bluuebunny/crossref_metadata_2025_split huggingface dataset:
Molecular‐orbital studies of the mechanism of xanthine oxidase‐catalyzed oxidation of purines, especially 2‐chloropurine
Authors: Song Pill‐Soon , Moore Thomas A. 
Published: December 2004

AbstractA detailed study to the xanthine oxidase‐catalyzed reactions of 2‐chloropurine and other substrate bases with various molecular‐orbital techniques such as HMO, ω‐SCFHMO, and ppp semiempirical SCFLCAOMO has shown that the enzyme reactions can be understood in terms of electronic reactivity indices. Furthermore, it appeared possible to suggest the enzyme specificity from a systematic analysis of discrepancy between mo theoretically predicted and observed reaction sites in substrates with 2‐ and 8‐oxy substituents. In other words, the discrepancy does not necessarily indicate the failure of the MO melthodes for such substrates, but it is possible to utilize the result in correlating with binding specificity of the ES complex. This has been done specifically for 2‐chloropurine.Among several electronic reacxtivity indices, frontier orbital density, superdelocalizability, and localization energy have been proved to be very useful. Diferent MO methods gave, in general, consistent results.
Nature of the chemisorption bond on semiconductor surfaces
Authors: Tomášek M. , Koutecký J. 
Published: December 2004

AbstractResults obtained by various quantum‐mechanical approaches in studying localized states on crystal surfaces are summarized and discussed. The one‐ and many‐electron aspects of the problem are compared and shown to lead to similar results. Emphasis is laid upon localized chemisorption states on intrinsic semiconductors. The problem of the calculation of chemisorption heat on solid surfaces is also mentioned.
Study of the convergence radius of the Rayleigh–Schrödinger perturbation series for the delta‐function model of H
Authors: Claverie Pierre 
Published: December 2004

AbstractThe convergence radius of the series expansion for the energy of H2+ in the δ‐function model (in terms of the perturbation parameter μ/λ, where μ is the charge of the perturbing nucleus and λ the fixed charge of the other nucleus) is investigated. A lower bound of this convergence radius (possibly equal to it) previously defined by Robinson [5] is studied analytically as a function of the internuclear distance R and computed numerically. The results differ strikingly from those previously obtained by Robinson who used a simplified but poorer lower bound: in contrast with this poorer bound, the one studied in the present paper is larger than for I every R, from which fact it may be concluded that, contrary to Robinson's previous result, the series expansion of the energy, in the δ‐function model under consideration, still converges when μ = λ for every R.
Spin–other–orbit and spin–orbit interactions in ƒ2 and ƒ3 electron configurations
Authors: Saxena K. M. S. , Malli Gulzari 
Published: December 2004

AbstractExpressions of the matrix elements of the spin–other–orbit and spin–orbit interactions for the various multiplets of all the states of ƒ2‐ and ƒ3‐electron configurations are reported and used to evaluate the Hartree–Fock values of these interactions in the neutral atoms Ce(4ƒ2), Pr(4ƒ3), Ho(4ƒ11) and Er(4ƒ12). The required values of the spin–spin parameters M, and the spin‐orbit parameter ζ for these atoms were obtained using numerical Hartree–Fock wave functions.
Electronic absorption spectra for free hydrazyl radicals, their ionic forms and initial hydrazines
Authors: Gubanov V. A. , Pereliaeva L. A. , Chirkov A. K. , Yastchenko G. N. , Matevosian R. O. 
Published: December 2004

AbstractThe results of the calculations of π–π* transition energies of some free hydrazyl radicals, their ionic forms and initial hydrazines by the SCF–MO–LCAO method with restricted configuration interaction are presented. The comparison of the data obtained with the electronic spectra experimentally found, revealed that the absorption of radical solutions in the visible area was due to the electronic structure of radical molecules, the contribution of ionic forms to the absorption being negligible.
Frequency distribution of radiation emitted from excited atomic systems
Authors: Hearn Dwight D. 
Published: December 2004

AbstractGeneral expressions for the time‐dependent probability amplitudes of the quantum states of two arbitrary, interacting atoms are calculated when one atom is initially in an excited p state and the other atom is in an s ground state. The lifetimes of the excited states and the line shape of the emitted radiation are obtained as functions of both the atomic separation and the energy difference between the excited states of the two atoms. The emission line shape is shown to be doubly peaked and to agree with the line shape of the radiation scattered by a system of two interacting atoms. The expressions for the lifetimes of the excited states are found to be identical to those obtained for the radiation scattering situation.
A theoretical investigation of the singly excited two‐electron atomic states 1s2s3S and 1s3s3S
Authors: Lyslo Gudmund , Aashamar Kjell , Midtdal John 
Published: December 2004

AbstractThe singly‐excited two‐electron states 1s2s 3S and ls3s 3S have been investigated by means of the variational perturbation theory procedure. The wave functions have been constructed as linear combinations of Hylleraas terms with hyperbolic factors in t, and the results obtained by carrying the computations through to 10th perturbation order and with 36(37)‐ 57(58)‐ and 85(86)‐term basic sets, respectively, are reported. These results compare favourably with the corresponding best values from previous conventional variational calculations.
Théorie quantique des phénomènes de biréfringence electrique et magnétique des molécules
Authors: Locqueneux R. 
Published: December 2004

AbstractA general formalism is set up to apply the variation‐perturbation method to the quantum theory of non‐linear optical effects, in particular to the electric and magnetic birefringence (Kerr and Cotton‐Mouton effects).The quantities describing these effects are obtained from the wave function Ψ of the ground state of the unperturbed molecule and the different vectors and tensors which are determined by a variation method.With this method an approximate computation of the Kerr and Cotton‐Mouton constants of some simple molecules has been made.
The green's functions of the Schrödinger equation for the simplest systems
Authors: Vetchinkin S. I. , Bachrach V. L. 
Published: December 2004

AbstractThe Green's functions for the simplest quantum mechanical systems the linear harmonic oscillator, the three‐dimensional isotropic oscillator, the Morse oscillator, the Kratcer potential, and the double‐minimum potential V(x) = (mw2/2)(/x/−R)2 are presented in closed analytical forms.
The general scheme of using non‐orthogonal radial orbitals in a complex electronic configuration of the atom
Authors: Jucys A. P. , Grudzinskas J. J. 
Published: December 2004

AbstractA theory for using non‐orthogonal radial orbitals between shells with identical orbital quantum numbers, in the case of complex configurations, is presented. Construction of the antisymmetric wave function of the whole configuration, with the help of antisymmetrical wave functions of individual shells, is described. General methods of calculating matrix elements of one‐ and two‐electron operators are given.
The cusp condition: Constraint on the electron density matrix
Authors: Clinton W. L. , Massa L. J. 
Published: December 2004

AbstractAttention is called to the attractive cusp conditions as useful constraints in fixing the elements of the electron density matrix. An equation for determination of pure state densities satisfying arbitrary constraints is reviewed, and a detailed formalism for using cusp conditions with this equation is displayed. A calculation is done using the nitrogen molecule as an example.
Study of the electronic structure of molecules. XV. Comments on the molecular orbital valency state and on the molecular orbital energies
Authors: Clementi Enrico , Routh Anne 
Published: December 2004

AbstractThe valency state (vs) concept is analyzed in the Hartree–Fock approximation. A valency state “standard” is defined for atoms at infinite separation. A molecular orbital valency state (Movs) is defined from a partitioning technique (bond energy analysis) previously introduced for the Hartree–Fock molecular wave functions. The Movs for a given atom in a molecule is much higher in energy than the vs and its energy varies from molecule to molecule depending on the exact field of the surrounding atoms. The examples selected in the discussion are the CH4 CH3F, CH2F2, CHF3 and CF4 molecules.An analysis of the orbital energies is then given in terms of the bond energy. The importance of the rearrangement effects following ionization of inner shell electrons (simulation of ESCA type experiments) is illustrated with computations of the positive ion for methane and its fluoroderivatives. It is concluded that rearrangement following ionization from inner shells is as important as rearrangements following ionization from valency electrons. A direct consequence is that the orbital energies should not be equated to the inner shell ionization potentials. The computation of such ionization potentials agrees to about 99.5% with ESCA data, when the energy of both the neutral and ionic species are computed; the use of the orbital energies limits this agreement to about 95%.
Some developments in the spin‐extended Hartree‐Fock method
Authors: Mestechkin M. M. 
Published: December 2004

AbstractAll the second‐order density matrix spin components for the spin‐extended Hartree‐Fock method are obtained. The coefficients in the final formulae are only ωsM, ωsM±1, ωsM±2, where ωsM are the weights of pure states of spin s in the initial unprojected determinant with spin projection M. The eigenvalue problem for the best electron density natural orbitals in the spin‐extended method is formulated. All the second‐order transition density matrix spin components between pure spin basis functions built of orthogonal orbitals and distinguished by different core choice are also found. This basis may be used on CI calculations.
A study of the AMO method as applied to the lithium metal. III. A method for calculating the total electronic energy of crystals in the LCAO approximation
Authors: Sperber Gunnar 
Published: December 2004

AbstractA method is given for accurate calculation of the expectation value of the Hamiltonian of a crystal with respect to a single‐determinant wave function of LCAO type. The method is partly based on numerical integration over one Wigner‐Seitz cell. In principle, the calculations involve infinite summations over lattice sites; suitable truncation methods are devised.
Optimization of molecular orbitals: Easy parametrizations of little rotations suitable for functional iteration
Authors: Gianinetti Ermanno 
Published: December 2004

AbstractThe method of functional iteration applied to orbital optimization is proposed. The main problem, i.e., the most economical parametrization of the rotations of the molecular orbitals, is resolved. First and second derivatives of rotations with respect to the parameters are given. The fact that the orbital space can be decomposed in subspaces such that energy is invariant against transformations of these subspaces into themselves is explicitly taken into consideration.
Gaussian orbitals optimized for lower bounds of hydrogenic atoms
Authors: Johnson J. W. , Poshusta R. D. 
Published: December 2004

AbstractVarious optimization criteria are compared for the hydrogen atom to find orbitals which improve lower bounds computed from the Weinstein, Temple, and Stevenson‐Crawford formulas. Minimization of squared energy deviation, “variance,” is recommended because the resulting lower bound orbitals give excellent lower bounds, converge to the exact wave function, are relatively easy to optimize, and are insensitive to the estimated energy eigenvalue. New linear combinations of Gaussian orbitals which minimize the variance are presented for the 1s, 2s, 2p, 3s, 3p, and 3d orbitals. These orbitals are compared with previous linear combinations with regard to their expectation values and local properties.
Interactions between molecules in the neighborhood of a long linear molecule
Authors: Krizan John E. , Lloyd Brian F. 
Published: December 2004

AbstractThe third‐order interaction between two nonpolar molecules due to the instantaneous induced dipole effect of an infinitely long linear molecule is assessed following arguments similar to earlier research on three‐body effects. It is found that an attractive force favors small polar angles in approach to the linear molecule; such behavior could be of interest to processes such as those involved in replication of DNA.
Alternative equivalent icosahedral irreducible tensors
Authors: Boyle L. L. , Schäuffer C. E. 
Published: December 2004

AbstractSome sets of icosahedral irreducible tensors can only be defined with respect to one of two seemingly equivalent choices of axes. The problem is explained and the cases when such an alternative choice may cause mixing of sets belonging to different irreducible representations are determined. It is found that the amount of mixing is a property of the spherical basis sets and is independent of the rank and of the permutation symmetry of the tensorial sets. It is further found that certain operators which change the handedness of the coordinate axes can similarly affect these sets, notwithstanding the centrosymmetry of the icosahedron.
Molecules‐in‐molecules calculations with fixed resonance integrals
Authors: Kiss A. I. , Martin A. 
Published: December 2004

AbstractA simple equation for the evaluation of resonance integrals from overlap integrals and ionization potentials of the molecular fragments is suggested for molecules‐in‐molecules π electron calculations. The singlet π → π* transition energies of some benzene derivatives containing donor substituents were calculated. The best results were obtained if in the expression of the resonance integral the first experimental ionization potential of the methyl derivative of the donor groups is used.
Approximate valence‐shell electron unrestricted Hartree‐Fock calculations with orthogonalized atomic orbitals: Proton hyperfine splittings in benzyl and related radicals
Authors: Nanda D. N. , Narasimhan P. T. 
Published: December 2004

AbstractA modified INDO procedure has been used to calculate the proton hyperfine splittings in benzyl and the isoelectronic anilino, phenoxy and 2‐azabenzyl as well as 2‐ and 3‐thenyl radicals. The present procedure differentiates between s‐, p‐ and d‐orbitals on an atom in estimating various integrals involving them, satisfies the rotational invariance requirements and employs an orthogonalized basis set of atomic orbitals for obtaining core‐Hamiltonian matrix elements. The calculations based on using the exponents which depend only on the type of orbital and the nature of atom fail to provide correct relative order of ortho and para proton splittings in benzyl as well as anilino, phenoxy and 2‐azabenzyl radicals. On the other hand, use of the exponents which are modified according to the charge densities in various orbitals leads to a high absolute value for para proton splitting compared to that for ortho proton splitting which in case of all these radicals is in agreement with experiment. A spin density calculation on benzyl, anilino and phenoxy radicals considering the variation of one‐center one‐electron and one‐center two‐electron integrals for different protons with their charges is found to yield further improvement in the relative order of ortho and para proton splittings in all these radicals. In 2‐ and 3‐thenyl radicals the role of 3d‐orbitals on sulfur has also been examined. To our knowledge, no unrestricted INDO calculations including 3d‐orbitals on sulfur have been reported in the literature so far.
Interaction of two H2 molecules: Comparative study of various perturbation procedures
Authors: Basilevsky M. V. , Berenfeld M. M. 
Published: December 2004

AbstractA variational calculation of the interaction between two H2 molecules using minimal basis set of 1s functions is performed to check the efficiency of various versions of the perturbation theory for intermolecular interactions. The matrix procedure starting with the zero‐order Hamiltonian which is symmetric with respect to intermolecular electron permutations shows better convergence than the procedures using nonsymmetric zero approximations. This conclusion follows from the calculations of ground state and four lowest excited states for three geometric configurations of the H4 system. The behaviour of the potential curves is interpreted in terms of symmetric perturbation theory. The various contributions to the interaction energy are considered in detail. The importance of charge transfer states for the description of the intermediate range of intermolecular separations is specially emphasized.
Ab initio calculations on porphin
Authors: Almlöf Jan 
Published: December 2004

AbstractAb initio SCF calculations are reported for the porphin molecule. The positions of the central protons have been optimized, and the equilibrium geometry is found to be a linear NH ⃛ HN arrangement. The NH vibrational frequencies have been computed and are compared to experimentally measured quantities. Several low ionized states have also been studied in separate spin‐restricted SCF calculations. The lowest state is found to have B1u symmetry with an ionization potential of 8.0 eV.
Analysis of the space correlation factors in a MCSCF representation of LiH and Li2 molecules
Authors: Besson M. A. , Suard M. 
Published: December 2004

AbstractThe space correlation factor is studied in LiH and Li2 molecules, using MCSCF wave‐functions. The shape of the Fermi hole is related to the localization of molecular space orbitals, while the Coulomb hole study indicates the importance of symmetry properties of the molecular orbitals involved in excited configurations for the representation of the electronic correlation inside the chemical bond.
Quantum chemical calculations on thioridazine
Authors: Kaufman Joyce J. , Kerman Ellen 
Published: December 2004

AbstractQuantum chemical CNDO/2 calculations for the conformational preference of the side chain of thioridazine as a function of angle indicated the crystallographically determined structure gave the lowest energy. There is also a small region of conformational flexibility within the first 90° of rotation of the side chain. This is commensurate with the results which we had obtained previously for our similar calculations for promazine and its Cl and CF3 derivatives, perazine and its Cl and CF3 derivatives, and for the hypothetical hitherto unknown N‐piperidinopromazine and its Cl and CF3 derivatives. The conformational profile of thioridazine resembles that of the perazines. The calculated gross atomic populations on the alkyl nitrogen in thioridazine was within the range we had previously found necessary for neuroleptic activity.
The simulated ab initio molecular orbital technique. VI. Open‐shell radicals in the spin restricted formalism
Authors: Collins Michael P. S. , Duke Brian J. , Eilers James E. , O'Leary Brian 
Published: December 2004

AbstractThe simulated ab initio molecular orbital (SAMO) method previously applied to RHF closed‐shell and UHF open‐shell situations has been applied to open‐shell radicals, such as the butyl radical and the pentyl radical, within the RHF open‐shell framework. The open‐shell Hartree–Fock theory is developed such that a rapidly convergent iterative method for evaluating the SAMO wave‐function can be employed. Results closely parallel those for the same systems using the UHF method and are of comparable accuracy to SAMO closed‐shell results.
Hulthén and slater type 2d functions in some excited configurations of sulphur and phosphorus
Authors: Chandler G. S. 
Published: December 2004

AbstractIt is shown that a substantial energy improvement is gained by the variational use of Hulthén orbitals, instead of single Slater orbitals, in the 3d shells of some excited configurations of sulphur and phosphorus. The energies obtained are close to those attained with two‐term Slater functions. In some cases the radial distribution functions from Hulthén orbitals are as good an approximation of SCF radial distributions as those from two‐term Slater orbitals. Single term 2d functions with only one parameter are found to give almost identical energies and radial distribution functions as those obtained from two‐parameter Hulthén orbitals. It is shown that the relationship between one‐term 2d orbitals and Hulthén orbitals gives a method of enforcing nuclear cusp conditions on the former with little effect on the energy.
An application of the Brillouin–Löwdin perturbation expansion: Lower bounds to the energy eigenvalues of the rigid rotator in an electric field
Authors: Grau D. 
Published: December 2004

AbstractOne of the variants of Löwdin's partitioning technique that makes use of a Brillouin‐type perturbation expansion for the study of lower bounds to the eigenvalues of a Hamiltonian has not been applied to any practical quantum mechanical problem so far. To illustrate how powerful this method can be, an application is made to the rigid rotator in an electric field, which has already been studied by Choi and Smith using the bracketing function of an intermediate Hamiltonian. It turns out that for a given order of the basis set of the Bazley space the Brillouin–Löwdin perturbation expansion gives closer bounds than the method of Choi and Smith, except for the case l = m, where the procedures can be shown to be mathematically equivalent. Especially for high l − m the number of basis functions needed to attain the same accuracy is by far larger for the method of Choi and Smith than for the Brillouin–Löwdin method.
Point‐group symmetry and transition metal superconductivity: NbMoTc alloys
Authors: Mueller F. M. , Myron H. W. 
Published: December 2004

AbstractUsing a factorization of the band density of states into point‐group symmetries, it is shown that the variation of the fractional density of states of one type—Γ12—correlates closely (ρ12 = 0.997) with variations in λM〈ω2〉. It is also shown that the Hopfield parameter η does not correlate with Np(0)Nd(0)/N(0), which would be true if superconductivity in d‐band materials was dominated by local atomic processes. It is concluded that bonding plays an important role in superconductivity in Nb‐Mo‐Tc alloys.
Perturbation theory in the path‐integral representation and its application to some simple quantum systems
Authors: Perlin Y. U. E. , Gifeisman S. H. N. , Van Quyet Pham 
Published: December 2004

AbstractIn the framework of the path‐integral formalism of nonrelativistic quantum theory the perturbation method is developed. This is based on the cumulative expansion of the partition function in powers of the difference between the classical actions of the exact and trial system. The trial system is constructed by replacing the Coulomb interactions by elastic ones, with parameters defined by Feynman's variational principle. The second order corrections to the ground‐state energy of the hydrogen and helium atoms, the hydrogen molecule, and molecular ion have been evaluated. The results have been found to coincide with the actual values with an accuracy of 4–6%, whereas the corresponding error of the variational solution is of the order of 15–20%.