bluuebunny/crossref_metadata_2025_split huggingface dataset:
Long‐ and intermediate‐range interaction between hydrogen atoms in the B1Σ state
Authors: Kołos W. 
Published: December 2004

AbstractThe energy of the hydrogen molecule in the B 1Σ state, for internuclear separations 12 ≦ R ≦ 20 a.u., has been computed using an 80‐term variational wave function depending explicitly on the interelectronic distance. The same type of wave function has been employed in the perturbation theory approach. Using the polarization approximation, and not expanding the interaction Hamiltonian, the first‐, second‐ and third‐order energies have been computed and higher‐order corrections have been estimated. The results show that in the region under consideration E and E represent the dominant contributions to the interaction energy in the B state.
On the use of Gaussian shell type basis orbitals for single‐center expansions. I. Evaluation of integrals
Authors: Poshusta R. D. , Agrawal V. P. , Moseley W. D. 
Published: December 2004

AbstractFormulas are derived for all Hamiltonian integrals required for molecular computations using a novel basis for single‐center expansions. The basis orbitals depend exponentially upon α(r − ρ)2 where r and ρ are the distance from center to electron and to a variationally scaled spherical shell, respectively. Comparisons are made between these so‐called Gaussian shell orbitals (GSO) and the conventional GTO and STO bases for single‐center calculations. A preliminary comparison on H using a single GSO, a non‐integer STO, and a GTO gives the optimized energies: −0.51089 a.u., −0.50504 a.u., and −0.50422 a.u., respectively.
Nonorthogonal radial orbitals for the configuration lnl
Authors: Jucys A. P. , Sabas K. V. , Kupliauskis Z. J. 
Published: December 2004

AbstractThe theory of treating the configuration n0ln1lNn2l by means of nonorthogonal radial orbitals is described. The expressions for the matrix elements of the nonrelativistic Hamiltonian are deduced. A table that enables one to obtain these expressions in terms of radial integrals for l = 0 and 1 are given. Numerical calculations are performed for the configuration 1s22s22p3p using generalized hydrogen‐like radial orbitals. The numerical values of the inter‐term energy differences are given and discussed.
Young operators and the Waller–Hartree spinfree method
Authors: Lim T. K. 
Published: December 2004

AbstractThe basic equivalent transformation Pσ = (−1)p (Pr)−1 as developed recently [6] is used to transform between the Young operators for a two‐row standard tableau and its conjugate two‐column standard tableau. These Young operators are shown to be the Löwdin spin projectors in the spin function‐dependent methods and the spin projectors in the spin function‐free methods. The sandwich operator of Gallup and the last diagonal element of the matric basis of Matsen are shown to be the special cases of our more general matric projectors. Several different forms of the Löwdin spin projectors, spin projectors, and matric projectors are shown and their relations are discussed.
Improved convergence of self‐consistence procedures in the MC SCF theory
Authors: Kuprievich V. A. , Shramko O. V. 
Published: December 2004

AbstractTo obtain optimized orbitals within the MC SCF theory, the energy surface near a chosen point is approximated by a quadratic function of independent matrix elements of a small orthogonal orbital transformation. The method of a second‐order one‐electron Hamiltonian (OEH) is developed on the basis of this approximation. A procedure is proposed to define step coordinates, insuring a rapid descent along an average‐energy surface also in the cases when the matrix of second energy derivatives has eigenvalues negative or close to zero. The results obtained in applying the OEH method for the calculation of ground and triplet states of uracile in the π‐electron approximation are discussed. When a complete matrix of the second energy derivatives is used, the self‐consistence procedure is quadratically convergent. An exponential, yet rapid enough convergence is provided by a simplified computation scheme neglecting cross derivatives.
Extended Hückel theory of hydrogen‐molecule interactions
Authors: Barriel J. M. , Riera J. M. , Sanz F. , Caballol R. , Vidal M. , Carbó R. 
Published: December 2004

AbstractA computational technique within the Extended Hückel frame‐work for the calculation of the hydrogen‐molecule interaction energy is described. The procedure is ten times faster than the usual diagonalization procedure.The algorithm consists of transforming the generalized secular equation to give a bordered‐diagonal matrix secular equation, which is solved by means of an efficient bisection technique.As an example, two problems are discussed: (i) glycine intra‐molecular proton transfer process; (ii) interaction of the molecular fragments CH and OH.An atomic SCF Gaussian basis set was used instead of the usual Slater type basis set.
Adiabatic perturbation theory for the degenerate case. II. Perturbation of degenerate bound states embedded in the continuum
Authors: Dmitriev Yuri 
Published: December 2004

AbstractAn adiabatic formula for the contracted Hamiltonian in a reference space containing bound‐state eigenfunctions of degenerate energy levels embedded in the continuum is derived. A general factorization theorem for the dynamic operatorSα(0, – ∞/λ) is proved, and the cancellation of the pole singularities in the perturbation series of the contracted Hamiltonian in adiabatic form is discussed.
A method for the construction of orthogonal spin eigenfunctions
Authors: Murty J. S. , Sarma C. R. 
Published: December 2004

AbstractA method for the construction of the essentially idempotent and Hermitian diagonal elements of the matric algebra of the permutation group Sn is proposed. For the irreducible representation [λ] = [λ1, λ2] characterising a spin state S of an n‐electron system, it is found that this method generates the complete set of spin projections from the appropriate primitive spin functions. The method is applied to a 7‐electron system in the spin state S = MS = 1/2 and the results are listed in the Appendix.
Spin projections for even electron systems
Authors: Sarma C. R. 
Published: December 2004

AbstractA new method is presented for obtaining the spin eigenfunctions of 2n electron systems in the spin state S and MS = 0. Using a modified Young operator the function for the state S, MS ( = S) of the system is first projected out. The projected function is then symmetrized over the last n particles and the weight lowering operator Š is then applied to it, resulting in a projection of the state S, 0. To within a multiplicative factor, the resulting function is identical with the one resulting from the vector coupling methods.
Angular correlation of electrons in the ground state of helium
Authors: Tatum J. Patrick 
Published: December 2004

AbstractThe spatial angular correlation of electrons in the ground state of the helium atom has been examined using configuration interaction and Hylleraas wave‐functions. It was found that, in general, the average angle between the electrons is not a maximum when the two electrons are at the same distance from the nucleus. For configuration interaction wave‐functions there is a position of the electrons for which the average value of the angle between the electrons is a maximum. Hylleraas wave‐functions do not show this behavior.
The N2 problem in molecular CI calculations
Authors: Meunier A. , Levy B. , Berthier G. 
Published: December 2004

AbstractThe possible existence of spurious terms increasing as the square of the number of electrons is theoretically proved for the CI treatments of molecules limited to single and double excitations, and is revealed by some numerical calculations (alcanes and juxtaposed hydrogen molecules). An improved perturbation scheme is suggested in order to evaluate the CI energy lowering more carefully. Covergence difficulties are found for larger systems (benzene).
Quantum mechanical tunneling in synaptic and ephaptic transmission
Authors: Walker Evan Harris 
Published: December 2004

AbstractQuantum mechanical tunneling theory is applied to the problem of synaptic vesicle release and to the problem of electric transmission at the ephaptic junction. In the synapse the tunneling produces conformational changes in macromolecules to open and close vesicle macrogates. Quantum mechanical tunneling as a basis for charge transfer and physical release of vesicles at junction membranes provides a unified concept of ephaptic and synaptic transmission. Details of this model are in agreement with experimental data for miniature endplate potential frequency and delay effects as a function of polarization, osmotic pressure, and temperature. The theory accounts for anatomical specializations at the synaptic cleft and the narrow junction observed for the ephapse.
Calculation of molecular electric polarizabilities and dipole moments. II. The LiH molecule
Authors: Haley Lawrence V. , Hameka Hendrik F. 
Published: December 2004

AbstractWe use a variation–perturbation method to calculate the electric polarizabilities and the electric dipole moment of the LiH molecule. We obtain 4.455 for the perpendicular polarizability and 4.001 (×10−24 cm3) for the parallel polarizability. Our result for the electric dipole moment at equilibrium nuclear distance is 5.866, which is in excellent agreement with the experimental value 5.828 debye units.
The electronic structure and properties of pyrazine, its cation, and anion
Authors: Hinchliffe Alan 
Published: December 2004

AbstractGaussian orbital LCAO MO calculations are reported for pyrazine in its ground and first excited triplet state, together with calculations on its anion and cation. Energies, population analysis indices, and one‐electron properties are compared across the series in order to investigate the differences in electron distribution.
Perturbation theory within the variation method
Authors: Epstein Saul T. 
Published: December 2004

AbstractSome of Moccia's formulas (Int. J. Quant. Chem. 8, 293 (1974)) for corrections to “not completely optimized variational parameters” are compared with those resulting from a perturbation theory analysis. It is not possible to choose the zero‐order Hamiltonian (at least not in any simple way) so as to get exact agreement, but one choice does come close.
Electron momentum distributions from valence‐bond wave functions
Authors: Gardre S. R. , Narasimhan P. T. 
Published: December 2004

AbstractMomentum densities obtained from the Heitler‐London (HL) wave functions for diatomic molecules and those from the corresponding valence‐bond (VB) wave functions including ionic terms are compared. In each case they shown maxima in the direction perpendicular to the bond. However, the dependence of momentum densities on mutual orientations of the two electronic momenta is quite complex in the latter case. The improvement in the Compton profile on including the ionic terms is illustrated with the example of H2. The momentum denmsities obtained from the VB wave function constructed from orthogonalized atomic orbitals (OAO) have also been examined. The HL wave function with OAOS leads to the same momentum distribution as the repulsive state HL wave function constructed from overlapping AOS.
On invariance requirements in approximate SCF MO Theory
Authors: Nanda D. N. , Narasimhan P. T. 
Published: December 2004

AbstractThe invariance question int he CNDO and INDO levels of approximation is discussed with particular reference to one‐center and two‐center two‐electron integrals and rotation of molecule‐fixed coordinate axes. It is shown that asufficient condition for rotational invariance for the cone‐center two‐electron integrals is Jμμ = Jμμ, +2Kμμ, where J and K are the Coulomb and exchange integrals over orbitals μ μ′ with the same azimuthal quantum number. CNDO and INDO procedures, which explicitly employ Löwdin's orthogonalized basis set of atomic ortbitals (OAO) and differentiate between s‐, p‐, and d‐orbitals on an atom in calculating various integrals, have also been examined in relation to the rotational invariance requirement. An expression which satisfies rotational invariance for two‐electron Coulomb repulsion integrals over OAOS is also given.
Discrete and continuum contributions to multipole polarizabilities and shielding factors of hydrogen
Authors: Tanner A. C. , Thakkar Ajit J. 
Published: December 2004

AbstractDiscrete and continuum contributions to the multipole polarizabilities and shielding factors of atomic hydrogen are computed. The discrete series show logarithmic convergence which can be accelerated by the u‐transform. The continuum contributions increase with increasing multipole order and are already dominant in the quadrupole polarizability and shielding factor. The shielding factors have greater continuum contributions than the polarizabilities.
Projected BCS Tamm‐Dancoff method for molecular electronic structures
Authors: Takahashi Mitsuo , Fukutome Hideo 
Published: December 2004

AbstractA new Tamm–Dancoff method for the ground and excited states of molecular electronic systems is developed. The method begins with a number‐projected BCS (PBCS) wave function and is generated by excitations of particle pairs from the degenerate geminals in the PBCS wave function. A direct optimization of the PBCS wave function is accomplished with successive Bogoliubov transformations so that one‐pair excitation terms in the Tamm‐Dancoff expansion of the ground state vanish (the generalized Brillouin theorem). The spin‐symmetry adapted first‐ and second‐order Tamm–Dancoff bases and matrix elements are calculated by means of the CI expansion of the PBCS wave function with natural orbitals that diagonalize the BCS geminal matrix. Numerical calculations are presented for the H4 system with D2h and D4h conformations and for methylene. The PBCS wave function is not a very good approximation for the ground state, accounting for only about half of the correlation energy. The second‐order Tamm–Dancoff correction improves the result as much as the double excitation CI. The Tamm–Dancoff terms consisting of two triplet pairs coupled to a singlet, and those relaxing the constraint imposed on the pairwise excitations in the PBCS wave function are important.
Shaping the density to fit one‐electron properties: Constrained RHF calculations on N2, FH, CO, and LiH
Authors: Zeiss G. D. , Whitehead M. A. 
Published: December 2004

AbstractThe molecular density required to give the correct values for one‐electron properties is rarely given by wave functions obtained from variation methods based on the total energy or the eigenvalues. Perhaps if we knew how the density should be shaped in any particular volume to fit a particular property, the whole molecular density might then be properly described to fit the whole volume. The secant‐parametrization procedure is used to constrain minimum basis set RHF wave functions for N2, FH, CO, and LiH to determine the effects of different constraints on RHF wave functions, and to see how constraints improve the quality of small basis set RHF wave functions. One‐electron property expectation values, energies, and unweighted and property weighted populations and electron density difference profiles are used to analyze the constrained wavefunctions. With the information from the constrained wave functions it should be possible to select a LCAO‐CI basis and states to give the correct density for all properties. This should map onto the constrained wave function in the region of the constraint and at the same time minimize the energy of the total molecular wave function. Such a density would be suitable for the density analyses favored by Bader and Nguyen‐Dang [Adv. Quantum Chem. 14, 113 (1981)], Mezey [Theor. Chim. Acta 54, 95 (1980); 58, 309 (1981); 59, 321 (1981)], and March [Theoretical Chemistry (Royal Society of Chemistry, London, 1981), Vol. 4, p. 158], and show the real atom needed to generate the molecule.
Molecular orbital studies on the structure of nucleoside analogs. IV. Conformation of 3‐deazapurine nucleosides
Authors: Saran Anil , Chatterjee C. L. 
Published: December 2004

AbstractConformational properties of 3‐deazapurine nucleosides (namely, 3‐deazaadenosine and 3‐deazaguanosine) have been investigated by the PCILO method. Both C(2′)‐endo and C(3′)‐endo sugar puckers have been considered and the results indicate that the conformational preferences of these nucleoside analogs are quite different from those of their parent nucleosides adenosine and guanosine, respectively. This result has been correlated in terms of the biological inactivity of these nucleoside analogs.
On the use of spin graphs for spin adapting many‐body perturbation theory
Authors: Mukhopadhyay Atri 
Published: December 2004

AbstractIn connection with spin adaptation in many‐body perturbation theory, this paper reexamines the use of spin graphs in view of a Hugenholtz‐like representation where both the orbital and the spin parts coexist. Together with the idea of essentially distinct diagrams, this representation leads to an economic handling of spin adaptation. As a side issue, the appropriate generalization of the Epstein–Nesbet partitioning for spin‐adapted perturbation theory is obtained. Compact formulas up to fourth order of the ground‐state energy, and up to third order for excitation energies and ionization potentials are given.
Magnetic properties and induced current density in acetylene
Authors: Lazzeretti P. , Rossi E. , Zanasi R. 
Published: December 2004

AbstractCoupled Hartree–Fock perturbation theory, within the framework of an accurate calculation, has been employed to visualize the electronic current density vector field induced by an external uniform magnetic field in the acetylene molecule. The current regime in a plane containing the molecular axis is rather different from those usually accounted for, evidencing the presence of a toroidal vortex in the CH bond. The maps are useful in rationalizing the experimentally observed proton and carbon NMR chemical shifts.
Usage de l'algkbre de lie su(n) dans l'etude des systèmes quantiques à n etats. III. Définition et obtention des constantes du mouvement conjoncturelles
Authors: Tillieu Jacques , Van Groenendael Augustin 
Published: December 2004

AbstractA partir de I'opérateur‐densité d'un système quantique à n états, sont définies et calcultes des constantes du mouvement pour lesquelles est proposée I'appellation nouvelle de conjoncturelles. On établit qu'il en existe au maximum n − 1 linéairement indépendantes. A I'aide des sous‐algèbres su(m) (m < n ), on montre comment il est possible d'introduire, suivant la forme de I'hamiltonien, des constantes du mouvement conjoncturelles reliées aux composantes du vecteur de cohérence.
Why is there a molecular relativistic effect?
Authors: Katriel J. , Feller D. , Davidson E. R. 
Published: December 2004

AbstractThe almost exclusive association of the molecular geometry dependence of the relativistic correction with the valence orbital contribution to the mass‐velocity and Darwin terms is investigated using SCF and MCSCF wavefunctions. The requirement of orthogonality of the valence orbitals to the core orbitals is confirmed to be the mechanism responsible for the increase in (the absolute value of) relativistic energy upon decrease of the internuclear distance. Certain “fingerprint”‐type features of the valence relativistic correction, revealing the identity of the particular core orbital giving rise to it, are identified.
Perturbation theory without wave function for multidimensional systems
Authors: Fernández Francisco M. , Castro Eduardo A. 
Published: December 2004

AbstractA new method to obtain perturbation corrections to the eigenvalues of multidimensional quantummechanical models is developed. It consists of rearranging the Rayleigh‐Schrodinger perturbation theory so that any coefficient of the perturbation series is obtained from a simple and compact recursion relationship. The Zeeman effect in hydrogen and the hydrogen molecule‐ion are used to illustrate the procedure.
Electron‐hole pair excitation in atom‐surface scattering
Authors: Iadonisi Giuseppe , Levi Andrea C. 
Published: December 2004

AbstractThe inelastic scattering of a light atom (helium) from a metallic surface with creation or annihilation of electron‐hole pairs is studied. Diagonal and off‐diagonal matrix elements of the atom‐metal interaction are discussed. The nonadiabatic contributions to the scattering are analyzed in terms of a statistical potential. Explicit calculations are performed for a jellium model at electron densities corresponding to Al and Na. The electron‐hole pair excitation probabilities turn out to be of the order of 10−5–10−3.
Continuum states of the hydrogen molecule with the R‐Matrix method
Authors: Tennyson Jonathan , Noble C. J. , Burke P. G. 
Published: December 2004

AbstractThe interaction of low‐energy electrons with H and ligh with H2 is analyzed using the R‐matrix method including both coupled state and polarization effects ab initio. Particular attention is paid to the energy region containing low‐lying H*2 resonances which converge to H A 2Σ. Resonances in both elastic scattering and the photoionization asymmetry parameter, β, are presented. At low photon eneries, satisfactory results for vibrationally resolved photoionization are obtained using the adiabatic nuclei approximation. The use of nonadiabatic techniques for higher energies is discussed.
Effect of hydrogen chemisorption on the magnetism of nickel clusters
Authors: Fournier R. , Salahub D. R. 
Published: December 2004

AbstractResults of self‐consistent field, local spin density, scattered wave calculations are reported for nickel clusters of 10, 13, and 14 atoms and these clusters interacting with one or two chemisorbed hydrogen atoms. The pure nickel clusters all have a reasonable average atomic magnetic moment (the average over all the clusters is 0.66µB) and the addition of hydrogen reduces this moment in each case. The reduction of magnetic moment is clearly larger on the nickel atoms that are nearest to hydrogen but there is also a noticeable change in the moments of the other atoms of the clusters. Three factors, of varying importance for the different clusters, contribute to the changes in the overall and local magnetic moments: (i) The extra electron brought in with the hydrogen goes into a down‐spin Ni d level, reducing the moment. (ii) The reelectron duced moment is accompained by a reduced exchange splitting and consequently some up‐spin d electrons, not directly involved in the bonding to hydrogen, are transferred to lower lying down‐spin d orbitals. (iii) For atoms close to the adsorbate, d character in the local density of states is pushed above the Fermi level through antibonding interactions with the hydrogen, further reducing the moments of these atoms.
The formation of Na+ and Na ions from Na*(2P) atoms near a tungsten surface. II. Extended diatomics‐in‐molecules models
Authors: Kuntz P. J. 
Published: December 2004

AbstractPotential energy surfaces (PES) for Na(2S, 2P) interacting with a tungsten surface partially covered with sodium ions are computed within the framework of the diatomics‐in‐molecules (DIM) method. A small number (1 to 10) of adsorbed sodium atoms are considered explicitly, the effect of the rest being taken into account through the fragment matrix elements in the DIM formulation. A physical model proposed previously to account for the experimental observation of Na+ and Na ions is supported by these calculations and, in addition, a new pathway to Na products is identified. The effect of including extra adsorbed atoms is discussed in terms of the molecular wave functions and a sensitivity analysis.