms have the same spectroscopic effects as the BPR‐6 mechanisms. Hence, the previous interpretation of IR and Raman spectra is consistent with tunneling through transverse digonal twists and turnstile mechanisms as well as through BPR‐6.
Ab initio molecular calculations with pseudopotentials: Higher quality calculations on Li2, LiH, and BeH2
Authors: Gáspár R. , Gáspár R. 
Published: December 2004

AbstractNew psudopotentials of higher quality have been used in ab initio molecular calculations. Simple molecular systems like Li2, LiH, and BeH2 have been investigated with varying basis sets. Energy, geometrical parameters, and dipole moments have been determined. The convergence properties of the various quantities have been investigated. They show characteristic behavior, e.g., stationary value for the energy where such behavior may be expected.
Treatment of hydrogen bonding within CNDO/2 and MINDO/3: CNDO/2H and MINDO/3H
Authors: Mohammad S. Noor , Hopfinger A. J. 
Published: December 2004

AbstractA formalism has been developed to treat hydrogen‐bonded A—H…︁B systems within the CNDO/2 and the MINDO/3 methodologies. In this formalism the interactions are divided into three distinct classes; those between (a) two hydrogen‐bonded atoms, (b) one hydrogen‐bonded and non‐hydrogen‐bonded atom, and (c) two non‐hydrogen‐bonded atoms. The last class of interactions is treated solely by the existing CNDO/2 or MINDO/3 method. For A –H…︁B systems, the core resonance integrals are individually parametrized depending upon the class of the interaction. Three types of A—H…︁B systems have been thus far parametrized. Nine hydrogen‐bonded dimers have been studied using the new formalism and the current CNDO/2 and the MINDO/3 methods. MINDO/3 predicts very large interatomic (A –B) distances for the equilibrium geometry, and relatively small stabilization values for the hydrogen‐bond energies. CNDO/2 predicts the reverse. The new formalism for both CNDO/2 and MINDO/3 predicts accurate geometries as well as energies for all nine dimers. The new formalisms are called CNDO/2H and MINDO/3H. A general discussion of the nature of hydrogen bonding as exhibited by CNDO/2H and MINDO/3H is presented.