bluuebunny/crossref_metadata_2025_split huggingface dataset:
New iterative scheme for a simultaneous calculation of m first eigenstates of a real symmetric matrix
Authors: Gołȩiewski A. 
Published: December 2004

AbstractA new iterative scheme for a simultaneous calculation of the m lowest eigenvalues together with their eigenvectors has been derived for a real symmetric matrix. The scheme is based on the orthogonal gradient method and is easy to use for large‐scale configuration‐interaction calculations of electronic wave functions. A variant of the scheme deals with nonorthogonal basis functions, which are particularly simple in the case of the bonded‐function method of Boys.
Space partitioning in multiple scattering techniques. I. Hydrogen molecular ion and hydrogen molecule
Authors: Garritz Andoni , Gázquez José Luis , Castro Miguel , Keller Jaime 
Published: December 2004

AbstractThe use of multiple scattering techniques combined with a statistical exchange potential for the description of the electronic structure of atoms, molecules, and solids depends strongly on the geometry (muffin tin, overlapping spheres, or cellular potentials) and on the form of the electron gas exchange. In this work we compare only the effects of using different geometries. For that purpose we have done calculations on the hydrogen molecular ion and Hartree‐type calculations on the hydrogen molecule so that no exchange effects are involved. To avoid arbitrariness in the choice of the sphere sizes we propose a nonempirical criterion that consists of using the set of radii that will minimize the charge in the interstitial region of the molecule or cluster. Some arguments are given to justify this criterion, and to clarify the differences between cellular, overlapping spheres, and muffin‐tin geometries. It is found that the cellular geometry gives a very good description around the equilibrium internuclear distance. However, for most systems of interest, exchange will be present. Thus, we have done, for comparison, the calculation on H2 using Xαβ statistical exchange. It is shown through this calculation that some of the correlation energy may be obtained by redefining the molecular orbitals in terms of non‐paired‐spins spatial orbitals, this formulation being required to obtain the correct free‐atom limit.
Difficulties in the computation of physical observables due to a truncated CI
Authors: Meunier A. , Levy B. 
Published: December 2004

AbstractIt is proved that the values of various types of physical observables issued from a truncated CI are not size consistent. These observables include electronic transitions, ionization potential, and all the quantities depending on the one‐electron density matrix γ. Numerical examples are given. It is found that the error due to this size inconsistency can be important for systems of moderate size (e.g., 16% on a matrix element of γ for a 20‐electron system).
Thermodynamic functions for vibronic systems
Authors: Crotov S. S. , Ischenko A. A. , Ivashkevich L. S. 
Published: December 2004

AbstractA method is introduced which simplifies the derivation of equations for the calculation of statistical sums for vibronic systems. Within the “independent ordering approximation” analytical expressions are found for thermodynamic functions for molecules with degenerate electronic ground state (E−β and E − e types of degeneracies). Vibronic interactions are shown to yield an additional thermodynamic stabilization. The computations were carried out for VCl4 and VBr4 with doubly degenerate electronic ground states. For these molecules the effect is shown to be small.
Valence‐bond studies of four‐electron three‐center bonding units. II. FNO, HNO, LiNO, LiON, and HF2−
Authors: Harcourt Richard D. , Roso Walter 
Published: December 2004

AbstractFor each of HNO, FNO, LiNO, LiON and HF2−, ab initio valence‐bond wave functions are reported for the four‐electron three‐center bonding unit which involves one of the following sets of atomic orbitals: (a) NX σ‐bond (X = H, For Li) and oxygen 2pπ′; (b) LiO σ‐bond and nitrogen 2pπ′; (c) hydrogen 1s and fluorine 2pσ. Six S = 0 valence‐bond structures pertain for this type of bonding unit. For HF2− and FNO, the calculated bond‐eigenfunction coefficients for some of these valence‐bond structures differ appreciably according to whether the Slater determinants include only the electrons of the four‐electron three‐center bonding unit, or all of the valence‐shell ± inner‐shell electrons. For HNO, LiNO, and LiON, the all‐electron and all valence‐shell electron calculations generate similar sets of bond‐eigenfunction coefficients. For each of the nitrosyl systems, the bond‐eigenfunction coefficient with largest magnitude is calculated to be that for which zero atomic formal charges occur in the valence‐bond structure. Qualitative valence‐bond and “increased‐valence” descriptions of the bonding are presented.
Tautomerism of oxopyridines and oxopyrimidines: Theoretical study with complete optimization of geometry
Authors: Czermiński R. , Lesyng B. , Pohorille A. 
Published: December 2004

AbstractThe semiempirical MINDO/3 method is employed to calculate the energies of various tautomers of model tautomeric compounds— 2‐oxo‐ and 4‐oxopyridines and pyrimidines. The results are compared with experimental data in the gas phase, where the solute–solvent interactions not included in theoretical calculations are absent. Although the relative energies obtained by the MINDO/3 method appear to be superior to other semiempirical and ab initio calculations, the accuracy of the method was determined to be as low as 3–4 kcal/mol. It indicates that it is exceedingly difficult to account theoretically for intrinsic stabilities of the tautomers. The importance of various factors influencing calculated free‐energy differences is discussed. Particular attention is paid to the problem of geometry optimization.
Electronic absorption and emission spectra of nucleic acids and their components: Some questions of theory and experiment
Authors: Danilov V. I. , Pechenaya V. I. , Zheltovsky N. V. 
Published: December 2004

AbstractThe results of calculations of the transition energies and polarization in nucleotide bases performed by the CNDO/S CI method are compared with experimental data for long‐wavelength and vacuumabsorption bands. The calculations and the analysis of experimental data testify to the existence of n − π* transitions in the first absorption bands of the bases. The study of double‐stranded polynucleotides and DNA hypochromism based on the theoretical electronic characteristics of the bases and perturbation theory is performed. The role of stacking and complementary interactions within the hypochromic effect is cleared up. Two mechanisms for the shift of the fluorescence band maximum are investigated: two‐proton transfer along H bonds and excimer formation. The study of H‐bond potential curves shows the disadvantage of two‐proton phototautomerism in the nucleotide base pairs in contrast to model systems. The possibility of excimer state formation in stacked homo‐ and heterodimers of nucleotide bases is shown within the extended‐Hückel treatment. The nature of excimer minimum for the excited‐state potential curve is analyzed.
π‐Orbital electron–energy contributions to chemical reaction heats, I. Reactions involving only linear molecules containing up to three first–row atoms
Authors: George Philip , Trachtman Mendel , Bock Charles W. 
Published: December 2004

AbstractAb initio calculations of various expectation energies have been made for the reactant and product species in six reactions that involve only small linear molecules. The reactions include fission by hydrogen, addition of hydrogen, exchange of triply bonded atoms, fluorination, and oxygen atom transfer. The change in total electronic energy is not invariably the result of changes in inner shell energy and outer shell σ‐ and π‐electron energies simply augmenting each other, but in several cases there is a complex interplay of opposing effects. This approach gives a different insight into the energetic aspects of changes in bonding from that derived from the concept of shared electron pairs in σ and π bonds together with lone pairs in valence shells. Changes in π‐electron energy are shown to be important in a reaction in which neither reactant nor product molecules contain π bonds in the usual chemical sense. While in a reaction in which there is a complete change in the nature of the triple bonds, and hence the π bonding, the change in π‐electron energy makes a smaller contribution than either the change in inner shell or the outer shell σ‐electron energies.
Variational principle for obtaining approximate analytical solutions of the temperature‐perturbed Thomas–Fermi equation for compressed atoms
Authors: Csavinszky P. , Tarr C. E. , Vosman F. 
Published: December 2004

AbstractA temperature correction to the Thomas–Fermi (TF) model of a neutral compressed atom has been given by Marshak and Bethe. The aim of the present work is to point out that, by formulating a variational principle, one may obtain approximate analytical solutions of the temperature‐perturbed TF equation. As a test of the proposed theory, the total energy of a Ne atom, confined to a spherical container, has been calculated as a function of the container radius at T = 0°K. Comparison of the data obtained with energy values calculated by Ludeña, using a self‐consistent‐field Hartree–Fock approach, shows reasonable agreement. It is, therefore, concluded that the variational approach suggested in this paper is a sound one.
Investigation of tertiary structure of a series of DNA compact forms by the small‐angle X‐ray scattering method
Authors: Polivtsev O. F. , Osika V. D. , Tsvankin D. Ya. 
Published: December 2004

AbstractThis work describes the investigation of the space organization of DNA compact forms and DNA–protein complexes by the small‐angle x‐ray scattering method. A plane texture was disclosed in dehydrated DNP films. Compact DNA particles formed in the presence of poly(ethylene glycol) reveal ordered structures having a periodicity of 84 Å. Various morphological forms of crystals of CTA‐DNA were used to obtain small‐angle x‐ray patterns of the single‐crystal type.
Perturbation and SCF calculations for excited states
Authors: Chang Tse‐Chiang 
Published: December 2004

AbstractOne can treat the linear variation calculation by the perturbation expansion; on the other hand, one can also relate the perturbed wave function for the mth state to the mth eigenvector and justify the upper bound nature of the perturbation calculations for excited states. One can also associate a special type linear variation wave function to the SCF calculation. The upper bound nature of the excited‐state SCF calculation can then be accessed by the help of this special wave function.
Ab initio studies of 1,3‐sigmatropic rearrangements: Effect of basis set and electron correlation
Authors: Rodwell William R. , Bouma Willem J. , Radom Leo 
Published: December 2004

AbstractAb initio molecular‐orbital theory has been used to study the 1,3‐sigmatropic hydrogen rearrangements: propene → propene, formic acic → formic acid, and vinyl alcohol → acetaldehyde. Fully optimized structures of stable molecules and transition states have been determined using gradient procedures and the 4‐31G basis set. Improved energies have been obtained using a variety of techniques with basis sets up to the size of double‐ζ plus polarization (DZP) and electron correlation up to the CEPA/DZP level. Although both polarization functions and electron correlation lead to a lowering of the calculated barriers, the values remain substantial for all three rearrangements.
CNDO‐MO calculation of NnFm molecules
Authors: Chen Cheng 
Published: December 2004

AbstractThe s‐p separation model CNDO‐MO method has been introduced and developed for both the open‐ and closed‐shell systems since 1975. This method has two chief advantages over most of the other CNDO methods. (1) The ns and np bases of the same atom may be considered independent to infer different bonding behavior. (2) Pariser–Parr and Nishimoto–Mataga approximations are applied to the Coulombic repulsion integrals, which not only simplifies the calculation and saves computer time but also gives reliable computational results. A series of nitrogen fluoride molecules such as NF, NF2, NF3, cis‐N2F2, trans‐N2F2, and N2F2 have been selected for this MO calculation. In each case, both ionization potential and dipole moment were calculated. The results are closer to the observed values than those reported in other works.
Relativistically parametrized extended‐Hückel calculations. II. Orbital energies of group‐IV tetrahalides and tetramethyls
Authors: Lohr L. L. , Hotokka M. , Pyykkö P. 
Published: December 2004

AbstractRelativistically parametrized extended‐Hückel (REX) calculations are reported for MX4 (M = Ti, Zr, Hf, C, Si, Ge, Sn, Pb; X = Cl, Br, I) and for M(CH3)4 (M = Ti, C, Si, Ge, Sn, Pb). Satisfactory agreement is obtained with experiment and with earlier assignments both for the energy levels and for their relativistic splittings. This encourages the use of REX for assigning PES spectra. Double‐ζ radial functions (REX and EHT) are given for Cl, Br, and I.
QCLDB—Quantum chemistry literature data base—a trial
Authors: Osamura Y. , Yamabe S. , Hirota F. , Hosoya H. , Iwata S. , Kashiwagi H. , Morokuma K. , Togasi M. , Obara S. , Tanaka K. , Ohno K. 
Published: December 2004

AbstractA brief account of a quantum chemistry literature data base (QCLDB) is described, which contains the key information of about 2000 papers of ab initio calculations of atoms and molecules published in 1977–1979. The QCLDB is stored in a computer and can be sorted and rearranged into author and substance indices. Selection of the items for each paper in a computer‐readable data base is discussed.
Purine–water interactions in base stacking
Authors: Langlet J. , Giessner‐Prettre C. , Pullman B. , Claverie P. , Piazzola D. 
Published: December 2004

AbstractThe intermolecular energy of two conformations of stacked hydrated purine dimers is computed for a varying number of molecules of water around the stacks. The results obtained show that hydration can modify the relative stability of the dimers as calculated in vacuo. In addition, the optimized arrangements of the molecules of water around the dimers depict that purine molecules strongly interact with the solvent and that bridges made of water dimers or trimers are formed between the stacked bases.
Quasirelativistic MO calculations on platinum complexes (anticancer drugs) and their interaction with DNA
Authors: Boudreaux E. A. , Carsey T. P. 
Published: December 2004

AbstractA self‐consistent modified extended Hückel (SC‐MEH) MO method has been extended to study the electronic structure and bonding in the complex ions: PtC42−, Pt(NH3)42+, cis‐Pt(NH3)2Cl2, trans‐Pt(NH3)2Cl2, and Pt(CN)42−, in which the atomic basis functions were derived from the published relativistic Dirac‐Fock functions. Both quasirelativistic (in the frozen spin approximation) and nonrelativistic calculations were made to include the 5s 5p 5d 6s and 6p levels of Pt and the nsnp valence levels of the ligands. Similar calculations have been completed in the nonrelativistic mode for Pt(NH3)2 (OH2)L and Pt(NH3)2L2, where L is the monodentated bond at the N‐7 position of guanine in DNA and L2 are the bidentated bonds at the N‐7 and O‐6 guanine sites, respectively. Based on these initial results specific conclusions have been made regarding a proposed biochemical mechanism that has been advanced to explain the unique anticancer activity of cis‐Pt(NH3)2 Cl2 (DDP).
Mechanism of binding of iron to potential therapeutic chelating agents
Authors: Spingarn Neil E. , Sartorelli Alan C. 
Published: December 2004

AbstractTo aid in designing new therapeutic iron chelating agents, the mechanism of iron binding to prototypic heterocyclic carboxaldehyde thiosemicarbazones has been studied. Based on molecular orbital and spectroscopic studies, iron (II) is found to bind in a covalent manner, while iron(III) seems to interact ionically. However, with both iron(II) and iron(III), chelate formation is dependent on charge interaction between the metal and the coordinating atoms of the ligand.
β−‐Decay effect on the proton movement in the pyridine–pyrrole hydrogen‐bonded system
Authors: Ikuta Shigeru , Hashimoto Shuichi , Imamura Masashi 
Published: December 2004

AbstractThe potential‐energy curves for the movement of proton along the hydrogen‐bond direction were calculated for the parent and cations produced from the β− decay of pyridine‐pyrrole labeled by tritium (3H) or carbon‐14(14C). For all cations the double minimum potential is predicted to be formed, and the activation energy for the proton transfer from one well to the other is about 10.0 kcal/mol, which is much lower than that in the (NH2HNH2)+ ion. A positional effect of nuclear transformation is scarcely expected in 3H β− decay, whereas a slight positional effect may be expected in 14C β− decay.
Electronic states near dislocations in transition metals: An application of quantum chemistry in technology
Authors: De Hosson J. Th. M. 
Published: December 2004

AbstractThis paper outlines a model for calculating the localized states of a 〈 100 〈 edge dislocation in α‐Fe. The model used for the calculations is based on the multiple‐scattering model (SCF‐X α‐SW). The purpose of this research is twofold: (1) To determine changes in electronic structure of the lattice near the core region of defects in α‐Fe. (2) The variations of hydrostatic pressure about an edge dislocation produce a rearrangement of the conduction electrons. The question is what electrical interaction might be expected between a dislocation and a charged solute atom. The calculations show that the electrons tend to flow away from the compression side toward the dilated regions. The electrical contribution to the binding energy of a solute atom and a dislocation in α‐Fe is of the order of 0.01 Ry/electronic unit charge of the atom.
Lattice relaxation and electric‐field‐gradient calculations in impurity‐doped single ionic crystals
Authors: Satoh Mitsuo , Taki Toshihiko 
Published: December 2004

AbstractThe lattice‐relaxation parameters in several ionic crystals doped with monovalent impurity ions are calculated by energy minimization, taking into account the Coulomb, overlap‐repulsive, and three‐body potential energies. The displaced 256 ions around one substitutional impurity ion are taken into consideration and the results are utilized to calculate the electric‐field gradients at sites similar to the (1,0,0), (1,1,0), and (1,1,1) sites relative to the impurity at (0,0,0), by using two different models for three‐body potentials.
Rotational tunneling of methane on the surface of graphite
Authors: Nosé Shuichi , Ozaki Yoshiaki , Maki Kazuo , Kataoka Yosuke 
Published: December 2004

AbstractNeutron scattering spectra from methane molecules adsorbed on a surface of graphite were observed by Newbery et al. They found four inelastic peaks in the temperature range lower than 30°K and assigned them to transitions between rotational tunneling states of methane molecules. This system was investigated by introducing both the interaction between methane molecules and that between a methane molecule and carbon atoms of graphite in order to explain the observed spectrum. It is shown that the system can be analyzed in terms of an effective field with tetrahedral symmetry.
Uniform quality gaussian basis sets for molecular calculations, I. C1 hydrocarbons
Authors: Poirier R. A. , Daudel R. , Mezey P. G. , Csizmadia I. G. 
Published: December 2004

AbstractThe optimality of MO basis sets of Gaussian functions, when constructed from AO basis sets optimized for the neutral atom or for atom ions, is investigated. A formal charge parameter Q is defined and used to adjust the AO basis sets to the molecular environment, by virtue of a simple quadratic expression. Calculations on a series of C1 hydrocarbons (CH2, CH3, CH3+, CH3−, CH4) using 3G basis sets indicate considerable variations in the optimum Q value with the molecular species. The proposed method offers a simple alternative technique to a full molecular basis set optimization.
Uniform quality gaussian basis sets for molecular calculations. II. C2 hydrocarbons
Authors: Poirier R. A. , Daudel R. , Csizmadia I. G. 
Published: December 2004

AbstractThis investigation is a continuation of a study on the optimality of MO basis sets of Gaussian functions, when constructed from AO basis sets optimized for the neutral atom or for ions. A formal charge parameter Q is used to adjust AO basis sets to the molecular environment, by virtue of a simple quadratic equation. Calculations are performed on a series of seven C2 hydrocarbons (C2H2, C2H4, C2H6, C2H3+ (open), C2H3+ (bridged), C2H5+ (bridged), and C2H4− radical anion). A simple rule is formulated to give approximate values of the charge parameter Q.
One‐ and two‐particle fractional parentage for arbitrary point groups, configurations, and coupling schemes
Authors: Fieck Gerhard , Wirsich Joachim 
Published: December 2004

AbstractThe fractional parentage coefficients (CFPS) for arbitrary symmetry (including non‐simply reducible point groups), different coupling schemes, and several open shells are discussed with emphasis on the common features. The differences between the coupling schemes arise merely from a different interpretation of the relevant symmetry group. The formulation uses the particle‐number representation (so‐called second quantization), in which the CFPS appear as the reduced matrix elements of the creation or annihilation operators. This shows, that there is no principal difference in the fractional parentage scheme of one or several open shells. For the latter case the theory of adjective CFPS is worked out and applied to an example of octahedral symmetry.
Electrostatic interaction for arbitrary point groups, configurations, and coupling schemes
Authors: Fieck Gerhard , Wirsich Joachim 
Published: December 2004

AbstractThe electrostatic interaction for arbitrary symmetry (including non‐simply reducible point groups), different coupling schemes, and several open shells is discussed from a common viewpoint using the particle‐number representation (second quantization). The different coupling schemes simply arise from a different interpretation of the relevant symmetry group. Using the adjective CFPS of the preceding paper, the interaction within several open shells can be calculated. In the case of one‐center expansions like ligand field theory the many‐particle matrix elements are directly expressed by the radial Slater integrals using isoscalar factors. An example of the formalism is worked out. By the way the unitary transformations between strong and weak field coupling schemes are expressed recursively in the number of particles.
Hypevirial theorem for open dynamical assemblies in the density matrix representation
Authors: Schweitz Jan‐Åke 
Published: December 2004

AbstractThe concepts of “open assembly” is defined, and the hypervirial theorem is derived in three steps: (i) An equation of motion of the reduced density matrix of the first order is derived. (ii) This equation is utilized to derive a general equation of continuity for single‐particle properties. (iii) The hypervirial equation for open assemblies is obtained as a long‐time average of the continuity equation. The hypervirial theorem is shown to be equivalent to a simple postulate: no physical property can keep on accumulating for ever in a limited volume. Finally, the hypervirial theorem is applied to the problems of force, torque, and virial balance in open dynamical assemblies, and the results are compared with the corresponding classical results.