bluuebunny/crossref_metadata_2025_split huggingface dataset:
Fifth‐order constant denominator perturbation theory within a localized bond model: Contributions from triple and quadruple excitations
Authors: Cullen J. M. 
Published: January 2005

AbstractThe contributions of the triple and quadruple excitations to the fifth‐order perturbation energy for the perturbation configuration interaction using localized orbitals (PCILO) method are derived. This completes the development of a fifth‐order constant denominator perturbation theory initiated in a previous paper [5] with the single and double excitations. This theory is tested on molecules containing strained ring geometries, stretched bonds, strongly polarized bonds, and delocalized pi systems: cases where the starting zero order reference wave function poorly describes the system. Although the perturbation expansions turn out to be slowly convergent, the Padé approximant taken from an energy series which itself is constructed from Padé approximants provides results accurate to within a few kilocalories/mole of benchmark calculations. Computational times as in the original PCILO procedure remain proportional to N3, where N is the number of bonds.
Explicit formulas for the Hamiltonian matrix elements
Authors: Lim Tiong‐Koon 
Published: January 2005

AbstractThe explicit formulas for the evaluation of the Hamiltonian matrix elements are presented. The calculation of the integral coefficients is independent of both the nature of the orbitals and th spin coupling schemes. It is fully automatic and only dependent on the number of doubly and singly occupied orbitals. Further‐more, the symmetric group representation matrices are not needed, and the N! problem can be avoided.
Recent progress in the understanding of light particle tunneling in metals
Authors: Karlsson Erik B. 
Published: January 2005

AbstractLight positive particles can be introduced in metallic lattices where they take up interstitial positions. The motion of particles between these sites is usually described as a phonon‐assisted tunneling process, which at high temperatures approaches a classical over‐barrier jump motion.The present paper reviews briefly the study of such phenomena at very low temperatures where phonon assistance is no longer effective. Experiments on proton transfer between certain sites in niobium metal and on positive muons which diffuse in copper or aluminium lattices show unusual temperature dependencies, which have been explained quantitatively with recent tunneling theories. These approaches take into account the simultaneous dissipation of energy to the electron bath, a phenomenon which was shown by J. Kondo to be determined by the screening electrons following the particle. The temperature dependence is essentially an effect of the Fermi distribution of the conduction electrons. The interaction of the particle with the itinerant electrons will also determine whether the particle wavefunction will be localized or form an extended (Bloch‐like) state.
Microscopic model for heavy‐ion nonrelativistic collisions
Authors: Köhler H. S. 
Published: January 2005

AbstractA microscopic model for nuclear dynamics has been formulated and applied to the collisions between heavy ions. It may be viewed as an extension of the collisionless time‐dependent Hartree–Fock (TDHF) method to include nucleon‐nucleon collisions. The collision term is written in the form of a time relaxation. The relaxation time is obtained from separate calculations. Some results are shown for the collisions between two 16O nuclei. The computations require a storage of several million words and require a super‐computer. Calculations have been done on the John von Neumann Center (JVNC) CYBER‐205. Future calculations are planned on the JVNC ETA‐10.
Self‐formation and evolution of singletons
Authors: Wilhelmsson H. 
Published: January 2005

AbstractNew solutions for reaction‐diffusion equations, called singletons and polytons, are found to be of fundamental significance. They exhibit properties of self‐formation. In the presence of nonlinear saturation effects their evolutions in time have pronounced maxima. Information about details of an initial spatial distribution is found to be carried along in time with the dynamic process of forming a growing singleton and to be restored at a later stage.
Spin adaptation of antisymmetrized geminal product wave functions
Authors: Sarma C. R. , Paldus J. , Öhrn Y. 
Published: January 2005

AbstractA spin‐free polynomial representation of antisymmetrized geminal products is presented for several cases. In particular, products of identical geminals, which possess different spin multiplicity, are considered. The cases of singlet geminals, singlet geminals with one or two triplet geminals coupled to the lowest possible spin multiplet, and triplet geminals coupled to an arbitrary multiplet are considered in detail, and explicit polynomial representation is given.
On the Ricatti equation for eigenvalue problems
Authors: Fernández Francisco M. , Castro Eduardo A. , Ogilvie John F. 
Published: January 2005

AbstractThe Riccati equation is shown to be suitable for obtaining implicit approximate analytic expressions for the eigenvalues of quantum‐mechanical systems. The Hamiltonian operator H = (1/2)p2 – (Z/r) + λr2 is used as a test example, and the resulting formulae are modified to deal with the Zeeman effect in hydrogen.
Improved intermolecular SCF theory and the BSSE problem
Authors: Mayer I. , Surján P. R. 
Published: January 2005

AbstractAnalytical and numerical studies are performed concerning the exclusion of the basis set superposition error (BSSE) from the SCF calculations of intermolecular interactions. Based on these studies a new procedure is proposed, which consists of the following steps: (1) determine the orbitals by the SCF scheme based on the recent “chemical Hamiltonian approach” (CHA‐SCF method), i.e., excluding the delocalization effects caused by BSSE, and then (2) calculate the usual energy expectation value. (This gives results superior to those obtained by the previous nonsymmetric CHA energy formula.) The actual numerical calculations performed for different simple systems (He2, water dimer) by using various basis sets indicate that the CHA/CE (CHA with “conventional energy” formula) potential curves are well‐balanced and are close to those obtained by the Boys–Bernardi (BB) method and usually (but not necessarily) go slightly beyond the latter. So our method gives results better than (or close to) those given by the BB method by performing only a single ∼N4 calculation at each geometrical arrangement of the system.
The stochastic theory and its relations to the general reaction rate theory
Authors: Christov S. G. 
Published: January 2005

AbstractA short review of the recent development of the stochastic approach to chemical reactions in condensed media is made. The relations of this approach to a general formulation of chemical kinetics are discussed on the basis of a many‐frequency oscillator model. It is shown that the classical transition‐state theory is not a particular case of the stochastic theory neither in the case of small viscosity nor in the case of large viscosity. The recent quantum generalizations of the stochastic theory are identical to earlier results of a quantum transition‐state theory that correspond to the case of large viscosity. Such generalizations based on previous results of a collision theory are made for the case of small viscosity. It is shown that these quantum generalizations of the stochastic theory are possible only in the temperature range of moderate tunneling defined in terms of a characteristic temperature.
Self‐interaction‐corrected electronegativities and hardness for atoms
Authors: Goycoolea Candelaria , Barrera Mauricio , Zuloaga Fernando 
Published: January 2005

AbstractElectronegativity χ and hardness η for 54 atoms and their positive and negative ions are calculated by means of self‐interaction‐corrected DFT including correlation terms. The exchange potential energy is treated by local spin density approximation corrected to account for self‐interaction effects as suggested by Rae. The highest occupied orbital eigenvalues for ions are identified to the chemical potential μ± for positive and negative charged atoms depending upon the developing charge process. Values of χ±δ and η± for the different ionic species are given for several values of δ. Average values for 〈χ〉 and 〈η〉 in the sense of Mulliken finite formula for neutral atoms are also tabulated and compared with Mulliken values from experimental data. The agreement among them is almost quantitative.
Are oxygen rings (On) and their negative ions (O) unstable?
Authors: Sinanoğlu Oktay 
Published: January 2005

AbstractOxygen clusters of the equilateral ring types On and their negative ions O are studied by the recently developed pictorial‐topological quantum chemistry method, VIF (valency interaction formulas), of this author. The species are found to be of high energy relative to separated oxygen atoms, the cause being evident from the VIF pictures. The odd n rings should be at local minima but are not likely to have observable negative ions. The even n rings have distortional instabilities, yet their negative ions are more likely to be observed as transients in beam experiments.
Application of box quantization to the determination of the dissociation rates of a system of two coupled morse oscillators
Authors: Lefebvre R. , Atabek O. , Garcia‐Sucre M. 
Published: January 2005

AbstractA well‐known model of two coupled Morse oscillators is studied within the context of box quantization. Two variants are considered: analysis of stabilization graphs or addition of an optical potential. These methods are shown to give efficient alternatives for the determination of unimolecular dissociation rates.
Symmetric and symmetry‐broken Hartree–Fock solutions of one‐dimensional models
Authors: Hu Chengzheng , Monkhorst Hendrik J. 
Published: January 2005

AbstractPariser–Parr–Pople Hartree–Fock crystal orbital calculations (both restricted and unrestricted versions) are performed for one‐dimensional models of π‐electron polymers. The π‐electron band structures corresponding to symmetric and nonsymmetric solutions have been calculated. To investigate the influence of the form of the interelectronic interaction, the calculations were performed with the Mataga–Nishimoto (MN) formula and a modified Mataga–Nishimoto (MMN) formula for two‐electron integrals. We have found that curves of the points of the minimum total energy per unit cell for the alternating models are very similar, but band structures and energy gaps are quite different when the MN formula was substituted with the MMN formula for two‐electron integrals.
The origin of energy functional in Roothaan open shell SCF theory
Authors: Klimko G. T. , Mestechkin M. M. , Plakhutin B. N. , Zhidomirov G. M. , Evarestov R. A. 
Published: January 2005

AbstractDifferent methods of averaging of energy over the states of electronic configurations γN (nγ = 1, 2, 3 and N = 1, 2, …, 2nγ − 1) leading to Roothaan' energy expression are considered. The consequent values of vector coupling coefficients (VCC) in energy functionals for various states as well as for average values of energy are presented.It is shown also that in molecular systems of cubic and tetragonal symmetry having electronic configurations tN (N = 2–4) and e2 there exist states for which VCC are dependent on the choice of basis set of degenerate open‐shell molecular orbitals. The origin of such “non‐Roothaan” terms and peculiarities of its calculation by the restricted Hartree–Fock method are discussed.
Calculation of integrals with slater basis from the one‐range expansion of the 0s function
Authors: Rico J. Fernández , López R. , Ramírez G. 
Published: January 2005

AbstractA one‐range expansion of the 0s‐STO—previously reported by us—is employed to derive explicit expressions for the elements of auxiliary matrices—appearing in the calculation of one‐ and two‐electron multicenter integrals with STOS—in the form of infinite series. Then, we are able to perform the formal summation of the series giving both closed expressions for these elements and several relations between them. A computational algorithm based on these formal developments is proposed and its numerical behavior is analyzed.
Graphical and color‐pairing symmetries
Authors: Klein D. J. , Živković T. P. 
Published: January 2005

AbstractSymmetries are considered for a class of Hamiltonian models with one (spin‐free) orbital per site. The models include common types of Parisier‐Parr‐Pople and valence‐bond Hamiltonians, defined over a continuous range of parametrizations. The symmetries investigated are linear canonical transformations and include the common point‐group and alternancy symmetries. We find “graphical” symmetries extending the usual point‐group symmetries and novel “color‐pairing” symmetries which involve hybrids of point‐group–like and alternancy symmetries of relevance for certain heteroatomic species. The occurence of recolorpairing transformations relating the eigensolutions of models for different molecules is also noted.
On the difference in bond orders between HMO and PPP methods
Authors: Randić Milan , Plavšića Dejan , Trinajstić Nenad 
Published: January 2005

AbstractWe have examined variations in bond orders in conjugated hydrocarbons that follow improvement in MO calculations on going from topological Hückel MO to self‐consistent Pariser‐Parr‐Pople–type calculations. Differences in individual bond orders can be interpreted as pointing to increased weights of selected Kekulé valence structures. Significantly, so indicated valence structures are those with the largest number of formal Kekulé benzene rings or the largest contribution to molecular resonance energy in terms of conjugated circuits.
PPP (Peel) calculations on iron porphyrins and iron phthalocyanine. Charge distribution and charge transfer
Authors: Henriksson‐Enflo A. 
Published: January 2005

AbstractSemiempirical molecular orbital calculations within a modified PPP method, the Peel method, have been performed in order to study the electronic structure and properties of porphyrins and phthalocyanines. It is found that the calculations can reproduce the results from more sophisticated ab initio calculations. It is found that the electronic reactions may take place at the metal atom and in the axial direction. The role of the porphine plane seems to be that of an electron buffer, keeping the electronic population in the porphyrin plane constant. A mechanism is suggested where electron transfer reactions can take place by change in the molecular geometry, as for example by vibrations.
Quantum‐chemical investigation of Buckminsterfullerene and related carbon clusters (I): The electronic structure and UV spectra of Buckminsterfullerene, and other C60 cages
Authors: Feng Jikang , Li Jun , Wang Zhizhong , Zerner Michael C. 
Published: January 2005

AbstractThe electronic structure of C60 cages are examined by use of the INDO and INDO/CI methods. The calculated spectrum of Buckminsterfullerene has one allowed low‐lying band at 27,300 cm−1, to be compared with an experimental value of 25,900 cm−1. These calculations suggest that other cage structures are not stable, or they are not ground state singlets.
Alternative implementation of the unitary group approach to the atomic shell theory
Authors: Zhenyi Wen , Yubin Wang , Zhiyong Zhang 
Published: January 2005

AbstractSubduction coefficients adapted to the group chain, which appeared in Racah's treatment of fn configurations, are defined and calculated in the unitary group approach. The coefficients are then utilized to construct successively adapted term functions and evaluate other interesting coefficients. In addition the simplified expressions for the Coulomb and spin‐orbit operators are obtained in terms of generators.
The use of localized molecular orbitals and the polarization propagator to identify transmission mechanisms in nuclear spin‐spin couplings
Authors: Diz A. C. , De Azua M. C. Ruiz , Giribet C. G. , Contreras R. H. 
Published: January 2005

AbstractAn extension of the IPPP (inner projections of the polarization propagator) method to theoretically analyze transmission mechanisms of indirect nuclear spin‐spin couplings is presented. The localization technique used is modified so that all the canonical molecular orbitals of a compound may be localized to represent chemical bonds, lone pairs, and the corresponding antibonding molecular orbitals. These localized molecular orbitals, together with the polarization propagator, are used to obtain an intuitive picture of how a coupling is generated as a sum of terms, each one consisting of two particle‐hole single excitations. This picture can be used to identify underlying transmission mechanisms and quantitatively evaluate their importance toward the total coupling. The paramagnetic spin‐orbit and the spin‐dipole interactions are studied in detail.
Computational models for proton transfer in biological systems
Authors: Pardo Leonardo , Mazurek Aleksander P. , Osman Roman 
Published: January 2005

AbstractA computational scheme based on a “mixed basis set” approach is applied to the study of the structure and the energetics in proton transfer systems. Five hydrogen‐bonded systems of the type (CH3HnA ‥ H ‥ BHmCH3)+, where A and B can be N, O, or S, have been investigated with various minimal and extended basis sets. Calculations with the extended basis set yield double‐well potential energy curves, which the minimal basis set is unable to reproduce. Calculations with the mixed basis set, constructed from an extended basis set on the atoms engaged in the hydrogen transfer part and a minimal basis set on the rest of the molecule, give predictions of geometries, potential energy curves, and relative energies similar to the results from the extended basis set. Inclusion of polarization functions in the mixed basis set becomes essential in systems that contain third row atoms. This scheme should become useful in studies of large molecules in which different parts can be represented at different levels of computational complexity.
Vibronic spectral behavior of molecules. XIII. Theoretical contribution to the vibronic coupling and the dushinsky effect on the S1 ← S0 absorption and the S1 → S0, S2 → S1, and S2 → S0 fluorescences of azulene
Authors: Gustav Klaus , Storch Michael 
Published: January 2005

AbstractBased on the completely optimized S0, S1, and S2 molecular geometries of azulene, the vibronic structure of the S1 ← S0 absorption as well as of the S1 → S0, S2 → S1, and S2 → S0 fluorescences is investigated theoretically within the adiabatic approximation. By means of theory‐experiment comparisons, the influence of non‐Condon terms and of the Dushinsky effect on the vibronic structure of azulene spectral behavior is discussed. Typical for the S1 ← S0 absorption and the S1 → S0 fluorescence are vibronic transition moment contributions of Condon type, whereas the interpretation of azulene S2 → S1 and S2 → S0 fluorescences is successful only within the scope of the Herzberg–Teller approach by taking into account vibronic coupling terms and, additionally, the Dushinsky effect in the latter case. An analysis of the relevant vibrational modes is given.
Computational chemistry on a PC
Authors: Náray‐Szabó Gábor , Ángyán János G. , Surján Péter R. , Szalóczy Zsolt , Ösapay Klára , Kövesdi István , Kolossváry István 
Published: January 2005

AbstractWe describe a package of some IBM PC programs that may find application in computer‐aided molecular design. PCGEOM constructs and visualizes molecular models from bond lengths, bond angles, and dihedral angles, from Cartesian coordinates, or from stored fragments. It may prepare output files to be used as input for other programs, like CNDOB (conventional CNDO/2) or PCMEP using the bond increment (BI) method for the calculation of molecular electrostatic potentials. PCPROT is in preparation and will use Protein Data Bank coordinates to visualize and manipulate protein molecular models. Starting from these, it will calculate electrostatic potentials using the BI method and/or monopoles adjusted to reproduce ab initio values for amino acid residues. FSCF is based on a CNDO‐type approximation and uses strictly localized molecular orbitals in order to partition large molecules into a central fragment, a polarizable region, and a fully transferable environment. The partition allows one to handle relatively large systems with up to 200 atoms. To illustrate applications, we present estimation of relative inhibitory potencies of a series of substituted triazines on chicken liver dihydrofolate reductase.
Localization of virtual orbitals
Authors: Bartha Ferenc , Bogár Ferenc , Kapuy Ede 
Published: January 2005

AbstractThe application of the MBPT in the localized representation requires that both the occupied and the virtual orbitals obtained by the canonical HF equation should be localized. The localization of the occupied orbitals is straightforward in general by any localization method. It is shown that by using Boys' method the localized virtual orbitals are spatially well separated and transferable not only in minimal basis sets.
Double bonds in the second and third periods. Ab initio study of the conjugation in HnX  YHmZH2 (X, Y = C, N, SI, and P; Z = B and N; n = 1, 2; m = 0, 1)
Authors: Korkin A. A. 
Published: January 2005

AbstractAb initio SCF calculations were performed to study the conjugation of C, N, Si, and P double bonds with BH2 (π‐acceptor) and NH2 (π‐donor). The variations of the energy, geometry, and electronic distribution on rotation ZH2 groups connected to the double bonds depend greatly on the polarities and polarizabilities of the molecules under study. The repulsive (attractive) interactions of the lone pairs lying in the plane of the double bond with donor (acceptor) orbital can modify strongly the relative stabilities of the conformations and the parameters of the molecule and electronic structures.
Nonstandard turn of MDGI protein binding site of fatty acids?
Authors: Suessmilch R. , Jaeger J. , Behlke J. 
Published: January 2005

AbstractThe growth‐regulating factor MDGI belongs to the fatty acid binding proteins of which the amino acid sequence is known. Starting from hydrodynamic and dichroic measurements which yield estimations about mass, diameter, thickness, and content of α and β structure of the MDGI protein, we used theoretical methods to calculate the structures and electrostatic properties of selected β‐chain sequences. The hydrophobic character of possible binding sites for fatty acids is demonstrated by the molecular electrostatic potential and field of this structures. A nonstandard turn element, formed by an H‐bridge between polar side chains, Lys‐Leu‐Gly‐Val‐Glu, which connects two extended β chains, was found. Bound oleic acid could be able to stabilize such turn structure, which seems to appear also at the entrance of the cavity of two other proteins. The optimization of an oleic acid‐turn van der Waals complex results in an interaction energy of about –50 kJ/mol originating mainly from the dispersion term and shows the participation of the four hydrophobic side chains Phe 64, Leu 66, Val 68, and Phe 70. Of these, at least Phe 64 and Phe 70 are known as possible binding sites of fatty acids.
1S core‐level shifts of Al and Ar atoms in aluminum clusters
Authors: Varga Zs. , Gyémánt I. K. 
Published: January 2005

AbstractTheoretical 1s core‐electron binding energies are presented for Al and Ar atoms in free space and in AlAl12, AlAl12Al6, and ArAl12 clusters. The binding energies have been calculated by the self‐consistent field Xα scattered‐wave (SCF Xα SW) method using various exchange parameters and different atomic‐sphere overlaps. The atom/cluster binding‐energy shifts have been obtained both from the Slater's transition‐state energies and from the total‐energy differences; these values are in better agreement with each other if calculated with proper overlapping than if with nonoverlapping spheres. A comparison with available experimental and theoretical data is given as well.