bluuebunny/crossref_metadata_2025_split huggingface dataset:
Dicretization to avoid singularities in vibration–rotation Hamiltonians: A bisector embedding for AB2 triatomics
Authors: Tennyson Jonathan , Sutcliffe Brian T. 
Published: January 2005

AbstractA previously proposed [Sutcliffe and Tennyson, Int. J. Quantum Chem. 29, 183 (1991)] body‐fixed Hamiltonian is applied to AB2 systems in Radau coordinates with the x‐axis embedded along the bisector of the angle. It is shown that by using a discrete variable representation for the angular coordinate it is possible to avoid singular regions of the Hamiltonian. A two‐step variational procedure is used to obtain rotationally excited states of the system. The results of test calculations H2S and D2S with J = 0, 1, 5, and 10 are discussed along with computer‐usage characteristics.
The structure of the bitetrahedryl molecule—A major shift due to electron correlation: Effects of carbonyl substituents, implications for the structure of coupled tricyclo[3.1.0.02,6]hexyl, and extension to cubylcubane
Authors: Xie Yaoming , Schaefer Henry F. , Aped Pinchas , Chen Kuohsiang , Allinger Norman L. 
Published: January 2005

AbstractAb initio molecular quantum mechanics has been applied to a number of coupled ring systems including biterahedryl and cubylcubane. Basis sets at least as large as double zeta augmented by carbon d functions (DZ+d) were used throughout. For biterahedryl, electron correlation effects qualitatively alter the molecular structure, decreasing the central CC bond distance while increasing the adjacent CC distance. Carbonyl substituents, contrary to some previous thinking, do not qualitatively alter the bitetrahedryl structure. This finding points to a structural problem with the monomer bicyclo[3.1.0.02,6]hexane that carries over to the coupled bicyclo[3.1.0.02,6]hexyl molecule. Finally, for the cubylcubane molecule synthesized in 1988, the only significant difference between Hartree–Fock theory and experiment occurs for the central CC distance, which, as in the other coupled molecules, is too long. These results significantly lengthen the (still very short) list of closed‐shell hydrocarbon molecules for which Hartree–Fock theory encounters structural difficulties.
Near‐equilibrium ordering of the crystalline phases of atomic hydrogen
Authors: Nobel J. A. , Wilson G. A. , Trickey S. B. 
Published: January 2005

AbstractSome time ago Min et al. [Phys. Rev. B 30, 5076 (1984)] found the near‐equilibrium sequence of phases of paramagnetic crystalline H to be (in order of increasing energy) simple cubic (SC), hexagonal close packed (HCP), body‐centered cubic (BCC), and face‐centered cubic (FCC), with FCC and HCP substantially separated (about 60 mRy/atom). This result is counterintuitive on three counts when compared with crystalline Li. That comparison suggests that HCP should lie only slightly below FCC and both should be substantially below BCC. (Simple cubic should lie lowest in H but not in Li because of the difference in orbital occupancy.) We have redone the calculations with our published full‐potential linearized augmented plane wave code WIEN. We find the sequence of phases with increasing energy to be SC, HCP, FCC, and BCC, the ordering expected from solid Li. By direct exploration of dense k‐space meshes, we show that the difference in predictions results from a k‐space mesh extrapolation technique used by Min et al.
Ab initio studies on the Ziegler–Natta catalytic process
Authors: Novaro O. 
Published: January 2005

AbstractA survey of the development of the application of quantum chemical techniques to Ziegler–Natta polymerization starting from the pioneering work of Clementi and co‐workers is presented. In the latter, the role of the catalyst was elucidated, a feasible reaction pathway was proposed, and several properties of the catalyzed reaction (heats of formation, activation barriers, physisorption and chemisorption energies, etc.) were obtained without semiempirical approximations. Furthermore, a few proposals for enhancing catalytic activity for the oligomerization process proved to be of great practical value and led to several patents. This turns out to be a good example of the usefulness of the computational techniques to provide intense interactions between theoretical and applied chemistry.
Effects of exchange on equilibrium bond lengths of heavy, almost spherical, tetrahedral molecules XH4
Authors: Egorov S. A. , March N. H. , Santamaria R. 
Published: January 2005

AbstractThe simplest nonrelativistic density functional theory, namely, that of Thomas, Fermi, and Dirac, is used to study the effect of exchange on the equilibrium bond lengths of heavy tetrahedral molecules XH4. In particular, the limiting bond length as the central atom X becomes infinitely heavy is shown to be reduced by exchange by some 0.34 Å. Comparison with experiment shows that the main features of the bond‐length variation through the series CH4PbH4 are reflected by the Thomas–Fermi–Dirac predictions, though the bond lengths remain too large for finite atomic number Z of the central atom. Therefore, a semiempirical correction is proposed, which, however, tends to zero as Z tends to infinity.
The calculation of frequency‐dependent hyperpolarizabilities including electron correlation effects
Authors: Rice Julia E. , Handy Nicholas C. 
Published: January 2005

AbstractThe theory for the calculation of the frequency‐dependent hyperpolarizabilities β(−2ω; 0, ω), β(−ω; 0, ω), and β(0; ω, −ω) is discussed. New relations between these tensors are derived for those wave functions that obey the time‐dependent Hellmann–Feynman theorem (e.g., the self‐consistent field [SCF] or the exact wave function). Using second‐order Møller–Plesset perturbation theory (MP2), expressions are obtained for the hyperpolarizabilities in terms of derivatives of appropriately defined linear polarizability tensors with respect to a static electric field. Results are presented for ammonia and formaldehyde for the optical Kerr effect and for secondharmonic generation. These results indicate that it is desirable to determine the frequency‐dependent contribution to the hyperopolarizability at the MP2 rather than the SCF level of theory, in cases where the static hyperpolarizability has a large contribution from electron correlation and/or where the frequency‐dependent contribution may be more significant, such as for secondharmonic generation.
Theory of molecular optoelectronics. IX. Calculation of χ(2) from theoretical hyperpolarizabilities for DAN
Authors: Munn R. W. 
Published: January 2005

AbstractComponents of the crystal quadratic susceptibility tensor χ(2) for second‐harmonic generation are calculated for the title compound, 4‐(N, N‐dimethylamino)‐2‐acetamido‐nitrobenzene. Input data are the crystal structure and refractive indices and CNDO hyperpolarizabilities; two different sets of refractive indices give similar results. The calculations yield effective polarizabilities as well as both rigorous and anisotropic Lorentz local electric fields. Susceptibility components are close to 10 pm V−1, rather smaller than those deduced experimentally and showing a weaker dependence on direction; Kleinman symmetry is mostly observed within 10%. Screened dipoledipole interactions using CNDO dipole moments amount to −80 kJ mol−1, implying permanent fields of 2 GV m−1.
Calculation of the probabilities for transitions into the continuous spectrum by using the L2 basis: A new method of solving the wave‐function renormalization problem
Authors: Smelov G. V. 
Published: January 2005

AbstractA new method is presented for calculating the normalization factor for continuous spectrum functions in the L2 basis. One variant of this method is applicable to operators given in the form of a tridiagonal matrix and does not require explicit calculation of the roots of the characteristic equation. The other variant of the method can be used in the general case, where after the basis is introduced, its length is gradually increased.
Superconductivity and the effective electron–electron interaction mechanism in the light of the configuration interaction study
Authors: Svrček Michal , Baňacký Pavol , Zajac Anton 
Published: January 2005

AbstractThe effect of electron–phonon interactions, described by the effective electron–electron interaction mechanism, on the electronic structure of molecular systems has been investigated by the configuration interaction method. For systems with strongly quasi‐degenerate electronic states near to the Fermi surface, this interaction mechanism, according to the BCS theory, results in the electronic ground‐state energy lowering and in the gap formation. A more rigorous treatment of the configuration interaction method as applied in this paper confirms the electronic ground‐state energy lowering, but the obtained results indicate that the effective electron–electron interaction term does not yield the experimentally detected temperature dependence of the gap. Accordingly, the two‐particle mechanism seems not to be the key factor for the gap formation and for the microscopic mechanism of superconductivity, which agrees with the results of the molecular nonadiabatic theory of electron–vibrational interactions.
Classical trajectories for two ring‐shaped potentials
Authors: Kibler Maurice , Lamot Georges‐Henri , Winternitz Pavel 
Published: January 2005

AbstractThe present paper deals with the classical trajectories for two superintegrable systems: a system known in quantum chemistry as the Hartmann system and a system of potential use in quantum chemistry and nuclear physics. Both systems correspond to ring‐shaped potentials. They admit two maximally superintegrable systems as limiting cases, viz., the isotropic harmonic oscillator system and the Coulomb–Kepler system in three dimensions. The planarity of the trajectories is studied in a systematic way. In general, the trajectories are quasi‐periodic rather than periodic. A constraint condition allows to pass from quasi‐periodic motions to periodic ones. When written in a quantum mechanical context, this constraint condition leads to new accidental degeneracies for the two systems studied. © 1992 John Wiley & Sons, Inc.
On the calculation of first‐order properties: Expectation value versus energy derivative approach
Authors: Ernzerhof M. , Marian C. M. , Peyerimhoff S. D. 
Published: January 2005

AbstractThe definition of so‐called first‐order properties is, in general, not unique for approximate solutions of the Schrödinger equation. In the present work, the determination of molecular properties as the expectation value or energy derivative are compared for a wide class of electron correlation treatments that are based on the stationarity of certain modified energy functionals. The case in which all perturbation‐dependent parameters in the energy expression are optimized is taken as a reference point, to which the derivative and the expectation value approaches are related by a sequence of approximations. By analyzing the validity of these approximations, we are in a position to give general criteria for the choice of the method to be employed. The derivative approach is shown to be a special case of a general method to improve the calculation of molecular properties. Furthermore, this method is extended to give a correction to the energy itself. © 1992 John Wiley & Sons, Inc.
The dynamic and the stochastic reaction rate approaches to nonradiative processes in condensed media
Authors: Christov S. G. 
Published: January 2005

AbstractA comparison is made between the exact classical dynamic theory and the stochastic theory of reaction rates in dense media. The conditions of their equivalence are derived on the basis of a harmonic oscillator model. It is shown in a direct way that in the general case of a multidimensional system the approximate activated complex (transition‐state) theory is not identical to the stochastic theory at any value of the viscosity coefficient. © 1992 John Wiley & Sons, Inc.
Empirical methods for computing molecular partition coefficients: II. Inclusion of conformational flexibility within fragment–based approaches
Authors: Richards Nigel G. J. , Williams Philip B. , Tute Michael S. 
Published: January 2005

AbstractA novel algorithm for computing the water/1‐octanol partition coefficient, logP, of conformationally flexible molecules, has been investigated using calculations upon a number of uncharged, linear dipeptides. In this method (which appears to be the first to consider explicitly the effects of the population of accessible conformational minima inbothphases), the partition coefficient for each dipeptide was calculated from the overall energy change associated with moving the relevant gas‐phase conformational distribution into waterandinto 1‐octanol. These energies were computed using solvation contributions based upon the solvent accessible molecular surface area and two sets of empirical parameters. In these initial studies, gas‐phase conformational minima were generated using systematic search methods. While the standard error in the computed logPvalues was disappointing, reasonable agreement was observed between calculated and experimental logPvalues for the set of model dipeptides, especially when specific hydration interactions involving polar fragments were correctly included in the empirical solvation term. These results indicate that the physical basis of many correction factors employed in the ClogP algorithm for computing logPprobably arise from neglect of the redistribution of conformer populations as flexible molecules partition between water and 1‐octanol.
An Ab initio study of tautomerism between formhydroxamic acid and formhydroximic acid
Authors: Yuanqi Tao , Wengui Duan 
Published: January 2005

AbstractAb initio MO calculations including electron correlation with the 4‐31G and 4‐31G** basis sets were performed in order to study the formhydroxamic acid‐formhydroximic acid tautomerism. The geometries, relative energies, and activation energy of the tautomer and transition state were determined. Based on total‐energy calculations at the MP4/4‐31G**//RHF/4‐31G** plus the scaled zero‐point vibration energy level, the energy of formhydroxamic acid is determined to be lower than that of formhydroximic acid by 40.7 kJ/mol. The activation energy of the formhydroxamic acid‐formhydroximic acid tautomerism via a 1,3‐intramolecular hydrogen shift is 151.4 kJ/mol. © 1992 John Wiley & Sons, Inc.
Substituents effects in phospha alkynes: A computational investigation
Authors: Jayasuriya Keerthi 
Published: January 2005

AbstractAb initio self‐consistent field (SCF) molecular orbital calculations were carried out at the MP2/431G* level to determine the effect of substituent groups, F, NH2, OH, CH3, CH2CH, and HCC—on the CP triple bond. Although there are considerable differences between the electronegativities of these substituent groups, a characteristic feature found in phospha alkyne derivatives is the weak sensitivity of the CP bond length to the substituent at the carbon atom. The molecular electrostatic potential analysis clearly reflects the strength of the electronegativity of these substituent groups and their ability to attract the polarizable charge from the CP triple bond. MEP analyzes further indicates that these derivatives of phospha alkynes are most likely to attract incoming electrophiles to the π‐electron region of the Ctriple bond. Transition metals are coordinated “sideways” with phospha alkynes, and in this configuration, the lone‐pair electrons of the phosphorus atom are unlikely to interact with the metal. © 1992 John Wiley & Sons, Inc.
Transition‐state structures for the reaction between styrene and tbutyl cyanoketene: Theoretical evidence for antarafacial characteristics in a π2 + π2 thermal cycloaddition
Authors: Rzepa Henry S. , Wylie William A. 
Published: January 2005

AbstractSemiempirical AM1 SCFMO models for the cycloaddition between monosubstituted alkenes (RCH = CH2, R = Me, Ph, CF3) and tbutyl cyanoketene predict the lowest‐energy transition state to be relatively polar and asynchronous in character and to have a cis relationship between the R and the tbutyl groups, in agreement with the experimentally determined product for R = Ph. We interpret this stereochemistry as resulting from the presence of a significant antarafacial component in the transition state, which enables these two bulky groups to adopt pseudoequatorial positions in which steric interactions are minimized. The alternative higher energy trans geometry forces the R and the cyano groups to adopt more sterically hindered pseudoaxial positions. Alternative biradical‐like transition states inferred by others from ab initio MCSCF calculations on the reaction between ketene and ethene do not retain any significant antarafacial component on the ketene and, therefore, form less satisfactory models for this specific reaction. © 1992 John Wiley & Sons, Inc.
Consistent parametrization of semiempirical MO methods
Authors: Jug Karl , Krack Matthias 
Published: January 2005

AbstractThe parametrization of semiempirical molecular orbital methods is reviewed. The parametrization procedures are classified in three categories. The newly proposed category of consistency of parameters in a row of elements is applied in SINDO1 to the Si atom. The calculations on test molecules and silicon clusters demonstrate that significant improvements in accuracy can be obtained in this way. © 1992 John Wiley & Sons, Inc.
A semiempirical SCF MO study of the interaction of odorant molecules with a biological substrate
Authors: Wu C. S. , Myers L. J. , Worley S. D. 
Published: January 2005

AbstractThe AM1 semiempirical SCF MOmethod developed by Dewar and co‐workers has been employed to compute possible interaction processes of the odorant molecules cocaine, methyl benzoate, and benzaldehyde with the biological substrate lysine. The only process predicted as possible in each case was nucleophilic attack on the carbonyl carbons of each odorant by the nitrogen lone pairs on lysine. The remote amino group of lysine in each case was predicted to react with lower activation enthalpy than did the α‐amino group, a prediction consistent with preliminary experimental data from these laboratories. Although the computed activation enthalpies were high (ca. 40 kcal mol−1) for the gas‐phase processes, it is conceivable that such processes could occur by alternate pathways with considerably lower ctivation energies in solvated media such as exists in the presence of mucus‐containing olfactory binding protein in vertebrate olfactory reception. © 1992 John Wiley & Sons, Inc.
Prototropic changes in cationic base‐pair adducts. II. Guanine methylation
Authors: Ford George P. , Wang Bingze 
Published: January 2005

AbstractAM1 calculations have been used to study the effects of CH attachment on the structures, energies, and, in some cases, proton transfer reactions of guanine cytosine base pairs. Methylation of both the guanine 3‐ and O6‐positions is predicted to lead to chemically significant concentrations of intermediate base pairs arising from proton transfer from the guanine 1‐ to the cytosine 3‐position. The possible biological implications of such intermediates in nucleic acids is discussed in relation to the formation of either doubly abasic sites or abasic sites opposite potentially miscoding DNA lesions. © 1992 John Wiley & Sons, Inc.
Spin‐independent three‐body effective valence‐shell operators: Application to molecular oxygen
Authors: Kanzler Alfred W. , Freed Karl F. , Sheppard Maurice G. 
Published: January 2005

AbstractA mathematical construction is presented that uniquely defines a set of spin‐independent effective valence‐shell Hamiltonian (Hv) three‐body matrix elements. These spin‐independent Hv matrix elements separate direct and exchange portions of the three‐body Hv matrix elements and therefore provide the most natural form for comparisons with parameterization schemes of semiempirical electronic structure methods in which the three‐body matrix elements are incorporated into semiempirical one‐ and two‐body Hamiltonian matrix elements in an averaged manner. Ab initio Hv three‐body matrix elements of O2 are computed through third order of quasidegenerate perturbation theory and are analyzed as a function of internuclear distance and atomic orbital overlap to aid in understanding how these three‐body matrix elements may be averaged into semiempirical one‐ and two‐body matrix elements. © 1992 John Wiley & Sons, Inc.
Magnetic susceptibility of some derivatives of cellulose and their mixtures: Theory and experiment
Authors: Sadykova A. Yu. , Saykaeva A. S. , Kostochko A. V. , Glebov A. N. , Moozyukov V. G. 
Published: January 2005

AbstractThe calculations of the magnetic susceptibility (χ) of the selected derivatives of cellulose have been performed using the localized molecular orbitals with the basis set of Gaussian functions. The advantages of the variational procedure in the framework of the approximation of the additivity of the localized molecular fragments for the evaluation of the magnetic susceptibility and nuclear magnetic shielding (σ) and the comparison with the NMR of high‐resolution data have been discussed. Theoretical values for χ of the 50 sizable organic molecules have been presented. © 1992 John Wiley & Sons, Inc.
Broken symmetry solutions of the two‐dimensional extended hubbard model and their instability conditions
Authors: Yamamoto Shoji , Ozaki Masa‐Aki 
Published: January 2005

AbstractInstabilities of the restricted Hartree–Fock (HF) state of the two‐dimensional extended Hubbard model under doping are studied with the linear response theory employing the linearized Bethe–Salpeter equation. Boundaries of the instabilities for the ordering vectors q = Q ≡ (π, π) and q = Q0 ≡ (0, 0) are given in terms of the interaction strength parameters at various doping levels. With doping, dominant instabilities generally change from ones for q = Q to ones for q = Q0. Broken symmetry phases obtained from the instabilities are also studied and classified using the group‐theoretical technique. For q = Q, two more new phases are obtained in addition to the four nonmagnetic and five magnetic phases that have been already derived by one of the authors. For q = Q0, three nonmagnetic and five magnetic phases are obtained. Deriving the explicit forms of the charge current and spin current operators in the HF approximation, charge current and spin current densities in the broken symmetry states are discussed in detail. © 1992 John Wiley & Sons, Inc.
A regional embedding method
Authors: Zou P. F. 
Published: January 2005

AbstractA new variational embedding method is derived. This method couples Nesbet's use of theR‐matrix in the determination of the electronic structure of a crystal with the energy variational technique. The procedure is based on the observation that in many cases the properties for a spatial region of a system change by relatively small amounts when the region is transferred to another system. The transfer of the region from one system to another is accomplished by the embedding potential that is obtained by the inversion of theR‐matrix and its energy derivative. It is shown that the interaction between two connected regions can be written as a surface term that is obtained by the continuity conditions on the wave function and its derivative on the surface. The existence of an identity resolution on the surface is demonstrated and this result is used to derive theR‐matrix and its inverse. An application of this method to His given, which shows that the method is accurate and reliable if one chooses the appropriate basis set to construct theR‐matrix and to perform the variation. © 1992 John Wiley & Sons, Inc.