bluuebunny/crossref_metadata_2025_split huggingface dataset:
Geometrical derivatives of dipole moments and polarizabilities
Authors: Simons Jack , Jørgensen Poul 
Published: December 2004

AbstractMolecular dipole moments and polarizabilities, as well as their geometrical derivatives, are given analytical expressions for multiconfiguration self‐consistent‐field and configuration interaction wavefunctions. By considering the response of the electronic wavefunction induced by electric field and geometrical displacement terms in the Hamiltonian, the response of the total electronic energy to these terms is analyzed. The dipole moment and polarizability are then identified through the factors in the energy which are linear and quadratic in the electric field, respectively. Derivatives with respect to molecular deformation are obtained by identifying factors in these moments which are linear, quadratic, etc., in the distortion parameter. The analytical derivative expressions obtained here are compared to those which arise through finite‐difference calculations, and it is shown how previous configuration‐interaction‐based finite difference dipole moment and polarizability derivatives are wrong. The proper means of treating such derivatives are detailed.
Electronic structure of the peptide linkage. I. Equilibrium geometry and electronic properties of formhydroxamic acid
Authors: Hilal Rifaat , Moustafa Hussein 
Published: December 2004

AbstractThe equilibrium geometry of formhydroxamic acid has been calculated within the framework of the INDO–MO formalism. Various structural factors are analyzed and discussed in terms of the calculated force constants and charge distribution. The possibility of internal rotation around the CN bond of formhydroxamic acid has been examined. The potential energy surface for the amide‐imide tautomerism is explored by calculating the geometries and characterizing saddle points on that surface. The cyclic and open dimers of formhydroxamic acid are examined and the hydrogenbond energy and length are calculated.
Ab initio studies of negative ion‐molecule(s) clusters present in the atmosphere. II. OH−(H2O)n for n = 0, 2, 3, 4
Authors: Sapse Anne‐Marie , Osorio Louis , Snyder Grace 
Published: December 2004

AbstractAb initio calculations are performed with 6–31G basis set to study the geometry and binding of the H3O, H5O, H7O, and H9O complexes. The H3O complex is also investigated with the 6–31 G* basis set and MP2 (Moller–Plesset perturbation theory of second order).
Theoretical invstigations of the electronic states of porphyrins. I. Basis set development and predicted spectrum of pyrrole
Authors: Rawlings Diane C. , Davidson Ernest R. , Gouterman Martin 
Published: December 2004

AbstractA new minimum basis set was developed for use in computing excitation energies of large molecules. It is particularly suited to calculating ionization potentials and Rydberg transitions. Pyrrole excitation energies and oscillator strengths calculated with this basis set are compared to larger basis set ab initio and semiempirical results. The 6‐eV band in the experimental spectrum is predicted to be the result of three Rydberg absorptions, with no underlying (π, π*) absorption. The calculations also provide an explanation for the observed NH stretch in the 6‐eV region.
Theoretical investigations of the electronic states of porphyrins. II. Normal and hyper phosphorus porphyrins
Authors: Rawlings Diane C. , Davidson Ernest R. , Gouterman Martin 
Published: December 2004

AbstractAb initio methods have been used to calculate the ground and excited states of “normal” and “hyper” porphyrins. Perturbation theory and CI methods were used to determine differential ground and excited‐state correlation effects for [Pv(P)F2]+ and [PIII(P)]+. A comparison is made to the INDO/S/CI predicted wavefunctions and spectra and to the experimental spectra of closely related molecules. The “hyper” [PIII(P)]+ calculations show some very low energy electronic transitions which provide an explanation for an anomalous “red” band in the spectrum and for the lack of fluorescence. Ab initio calculations also predict that (1) the lowest energy 1A1 state is a two‐configuration wavefunction which can be described as a diradical, (2) the two lowest‐energy singlet excited states are double excitations from the closed shell SCF configuration, and (3) a 3B2 state is very close in energy to the lowest 1A1 state.
A second‐quantization approach to the analytical evaluation of response properties for perturbation‐dependent basis sets
Authors: Helgaker Trygve U. , Almlöf Jan 
Published: December 2004

AbstractA general theory for response properties is presented which is applicable to perturbations affecting the molecular basis set, notably nuclear derivatives. A perturbation‐independent Fock space is introduced, and the necessary reorthonormalization of a truncated basis set after a perturbation is explicitly incorporated in the Hamiltonian. Explicit formulas for MCSCF first‐ and second‐order properties are presented, and some computational aspects are briefly discussed. A brief comparison with previous results is given.
A spin‐dependent unitary group approach to many‐electron systems
Authors: Gould M. D. , Chandler G. S. 
Published: December 2004

AbstractIn this article we derive a segment‐level formula for the matrix elements of the U(2n) generators in a basis symmetry adapted to the subgroup U(n) × U(2) (i.e., spin‐orbit basis), for the representations appropriate to many‐electron systems. This enables the direct evaluation of the matrix elements of spin‐dependent Hamiltonians.
Brillouin‐zone double integrals
Authors: Oriade J. O. O. 
Published: December 2004

AbstractA semianalytic scheme for the evaluation of Brillouin‐zone double integrals, such as are met with in the evaluation of quantities such as the Compton profiles and the angular distribution of annihilation photons is proposed. We give an alternative approach to the handling of the jump discontinuities at the generalized Fermi surfaces.
Multibody analysis of potential energy surfaces for first‐ and second‐row tetramers. II. The cases of O4 and S4
Authors: Feng W. L. , Novaro O. 
Published: December 2004

AbstractCalculations on different geometries of O3, S3, O4, and S4 clusters show that oxygen and sulfur present quite different behavior as concerns cluster formation. O3 has a C2v, symmetry while S3 is equilateral D3h. O4 cannot form a structure with near‐equal bond lengths while S3 can form several such structures, of which the ring structures are more stable than the chain or branched structures. A multibody analysis of the cluster energies gives a cogent rationale of these differences, showing for instance that large three‐body nonadditive repulsions make O4 unstable, while three‐body effects turn out to be attractive for S4.
The mechanisms of elementary physicochemical processes: An introductory report
Authors: Re Giuseppe Del , Barone Vincenzo , Lelj Francesco 
Published: December 2004

AbstractMost of the work of theoretical physical chemists and chemical physicists relates at least indirectly to the mechanisms of physicochemical processes. Therefore it is important to examine the meaning and the scope of that notion in the context of recent developments in computational studies. After a brief mention of the meaning of the expression “elementary” physicochemical processes (EPCP), the authors adopt as a definition of mechanism a cause—effect description of anEPCPbased on metastable and transient states corresponding to minima and saddle points of the potential energy surface; these states transform into one another according to appropriate selection rules. The so‐called reaction‐path Hamiltonian can be seen as the starting point for a quantum interpretation of the mechanism concept. On this basis the reaction coordinate, the mechanism profile, and the transition state can be fitted into the same framework. Selection rules are illustrated on the symmetry rules, with a few recent examples of applications which also show their limitations. “Propensity” rules allowing surmises on the nature of a transition state from a static picture of the initial state are also considered and their connection with “reactivity indices” emphasized. Processes involving excitation of electronic states as well as environmental effects are briefly examined. Finally, a specific example taken from surface studies is described in some detail to provide the grounds for further reflection.
Gas phase unimolecular 1,1‐hydrogen elimination: Reaction mechanism and isotope effect
Authors: Barone Vincenzo , Bianchi Nadia , Lelj Francesco , Russo Nino 
Published: December 2004

AbstractThe mechanism of unimolecular 1,1‐elimination of H2 from carbocations has been investigated by the semiempirical MNDO method in view of its very good performances in the analogous elimination from H2CO. Contrary to previous suggestions, the critical configuration obtained at the MNDO level is characterized by a reduced symmetry with respect to the reacting molecule and by a very short HH distance. RRKM computations of the rate constants and isotope effect employing MNDO results for the activation energies and vibrational frequencies indicate also that the present, nonsynchronous mechanism is compatible with all the available experimental data.
Simple models for large‐molecule calculations
Authors: McWeeny R. 
Published: December 2004

AbstractIn spite of tremendous advances in computational quantum chemistry during recent years, there remains the problem of how to deal with molecules so large that calculations of electronic structure and properties require the use of simplified models. This review contains (i) an appraisal of some currently available theoretical models together with proposals for further development, and (ii) a discussion of the use of such models (via variation‐perturbation theory) in the interpretation and prediction of electronic properties. The great majority of electronic properties can be discussed in terms of three distribution functions: the electron density, the spin density, and the current density. The electron density is “observed” in x‐ray scattering and the spin density in (spin‐polarized) neutron scattering, while the current density (present whenever a magnetic field is applied) gives rise to effects observed typically in ESR and NMR experiments. The aim of model calculations should be to obtain a realistic picture of the distribution functions, sufficient for semiquantitative property calculations. For this purpose, as history suggests, it may not always be essential to proceed to the limits of computational feasibility.
Pseudopotential calculations including core‐valence correlation: Alkali and noble‐metal compounds
Authors: Stoll Hermann , Szentpály Lászlö V. , Fuentealba Patricio , Flad Jürgen , Dolg Michael , Fraschio Franz‐xaver , Schwerdtfeger Peter , Igel Gudrun , Preuss Heinzwerner 
Published: December 2004

AbstractA summary is given of our recently developed semiempirical pseudopotentials and their applications, and new results are presented on the K2 molecule.
On the interplay between theory and experiment in molecular structure problems: Some case studies
Authors: Skancke Per N. 
Published: December 2004

AbstractSome brief, general comments on the concept of molecular structure will be given. Some important points connected to the use of models in the interpretation of molecular structures will also be mentioned. The main theme in one part of the presentation will be the cooperative efforts made by experimentalists (mainly spectroscopists) and theoreticians (computational chemists) in order to measure, predict, and analyze the perturbations on the cyclopropane ring by different substituents. The aim is to demonstrate that ab initio calculations of a certain quality can constitute an important support for experimental studies in that they are able to discriminate between different models that otherwise are equally probable. The second part of the presentation will be concerned with a class of molecules that gives both experimental structural chemists and computational chemists a great challenge, namely, the metallocenes. A discussion of some of the grave discrepancies between theory and experiment regarding their geometry will be given.
The effect of an impurity in quasi‐one‐dimensional systems: Green's function and cluster model approach in comparison
Authors: Seel M. 
Published: December 2004

AbstractThe problem of an impurity in a quasi‐one‐dimensional system is addressed. A hydrogen atom in a lithium chain serves as physical model. Two complementary descriptions, the cluster model approach with focus on local interactions, and the self‐consistent Green's function approach which takes into account the extended nature of the system will be presented at the Hartree‐Fock level. The results of both approaches are discussed in a comparative analysis.
Charge transfer in adsorbate‐substrate systems: Extension of the Anderson–Newns treatment to two‐ and three‐dimensional substrates
Authors: Montella Nicola , Barone Vincenzo , Re Giuseppe Del 
Published: December 2004

AbstractCharge transfer in adsorbate‐substrate systems has been studied in the limiting case of an unperturbed substrate extending the Anderson‐Newns treatment to two‐ and three‐dimensional substrates. This allows comparison of different chemisorption sites within an essentially analytic treatment. Important changes of the local densities of states with respect to the usual one‐dimensional model are obtained. The resulting charge transfers are of the same order of magnitude as those obtained by adding correlation effects to the one‐dimensional Anderson‐Newns model.
Dimensional and geometrical effects on the electronic structure of polycyclic hydrocarbons
Authors: Pucci R. , Baldo M. , Martin‐Rodero A. , Piccitto G. , Tomasello P. 
Published: December 2004

AbstractIn the framework of the SCF‐RPA scheme and ppp approximation we studied the excitation spectrum of pyrene, coronene, ovalene, and hexabenzocoronene. The triplet stability matrix of the corresponding Hartree–Fock solution was also studied for each molecule. At variance with the case of long linear polyacenes [M. Baldo, G. Piccitto, R. Pucci, and P. Tomasello, Phys. Lett. 95A, 201 (1983)], all these molecules do not display Hartree–Fock triplet instability. Furthermore, the theoretical analysis indicates that the electronic structure of the molecular series can be characterized by a dimensionality intermediate between one and two, with a smooth transition between the two limits. The transition is essentially determined by the molecular shape. The analysis is supported by the fairly good agreement of the theoretical spectra with the available experimental data.
Quantum chemical studies on zeolites and silica
Authors: Sauer Joachim , Zahradník Rudolf 
Published: December 2004

AbstractAfter mentioning differences in CO and SiO bonding and different structural types of silicates, the conclusion that interactions between external partners and surfaces of silica and zeolites are mostly dominated by van der Waals forces is discussed. Consequently, the theoretical treatment includes (i) selection of appropriate cluster models, (ii) application of nonempirical quantum chemical methods for obtaining interaction potentials, and (iii) statistical thermodynamic evaluation of adsorption characteristics. As examples vibrational frequencies of H2O and NH3 adsorbed on cationic sites, the interaction of conjugated hydrocarbons with Na+ sites, and the interaction of H2O with various sites on silica and zeolite surfaces are considered.
A review of a theoretical model of the excitonic superconductivity in CdS and CuCl
Authors: Collins T. C. , Chandrasekhar M. , Seel M. 
Published: December 2004

AbstractIn this review we briefly outline the model developed by Collins and co‐workers [T. C. Collins, A. B. Kunz, and R. S. Weidman, Recent Advances in Quantum Theory of Polymers, J. M. Andre et al., Eds., Lecture Notes in Physics (Springer‐Verlag, New York, 1979), Vol. 113, p. 240; T. C. Collins, M. Seel, J. J. Ladik, M. Chandrasekhar, and H. R. Chandrasekhar, Phys. Rev. B 27, 140 (1983)] for CdS and CuCl. The model should be applicable to organic semi‐conductors. It is the outgrowth of the work of Little [W. A. Little, Phys. Rev. 134, 1416A (1964)], Devreese et al. [J. T. Devreese, A. B. Kunz, and T. C. Collins, Solid State Commun. 11, 673 (1972)] and a large amount of literature concerning He3 p‐state [A. J. Leggett, Rev. Mod. Phys. 47, 331 (1975)]. This model is built around calculated band structures and the polarization of the valence band by conduction electrons at moderately high impurity concentrations. We extend the model to include both singlet and triplet paired spin states. The parallel paired states contribute a paramagnetic term to the susceptibility, and we discuss the conditions for this term to dominate over the diamagnetic term.
A model for biological specificity
Authors: Avery John 
Published: December 2004

AbstractThe phenomenon of biological specificity is described, and a history of discoveries related to the phenomenon is presented. Aspects of biological specificity described include the mechanism of the immune system, chemotherapy, enzyme‐substrate specificity, neurotransmitters, autoassembly of viruses, autoassembly of subcellular organelles, differentiation, and cellular recognition. A model for biological specificity involving both steric and electrostatic complementarity is presented and the role of structured water and hydrophobic forces is also discussed.
Origin of high efficiency and specificity of biochemical reactions
Authors: Hobza Pavel , Zahradník Rudolf , Ladik Janos 
Published: December 2004

AbstractIt is suggested that the extreme efficiency and specificity observed for chemical reactions in biological systems (compared to chemical reactions occurring under common chemical conditions) is mainly due to special features of these reactions. A key position is ascribed to the role of weak intermolecular interactions (also called noncovalent or van der Waals); in addition, the important role of the entropy and of cyclic or cavitylike shapes of numerous reaction sites, together with the influence of strong local electric fields on transport phenomena and reactivity are discussed. The greatest attention is paid to the applicability of computational methods for evaluation of weak intermolecular interactions, ranging from beyond Hartree—Fock methods to empirical potentials.
The photochemical primary step and the potential in the proton bridge of the chromophore of visual pigments and bacteriorhodopsin
Authors: Sandorfy C. 
Published: December 2004

AbstractThe primary photochemical step in the mechanism of vision is likely to involve cis → trans isomerization as well as proton transfer. Our understanding of this is connected with the problem as to whether the nitrogen of the chromophore Schiff base is protonated or not. More precisely we need to know the shape of the potential in the CN···H+···X group, where X is a proton donor. It is proposed that this potential is double well. The consequences of this relating to the resonance Raman spectra are discussed.
Dynamics of enzyme reactions in solution and in heterogeneous media
Authors: Ricard Jacques , Noat Georges 
Published: December 2004

AbstractEnzymes are catalysts occurring in living cells. The aim of this paper is to describe some basic properties of enzymes and to understand on a physical basis how the intracellular milieu may control the kinetics of enzyme reactions. We shall therefore consider in succession some kinetic properties of enzymes in solution and the way cell surfaces and biological membranes control the dynamics of enzyme reactions. The choice of topics which have been selected in this paper is somewhat arbitrary and certainly reflects the tastes and the personal interests of the authors.
Probable physical mechanisms of the activation of oncogenes through carcinogens
Authors: Ladik János , Čížek Jiří 
Published: December 2004

AbstractA brief review is given of the three main known biochemical mechanisms of human oncogene activation. The underlying possible physical and chemical mechanisms (both short‐ and long‐range) caused by chemical carcinogens are also briefly discussed. The probable role in carcinogenesis of conformational solitons generated after the release of carcinogens previously bound to nucleotide bases is pointed out. For such a soliton the Hamiltonian is written down and the solution of classical equations of motion is outlined.
Structure and properties of transition‐metal compounds. A systematic study of basis set effects in ab initioSCF calculations
Authors: Barandiaran Z. , Seijo L. , Huzinaga S. , Klobukowski M. 
Published: December 2004

AbstractThe recently developed Gaussian basis functions [2] were used in calculations on the ground electronic states of molecules containing transition‐metal atoms: ScF3, TiCl4, ZrCl4, Cr(CO)6, Ni(CO)4, CuF, CuCl, Zn(CH3)2, and Cd(CH3)2. The usefulness of minimal basis sets, the importance of splitting of the valence part of the minimal basis sets, the role of the triple splitting of the d‐block functions, and the need for p‐, d‐, and f‐type polarization functions were discussed in the context of the geometrical structure and the firstorder electronic properties of the transition‐metal atom compounds.
Chemisorption of CO on Pd(100): An lcgto‐lsd cluster study
Authors: Andzelm J. , Salahub D. R. 
Published: December 2004

AbstractLCGTO‐LSD model potential calculations have been performed for CO interacting with two‐, four‐, and eight‐atom clusters of Pd, chosen to model the bridge site of the (100) surface. The geometry and vibrational frequencies are not very sensitive to the cluster size. For Pd8 + CO we obtain dC—O = 1.18 Å (1.13 ± 0.1 exp.), dPd—C = 1.87 Å (1.93 ± 0.07 exp.), and (uncoupled) estimates for ωC—O = 1828 cm−1 (1895 exp.) and ωPd—CO = 454 cm−1 (339 exp.) Binding energies of 4.8, 3.8, and 2.6 eV are calculated, respectively, for Pd2 + CO, Pd4 + CO, and Pd8 + CO which may be compared with the experimental initial heat of adsorption of 1.6 eV. Ionization potentials for CO‐derived levels are in excellent agreement with experiment (relative to ϵF: 4σ (‐11.0 eV, ‐11.2 exp.); 5σ (‐8.0, −8.2 exp.); 1π [−7.5 (b1), −7.3 (b2), −7.5 exp.]). The main negative ion states of 2π* character are calculated at 2.8 eV (b1) and 2.7 eV (b2) above EF. Other states with appreciable 2π* character are found near 5 eV. These may be compared with inverse photoemission results which show a broad peak centered at 4.8 eV. Interactions of the 4σ, 5σ, 1π, and 2π* orbitals of CO with the metal are discussed. The 4σ and 5σ levels are highly mixed, each receiving appreciable contributions from the 4σ and 5σ orbitals of isolated CO. This is discussed in relation to the dispersion of the 4σ and 5σ levels observed in UPS and to the photon‐energy dependent intensities of the 4σ and 5σ resonances. The 2π* component of the backbonding comes through several levels in the upper part of the d band which contain small 2π* contributions in bonding combination with Pd d orbitals. The main 2π* orbitals (contaminated by small antibonding contributions from the metal) are empty (see above).
Electronic structure of highly doped conducting polymers
Authors: Kertesz Miklos 
Published: December 2004

AbstractThree‐dimensional energy‐band calculations on highly doped donor and acceptor compounds of polyacetylenes have been carried out using experimentally suggested channel and intercalation models for the structure of these conducting polymers. The band structures can be very well approximated by the superposition of bands calculated for the polymer chains without the presence of dopants and the bands calculated for the guest chains or layers, strongly supporting the rigid‐band model (RBM) for these structures. The absence of strong mixing between the guest and host orbitals explains the small perpendicular to the chain band widths (and corresponding resonance integrals as well) supporting interchain hopping models for the charge transport in these systems. The results are compared with calculations on graphite intercalation compounds (C8K and C7Br).