bluuebunny/crossref_metadata_2025_split huggingface dataset:
Ab initio calculations on the barrier to pseudorotation of model 2′‐deoxyfuranose and 2′‐deoxy‐2′‐fluorofuranose rings
Authors: Lesyng Bogdan , Marck Christian , Guschlbauer Wilhelm 
Published: December 2004

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

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

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

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

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

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

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

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

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

Abstract2,4‐pentadienal and 2,4,6,8‐nonatetraenal were studied as simple model systems of retinal. Four kinds of CI were performed on low‐lying excited states of 2,4‐pentadienal by using a split valence basis set. The results show that MR SD π CI is not adequate for the π–π* state and the single excitation σπ CI is a good compromise between cost and effectiveness as far as singly excited states are concerned. This CI was applied to the bigger model system. All‐trans and 11‐cis forms of aldehyde, Schiff base, and protonated Schiff base of the model system were studied. A fairly large energy lowering of about 1 eV of the first allowed excited state (π → π*) occurs when the Schiff base is protonated for both all‐trans and 11‐cis forms.
Inclusion of relativistic effects into ZDO methods. V. Effect of core and polarization functions in bonding of first‐group elements
Authors: Boča Roman 
Published: December 2004

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

AbstractIn this article, we discuss how to incorporate correlation aspects in the interaction of the electron with a “frozen” correlated target. We discuss the important aspects of correlation for e− + N2 scattering using configuration interaction, coupled cluster, and perturbative methods to describe the correlation in the target molecule. © 1995 John Wiley & Sons, Inc.