bluuebunny/crossref_metadata_2025_split huggingface dataset:
The basis set dependence of structures and energies of various states of cyclodisiloxane
Authors: Brenstein Robert J. , Scheiner Steve 
Published: December 2004

AbstractSeveral common basis sets, ranging from minimal to double‐zeta, are applied to study the neutral singlet and triplet as well as positive‐ and negative‐ion doublet states of cyclodisiloxane. The effect of d‐polarization function exponents on the equilibrium geometries and energies is analyzed. The d‐type functions seem to be essential in the basis set of silicon, whereas their presence on oxygen is less critical. The optimum exponents (with respect to SCF energy) are determined to be 0.45 for Si and 0.60 for O, very close to those recommended for the 6–31G** basis set. The best structural predictions are obtained with the 6–31G(2d, p) basis set, which contains two sets of d functions on the heavy atoms. The predicted SiO bond length is 166 pm; the SiSi and OO distances are 237 and 232 pm, respectively, which correspond to an O—SiO angle of 88.6°. The ground state is found to be a singlet. All higher states have longer SiO bonds and Si—Si distances, whereas O—O distances are shorter. The energy separation between the singlet and other states is modified by electron correlation (MP treatment) by only a few kcal/mol.
Rayleigh–Schrödinger perturbation theory for many‐electron systems with partitioning of configurations by orders of excitation. II. Analysis of the ground state wavefunction and energy
Authors: Taylor William J. 
Published: December 2004

AbstractRayleigh‐Schrödinger perturbation theory has been rederived by an extension of Löwdin's partitioning method in which excited configurations are partitioned by their orders. The resulting equations show explicityly how first‐, second‐, and third‐order configurations contribute to the first three orders of perturbative corrections to the ground‐state wavefunction, and the first seven orders of corrections to the groundstate energy. The corrections to the energy have also been expressed in terms of the coefficients in the wavefunction by an independent derivation; this yields more compact expressions for the energy corrections. The results are summarized in three convenient tables. The relation of the results for the energy to the requirement of size consistency, and the linked cluster theorem of many‐body perturbation theory, is discussed.
Simple geometric generation of special points in brillouin‐zone integrations. Two‐dimensional bravais lattices
Authors: Ramíarez Rafael , Böhm Michael C. 
Published: January 2005

AbstractA clear tabulation of sets of special points in numerical integrations over Brillouin zones (BZ) is given for the five two‐dimensional Bravais lattices. The special points derived on the basis of plain geometrical criteria coincide with a special case of the large unit cell (LUC) method. This similarity is also found for other generation procedures of k‐grids reported in the literature; that is, the corresponding techniques have their common origin in the LUC approach. The underlying integration procedure of the described scheme is based on the discrete summation, which implies that the weighting factors at the k‐points are only determined by the point symmetry in reciprocal space not by details of the adopted numerical integration formalism. The method can be applied to semiconducting and insulating solids—that is, to materials with finite band gaps. Grids in reciprocal space are generated on the basis of simple geometric criteria. The numerical effort of the present geometrical approach is negligible in comparison to the computational steps necessary in generation procedures of optimized k‐grids that have been published in the literature. The application of the suggested integration procedure is extremely simple; the generation of the k‐grids is only based on a few definitions and plain geometrical formulas. Modifications of the grid size are possible without difficulties. A collection of weighting factors ωi and special and general positions in the BZ is given; different grids are tabulated together with their geometrical efficiencies. The generated integration domains are compared with the results of elaborate schemes of finding grids with maximal efficiency.
Structure of MCSCF electronic energy domain
Authors: Panin A. I. , Khait Yu. G. , Gorinchoy N. N. 
Published: January 2005

AbstractThe modern version of the process of removing redundant variables in an MCSCF optimization problem is studied on the basis of the manifold theory. It is shown that there exists a simple parametrization of the MCSCF orbital manifold that is convenient for computer implementation of quasi‐Newton optimization schemes. A sequential unconstrained optimization technique for minimizing electronic energy with respect to local coordinates is described.
Exact results on homopolar singlet excitation energies of the hubbard model for linear polyenes
Authors: Hashimoto Katsufumi 
Published: January 2005

AbstractThe Lieb–Wu equations for homopolar singlet excited states of the Hubbard model of linear polyenes are formulated in a form tractable by numerical calculation. They are solved to obtain exact excitation energies of the lowest two excited states, corresponding to the 1B and 1E states of the benzene molecule, for systems with up to 50 and 30 electrons, respectively. These results may be particularly useful for checking accuracy of approximate methods for the homopolar states for which the conventional CI method is very difficult to apply. Calculation is also carried out for the 1B+ ionic excited state involving a spin excitation, which corresponds to the 1B state of the benzene molecule. The length dependence of the excitation energies is examined and shown to have no N−1 dependence for large N, where N is the number of electrons. This supplements the result on the 1E ionic state obtained in our previous paper.
The quality of s‐orbitals determined by least‐squares fitting and constrained variational methods
Authors: Robertson Donald D. , Smith Vedene H. , Simas Alfredo M. , Thakkar Ajit J. 
Published: January 2005

AbstractCalculations of a wide range of position and momentum space properties of 23 orbital basis sets for atomic helium are performed to supplement our previous work. The combined results on 94 orbital basis sets are used to demonstrate that, as in the case of d‐orbitals, the least squares error is not as discriminating as other information theoretic measures of overall quality for s‐orbitals. Detailed comparisons suggest that constrained variation can improve the overall quality of the resulting orbital provided that the constraints are selected carefully to complement the bias inherent in the energy expectation value and the functional form of the basis functions.
The dirac second‐order equation and an improved quasirelativistic theory of atoms
Authors: Karwowski J. , Kobus J. 
Published: January 2005

AbstractA transformation decoupling the Dirac second‐order equations for the large and small components of the radial atomic wavefunction has been derived. The decoupling procedure is exact for one‐electron atoms and approximate, though very accurate, for a general case of an atomic spherical potential. Its connection with the quasirelativistic theory of atoms has been discussed. The transformation has been applied to deriving an optimized quasirelativistic equation and to establishing relations between the quasirelativistic and the Dirac wavefunctions. Formulas resulting from these considerations allow improvement of quasirelativistic expectation values of operators so that the error, relative to the Dirac values, is reduced by a factor of 10.
The correlation effects at a local level. CI partitions
Authors: Largo‐Cabrerizo A. , Largo‐Cabrerizo J. 
Published: January 2005

AbstractMaking use of the local pair distribution function, h(r12+;r1, α), we have carried out a detailed study of the local mean values of r obtained from a CI wavefunction and from its different possible partitions for the helium isoelectronic series. Through comparison of these results with those provided by Hartree‐Fock and Hylleraas‐type explicitly correlated wavefunctions, we reached several conclusions concerning the relative importance of different partitions of the CI wavefunction in order to get a proper description of local correlation effects.
Some recent experiments on inelastic electron‐molecule collisions at low energies
Authors: Allan Michael 
Published: January 2005

AbstractThis work describes a novel trochoidal electron spectrometer and illustrates its properties on a few examples. In the first example vibrational excitation in the small molecules H2 and N2 is measured. Comparison of the results with theoretical work points to the neccessity of using non local methods for correct treatment of near‐threshold region and of extremely short‐lived resonances. In the second example the observation of resonant autodetachment of Feshbach resonances in many organic molecules is reported.
A molecular orbital study ofcisandtransdiaminodichloroplatinum(II)
Authors: El‐Issa B. D. , Makhyoun M. A. , Salsa' B. A. 
Published: January 2005

AbstractThe relativistic multiple scatteringXα method has been applied in order to calculate the eigenvalues and the charge densities of cis and trans diaminodichloroplatinum(II). The activity of the cis isomer as an antitumor agent is discussed in terms of the possible modes of coordination of certain fragments of the DNA molecule through the LUMO state of the cis Pt complex. Although possible, coordination of the trans isomer is rendered unlikely as a result of possible strain effects.
On the contribution of A. C. Hurley to theoretical chemistry
Authors: Maslen V. W. 
Published: January 2005

AbstractAndrew Crowther Hurley was born in Melbourne, Australia, on 11 July 1926 and received his early education at Melbourne Church of England Grammar School. He graduated from the University of Melbourne with high honors in mathematics and natural philosophy and, in 1949, was awarded his M.A. with first class honors in the school of mathematics for his thesis “The Irreducible Crystal Classes in Four Dimensions,” his supervisor being Dr. Hans Schwerdtfeger. From 1950–1952 he was a member of Trinity College, Cambridge, and was awarded a Ph.D. for his research in theoretical chemistry under Sir John Lennard‐Jones. In 1953 he returned to Melbourne and joined what was to become the Division of Chemical Physics of the Commonwealth Scientific and Industrial Research Organization (CSIRO). The award of a fellowship by Trinity College enabled him to spend two more years, 1954–1956, in the Theoretical Chemistry Department of the University of Cambridge, and this was followed by a year with Professor J.C. Slater's solid state and molecular theory group at the Massachusetts Institute of Technology. In 1957 Andrew again returned to the Division of Chemical Physics in Melbourne, where he has remained, except for the academic year 1962–1963, when he was Visiting Professor in Theoretical Chemistry at Iowa State University.
Diabatic surfaces which permute into one another
Authors: Hall G. G. 
Published: January 2005

AbstractSome of the problems, discussed recently, with the idea that every molecule has a structure can be removed by adopting the idea that some molecules have several structures with probabilities of transfer between them. This is supported theoretically by using diabatic energy surfaces and wave functions. Since these are not easy to define in general, their definition is given in the special case where the surfaces permute into one another. Two kinds of permutation operations are required. The analysis is given more fully for two intersecting diabatic surfaces. The analogy to the double‐valued property of the wave function near a conical intersection is given.
The 3‐21G(N*) basis set: An economical polarized basis set for ab initio studies on nitrogen‐containing molecules
Authors: Riggs Noel V. , Radom Leo 
Published: January 2005

AbstractThe 3‐21G basis set shares with its older cousin, the 4‐31G basis set, a tendency to overestimate valence angles at nitrogen atoms and to underestimate seriously barriers to inversion at such atoms. The 6‐31G* basis set generally yields greatly improved results in these respects. It is here shown that, for a variety of molecules, supplementation of the 3‐21G basis set at three‐ or two‐coordinate nitrogen atoms with a set of six d‐functions having exponent 1.0 leads to optimized geometries and inversion barriers at such nitrogen centers in good agreement with results obtained with the 6‐31G* basis set. This supplemented basis set, designated as 3‐21G(N*), also leads to calculated vibrational frequencies in good agreement with those calculated with the 6‐31G* basis set. The 3‐21G(N*) basis set offers an economical alternative to the 6‐31G* basis set, particularly for molecules containing several first‐row atoms other than nitrogen.
Unified analytical evaluation of the two‐center coulomb and hybrid integrals over slater‐type orbitals
Authors: Guseinov I. I. 
Published: January 2005

AbstractTwo‐center Coulomb and hybrid integrals are combined into a single expression by use of the formulas given by the author for the expansion of real and complex STO's about a point displaced from the orbital center. The final results are expressed in terms of the overlap integrals between STO's. Analytical formulas for the evaluation of these integrals have been established recently by the author.
A unified theory of the stability of benzenoid hydrocarbons
Authors: Cioslowski J. 
Published: January 2005

AbstractThe spectral density operator technique is proved to be a convenient tool for derivation of approximate topological formulae for total pi‐electron energy (Epi) of benzenoid hydrocarbons (BHs). Developed mathematical formalism points out a common origin of three different measures for the stability of BHs, namely: resonance energy (RE), number of Kekule structures (K) and HOMO‐LUMO separation (XHL). In turn, a novel topological invariant corresponding to “normalized” RE is derived. Numerical calculations for a representative set of BHs demonstrate effectiveness of the present approach. Various approaches to an estimation of BH stabilities are discussed.
Resonance energies of large conjugated hydrocarbons by a statistical method
Authors: Randić Milan , Solomon Vasanth , Grossman Stuart C. , Klein Douglas J. , Trinajsti Nenad 
Published: January 2005

AbstractWe developed a theoretical method for studying the aromatic stability of large molecules, molecules having a dozen and more fused benzene rings. Such molecules have so far often been outside the domain of theoretical studies. Combining the statistical approach and a particular graph theoretical analysis, it is possible to derive the expressions for molecular resonance energy for molecules of any size. The basis of the method is enumeration of conjugated circuits in random Kekulé valence structures. The method has been applied to evaluation of the resonance energies of conjugated hydrocarbons having about a dozen fused benzene rings. The approach consists of (1) construction of random Kekulé valence structures, (2) enumeration of conjugated circuits within the generated random valence structures, and (3) application of standard statistical analysis to a sufficiently large sample of structures. The construction of random valence forms is nontrivial, and some problems in generating random structures are discussed. The random Kekulé valence structures allow one not only to obtain the expression for molecular resonance energies (RE) and numerical estimates for RE, but also they provide the basis for discussion of local molecular features, such as ring characterization and Pauling bond orders.
Fixed closest distance of approach dipole potential mapping: An effective alternative to the monopole isopotential approach
Authors: Mishra P. C. , Tewari R. D. 
Published: January 2005

AbstractIt has been shown that an easier procedure to the study of molecular electrostatic potentials than the prevalent monopole isopotential approach is to use a dipole as the mapping device: allow the dipole to move on a geometrical surface enclosing the molecule such that the distance between the surface and the nearest atoms of the molecule (closest distance of approach) is fixed, say at 2 Å, calculate the electrostatic interaction energy between the molecule and the dipole at different points of the surface, and let the dipole orient itself along the minimum energy direction and treat these minimum energies as the desired electrostatic potentials. Thus the favorable binding sites of a given molecule for other species along with their relative orientations can be obtained. This procedure has been applied to the nucleic acid bases using the necessary input data from two independent sources. Thus success of the procedure in predicting important features of molecular electrostatic potentials has been demonstrated.
The effect of “full” and “limited” counterpoise corrections with different basis sets on the energy and the equilibrium distance of hydrogen bonded dimers
Authors: Alagona Giuliano , Ghio Caterina , Cammi Roberto , Tomasi Jacopo 
Published: January 2005

AbstractA detailed analysis of the ΔE(R) curves for nine hydrogen bonded dimers involving HF, H2O, and NH3 as partners, computed with five basis sets (STO‐3G, MINI‐1, 3‐21G, 4‐31G, and 6‐31G**) and subjected to counterpoise (CP) corrections with three different methods is reported. Using several criteria and tests, the positive effect of full CP corrections for the description obtained with all the basis sets (with the exception of the STO‐3G one) is pointed out. The CP correction at the 6‐31G** levels is still sizable and improves the results with respect to the estimated Hartree–Fock limits of ΔE(Req) and Req. The results obtained with the application of the full CP correction to the MINI‐I energies are of relatively good quality. The STO‐3G energies at the SCF level are, on the contrary, overcorrected by the full CP correction and slightly improved by “limited” CP corrections: this basis set, however, is not recommended for calculations of geometries and stabilization energies of H‐bonded dimers of this size.
A gauge‐variational calculation of molecular diamagnetic susceptibility
Authors: Sinha S. K. , Ghosh R. 
Published: January 2005

AbstractIn order to avoid the determination of the perturbed wave function in the susceptibility calculation of closed shell molecules, several authors have developed gauge‐variation methods based on Rebane's work. The present work illustrates that introduction of a certain restriction in the gauge‐variation method makes the susceptibility calculations exceptionally simple. The restriction involves the use of the Coulomb gauge so that the gauge‐function satisfies Laplace's equation. The restriction was dictated by the demonstration of gauge‐invariance of the Rayleigh–Schrouml;dinger perturbation theory of diamagnetism. The method with the prescribed restriction has been applied to the H2O molecule, as an example, to demonstrate the advantages.
The complexes of phenyl acetylene with HF, H2O, and NH3: An ab initio study
Authors: Sapse A. M. , Jain Duli C. 
Published: January 2005

AbstractPhenyl acetylene complexes with HF, H2O, and NH3 are investigated with ab initio molecular orbital calculations using the 6‐31G Gaussian basis set. HF is found to form a π complex, whereas H2O and NH3 form σ complexes. Observations of experimental spectroscopic shifts are rationalized.
EHMO Calculations for a pyridine molecule adsorbed on a Ag adatom: Implications for surface enhanced Raman spectroscopy
Authors: Tian Bogang , Wu Guozhen 
Published: January 2005

AbstractExtended Hückel molecular orbital calculations have been made for the silver adatom‐pyridine complex in order to reveal the very chemical nature of the surface‐enhanced Raman scattering. The variation of the behaviors of the bond orders and charge densities as functions of the AgN distance and the net charge on the Ag atom, which is applied voltage dependent, is demonstrated. The calculated optical transition of the system can accommodate the experimental observation that as the wavelength of the exciting line shifts from 4880 to 6328 Å, the surface‐enhanced Raman signal is enhanced more. Most interesting is the fact that the calculations reproduce well the experimental observation that there is an applied voltage on the electrode at which the surface‐enhanced Raman signal is a maximum. The calculations show that the charge transfer enhanced mechanism is mostly due to the transfer of electrons from the silver atom to the pyridine molecule. Several useful overlap integrals involving d orbitals not available in the literature before are listed in the appendices for convenient reference.
A regular periodic table of the elements and its quantum mechanical requirement
Authors: Purdela D. 
Published: January 2005

AbstractBy using the n + (1/2)l filling rule of the atomic Aufbau principle, where n is the principal quantum number and l is the azimuthal quantum number, a new periodic table is presented, its periods having, in order, 8, 18, 18, 32, 42, 50, … elements. The mentioned rule is proposed instead of the n + l rule (or Madelung's rule) which constitutes the quantum mechanical basis of the current periodic table and predicts periods having, in order, 2, 8, 8, 18, 18, 32, 32, 50, … elements. The new periodic table is called “regular” because its groups are formed according to a single rule (namely, the first elements of each period are placed in the same order as the elements of the preceding period), in contrast with the current periodic table, where no simple rule can be applied for the same purpose. The most characteristic feature of the regular periodic table is the fact that its groups are also related in a periodic manner.
C. A. Coulson and the surface energy of metals: A further comment
Authors: Hall George G. 
Published: January 2005

AbstractThe role of the boundary conditions is emphasized in determining the relation between the counting problem, posed by Coulson and solved by the number theorists, and the physical problem from which it arises. The surface energy term, which Coulson sought, does exist, but only when the periodic boundary conditions, which he used, are replaced by the finite box conditions.