bluuebunny/crossref_metadata_2025 huggingface dataset:
Highly correlated particle densities and idempotent one‐densities
Authors: Henderson George A.
Published: December 2004

The problem of determining idempotent one‐densities which integrate to the exact or to a highly correlated particle density is considered. A method for obtaining the minimum energy idempotent one‐density integrating to a given correlated particle density within a finite basis is described. The implications of this are twofold. First, Hartree–Fock accuracy can be exceeded in describing the electron density with an idempotent one‐density; this is particularly relevant to the problem of constructing orbitals from experimental x‐ray scattering data. Second, electron densities from analytic CI or MCSCF wave functions can be made available in a form as compact as the Hartree–Fock density by reporting the orbitals which define the correlated density via an idempotent one‐density. A numerical example of the new method is given in which an accurate correlated density for He is “fitted” by an idempotent one‐density represented in a finite (near Hartree–Fock) basis. Considering the deficiencies of the basis for this purpose, a technique is suggested for constructing basis sets optimized for prediction of one‐electron properties rather than for energy.
Restricted hartree–fock method instability
Authors: Mestechkin M. M.
Published: December 2004

The explicit forms of the spin density matrix variations which are responsible for the external instability in the restricted Hartree–Fock (RHF) method are found. The RHF open shell instability matrix which guarantees the distorted wave function to belong to the same space as the initial one is derived for arbitrary spin.
Vers une nouvelle interprétation de la liaison chimique?
Authors: Julg Andre , Julg Pierre
Published: December 2004

Le découpage de la densité électronique totale dans une molécule, en contributions provenant des diverses orbitales, permet de définir des régions orbitalaires de caractère conventionnel dont la charge totale est égale à la somme des populations de Mulliken des orbitales constituant les diverses régions. La fluctuation relative de la charge de ces régions s'exprime simplement en fonction des populations de Mulliken et des indices de liaisons. Cette analyse de la densité dans des molécules variées (saturées ou conjuguées) permet de retrouver les liaisons chimiques de la graphie classique comme correspondant à des régions orbitalaires de charge voisine de 2 et présentant une fluctuation relative fiable (⩽0, 10). Compte tenu du caractère dynamique que contient la notion de liaison, les résultats obtenus trouveraient une interprétation simple dans l'espace des phases. L'impuissance de la Mécanique quantique de donner une description dans cet espace expliquerait l'impossibilité de déduire de calculs quantiques la notion měme de liaison chimique.
Thermodynamics of the DNA helix‐coil transition
Authors: Porta Jordi
Published: December 2004

The thermodynamics of the DNA helix‐coil transition is studied, starting from the thermodynamical potential difference between the states helix and coil; this potential difference is understood as the difference in free energies. With only three parameters obtained from experimental data different quantities of the T2 phage DNA molecule are calculated. It is observed that the phase transition is of second order.
Relativistic symmetry orbitals for the double groups C2v, C∞v, D∞h, and Oh
Authors: Rosén Arne
Published: December 2004

Relativistic symmetry orbitals are given for molecular LCAO calculations for the double groups: C2v, C∞v, D∞h, and Oh (6‐fold) coordination. The atomic orbitals used in the LCAO are of the four component form. A discussion of the comparison between nonre ativistic and relativistic molecular eigenvalues is presented.
Floating spherical gaussian orbital model study of some organometallic systems. I. LiCH3, HBeCH3, and CH3BeCH3
Authors: Ray N. K. , Mehandru S. P. , Bhargava Shobha
Published: December 2004

The floating spherical Gaussian orbital method has been used to study the equilibrium geometries and electronic structures of some organometallic systems like LiCH3, HBeCH3, and CH3BeCH3. The predicted geometries are in good agreement with other theoretical estimates and available experimental results. Electron transfer values from carbon to metal atom and bond energies of LiC and BeC bonds are estimated and discussed. The wave functions obtained here have also been used to predict the electron momentum distributions and Compton profiles for LiCH3, HBeCH3, and CH3BeCH3.
Dependence of the electrostatic molecular potential upon the basis set and the method of calculation of the wave function. Case of the ground 3A1(π → π*) and 1A1(π → π*) states of formaldehyde
Authors: Daudel R. , Le Rouzo H. , Cimiraglia R. , Tomasi J.
Published: December 2004

The dependence of the molecular electrostatic potential V on the method of calculating the wave function and on the basis set has been examined for three electronic states of H2CO. The calculations analyzed here refer to two different basis sets: a minimal set and a split‐valence shell set, both supplemented with diffuse p orbitals. The methods of calculation include the SCF procedure (for the ground state), CI procedures of increasing complexity, which practically reach their asymptotic limits, and, for the excited states, simpler methods (rigid orbital excitation, complete CI of single excitations, electron‐hole potential methods). It appears that in the ground state the two bases give equivalent descriptions of V when the CI process reaches the asymptotic limit and that the SCF descriptions approximate fairly well the more accurate ones. For the 3A1(π → π*) state the conclusions on the two basis sets are similar and, in addition, it is shown that a simple method (EHP) gives an approximation of V that reproduces the essential features of the complete CI calculations. The inadequacy of both basis sets for the representation of the 1A1(π → π*) state turns out to be evident. A discussion is presented concerning the possibility of adopting the calculation of V as an auxiliary tool to compare the accuracy of different descriptions of the same electronic state.
Quasidegenerate coupled‐cluster approach with hermitian model hamiltonian
Authors: Kvasnička Vladimír
Published: December 2004

The quasidegenerate coupled‐cluster approach with a hermitian model Hamiltonian is constructed. This theory provides an efficient method for simultaneous construction and evaluation of the hermitian diagrammatic model Hamiltonian up to an arbitrary high order of perturbation theory and prescribed form of topology.
On the electrostatic properties of papain in relation to its enzymatic activity
Authors: Lavery R. , Pullman A. , Wen Y. K.
Published: December 2004

The electrostatic potentials and fields created by the thiol protease, papain, are computed on the surface envelope of the protein and in the region of its active site using a technique based on accurate multipole expansions of the electron density of appropriately chosen subunits. The effect of binding counterions to the protein is considered specifically. The possible role of the “electrostatic environment” in the functioning of the enzyme is discussed and it is found that this environment can favor the proton transfer between Cys 25 and His 159 that is thought to initiate its mode of action, but that the reasons for this appear more complicated than earlier models would suggest.
Bond length alternation in cyclic polyenes. II. Unrestricted hartree–fock method
Authors: Paldus J. , Chin E. , Grey M. G.
Published: December 2004

The problem of bond length alternation in cyclic polyene models as described by the Pariser–Parr–Pople π‐electron Hamiltonian, together with an empirical quasi harmonic σ‐core potential is investigated using the unrestricted Hartree–Fock wave function employing different spatial orbitals for different spins. It is shown that in contrast to the restricted Hartree–Fock method, which favors bond alternation in large cyclic polyenes, the unrestricted Hartree–Fock method stabilizes the symmetric structures with equidistant internuclear separation. An assessment of the amount of correlation error recovered by the unrestricted Hartree–Fock procedure is examined and the qualitatively different behavior of the cyclic polyene models when described by restricted and unrestricted Hartree–Fock wave functions is discussed from this viewpoint.
Bond length alternation in cyclic polyenes. III. Alternant molecular orbital method
Authors: Pauncz R. , Paldus J.
Published: December 2004

The bond length alternation problem in cyclic polyene models as described by the Pariser–Parr–Pople Hamiltonian and an empirical quasiharmonic π‐core potential is investigated using the one‐parameter alternant molecular orbital (AMO) method. It is shown that in contrast to the unrestricted Hartree–Fock (UHF) results, which lead to symmetric equidistant structures, the one‐parameter AMO results yield bond length alternating structures similar to those obtained with the restricted HF approach. The correlation energy recovered by the AMO method is examined for the symmetric polyenes in the whole range of coupling constants for both the Pariser–Parr–Pople and Hubbard Hamiltonians and compared with exact full configuration interaction (FCI) results. For the first member of the cyclic polyene series we also compared the FCI and AMO correlation energies for different nuclear framework distortions. This comparison indicates that in contrast to the UHF results the fraction of the correlation energy recovered by the AMO approach is very uniform over the range of nuclear distortions considered. The AMO results thus strongly indicate the dimerization in the polyenic chains.
Protonation of aminopyrone: A four centers problem, CNDO/2 versus MNDO study
Authors: Meghezzi H. , Boucekkine A. , Kolli B. , Hamdi M.
Published: December 2004

The mechanism of protonation of aminopyrones and the nature of the protonated forms are of some interest because of the biological importance of these compounds. The use of CNDO/2 molecular electrostatic potentials contour maps shows that the most reactive centre is the extracyclic oxygen. MNDO protonation energies confirm the protonation of this atom and elucidate the basicity of the nitrogen atom. The effect of a N substitution on the basicity of the N13 atom has been studied. The calculations are in agreement with preliminary experimental results.
A molecular geometry invariant property of energy level set boundaries in z space
Authors: Mezey Paul G.
Published: December 2004

Boundaries of electronic energy level sets in the nuclear charge space wZ can be parametrized as single valued functionals for any nuclear geometry of molecules, which property, combined with the general convexity of such level sets, leads to various electronic energy inequalities.
Basis set quality. II. Information theoretic appraisal of various s‐ orbitals
Authors: Simas Alfredo M. , Thakkar Ajit J. , Smith Vedene H.
Published: December 2004

The expectation values 〈rk〉 (−2 ⩽ k ⩽ 4, k = 10), values of the charge density ρ(r) at selected points, and coefficients in the MacLaurin expansion of ρ(r) are used to test the quality of 71 orbital basis sets used for the atomic helium Hartree–Fock problem. These tests in position space are complementary to the momentum space tests previously carried out [Int. J. Quantum Chem. 21, 419 (1982)]. Information theoretic measures with respect to either or both position and momentum space properties are subsequently defined and the orbitals are ranked accordingly. These measures indicate that, for a given orbital, momentum space properties are more poorly predicted than position space ones. Moreover an improvement in the virial ratio does not necessarily lead to a more balanced orbital with respect to position and momentum space properties. Basis sets containing Slater‐type orbitals are again found to be rather accurate. The exponentially damped rational function is confirmed to be the outstanding two‐parameter unconventional orbital.
Ordering of Kekulé structures using nonadjacent numbers
Authors: El‐Basil Sherif , Osman A. N.
Published: December 2004

Kekulé indices and conjugated circuits are computed for 36 Kekulé structures, together with two VB quantities associated with the corresponding factor graphs (previously called submolecules). These latter quantitites are nonadjacent numbers of Hosoya and the reciprocal of the connectivity indices of Randić. It was found that the index of Hosoya successfully orders a set of Kekulé structures belonging to the same hydrocarbon in a parallel order as their Kekulé indices and branching indices. This substantiates the relation between VB and MO theories. A code is derived by summing contributions of nonadjacent numbers in all Kekulé stuctures of a hydrocarbon. The order of the resulting codes is found to be identical to the order of the molecular properties (resonance energies, π‐energies, and eigenvalues) of the hydrocarbons.
Electron correlation effects on magnetic properties of BH
Authors: Iwai Masahiro , Saika A.
Published: December 2004

The nuclear magnetic shielding and magnetizability tensors for the BH molecule are calculated by the coupled‐Hartree–Fock method and many‐body perturbation theory. As in the case of H2, HF, and F2, the second‐order non‐CHF diagrams make an inappreciable contribution.
A Hermitean reformulation of the Born–Oppenheimer nonadiabatic coupling terms for diatomic molecules
Authors: Römelt Joachim
Published: December 2004

A reformulation of the Born–Oppenheimer nonadiabatic coupling for diatomic molecules is presented, which clearly demonstrates its hermiticity and allows a computation of the coupling elements using very accurate CI functions for the description of the electronic motion.
Calculation in k space of integrals arising in the theory of Van der Waals forces. II. Formulas for arbitrary atoms
Authors: Ishida Kazuhiro
Published: December 2004

Basic integrals arising in the momentum space formulation of the van der Waals forces are derived for arbitrary two atoms. These integrals are essentially the matrix elements of the Fourier transformed Coulomb operator and the Fourier transformed square of the Coulomb operator between any two Slater orbitals. The derivation is completely analytic and the results are expressed as finite series expansions in terms of auxiliary integrals. Recursion relations among the auxiliary integrals are developed.
A possible interpretation of atomic valency as a spontaneously broken symmetry response
Authors: Prat R. F.
Published: December 2004

The concept of atomic valency is looked at as a specific response of an atomic system to an arbitrary environment. It is shown, on the basis of a mathematical result in group actions theory, how valency might be related both to previously found Hartree–Fock deformed states and to the isotropy groups (up to a conjugation) of the extrema of the electronic energy functional.
Perturbation theory and electron correlation in extended systems: Cyclic polyene model
Authors: Takahashi M. , Paldus J. , Čížek J.
Published: December 2004

The applicability of the finite‐order many‐body perturbation theory to the electron correlation problem in extended one‐dimensional systems is examined. The cyclic polyenes CNHN, N = 4ν + 2, ν = 1, 2, …, with the DNh geometry as described by both the Pariser–Parr–Pople and Hubbard Hamiltonians, are employed to model the metallic‐like one‐dimensional systems. The second‐order perturbation theory contributions to the correlation energy are obtained with three different partitionings of the Hamiltonian (Hückel, M⊘ller–Plesset, and Epstein–Nesbet). The third‐ and fourth‐order contributions are also calculated in special cases. A comparison with other methods is given wherever available. For the Hubbard Hamiltonian the asymptotic behavior of the perturbation theory expansion is examined numerically. It is shown that the finite‐order perturbation expansion can provide reliable results for the correlation energy of one‐dimensional systems even in the correlation region which corresponds to the spectroscopically determined physical value of the coupling constant.
Analysis of bonding properties in molecular ground and excited states by a Cohen‐type bond order
Authors: Dick Bernhard , Freund Hans‐Joachim
Published: December 2004

We propose a Cohen‐type bond order analysis in terms of orthogonalized atomic basis functions which can be used to analyze NDO wave functions of large organic and metal–organic molecules. It is shown that for small molecules the results gained with this method are in excellent agreement with the same analysis based on ab initio STO‐3G wavefunctions. For large planar aromatic systems these all‐valence electron bond orders are found to be a consistent generalization of the π‐bond order. A simple relation between these bond orders and the corresponding covalent bond energies is established. The method can be easily extended to study excited state multiconfiguration wave functions. We present calculations for C2H2, C2H4, C2H6, and Mn2(CO)10. The results indicate that the method can be used to discuss the photochemistry of organic and metal–organic compounds.
Density functional theory: Approximating quasiparticle density functional
Authors: Beznosjuk Serge A. , Kryachko Eugene S.
Published: December 2004

A constructive approach for deriving the approximating quasiparticle energy density functional is proposed. As a matter of fact, the proposed approach is the direct development of the Kohn–Sham quasiparticle concept and the Levy–Valone approach. The approach presented takes into account a pseudopotential character of the exchange‐correlation part of the density functional and results in a system of functional equations to obtain ground‐state energies of many‐electron systems.
A procedure for determining reaction paths and saddle points
Authors: Bálint Imre , Bán Miklós I.
Published: December 2004

To determine reaction paths, a method that does not require the previous location of extrema is presented and illustrated by an example. The procedure is based on a local symmetry property of the potential surface.
Fourier transforms of atomic orbitals. II. Convolution theorems
Authors: Niukkanen A. W.
Published: December 2004

Analytic expressions have been derived for tensor convolutions of basis functions of exponential class (the reduced Bessel functions, the Slater, and the hydrogen‐like functions). For reduced Bessel functions, in particular, the method suggested in the paper is a more clear and compact alternative to the laborious proof of the important Filter and Steinborn convolution theorem.
Diagrammatic valence‐bond theory for finite model Hamiltonians
Authors: Ramasesha S. , Soos Z. G.
Published: December 2004

Valence bond (VB) diagrams form a complete basis for model Hamiltonians that conserve total spin, S, and have one valence state, ϕp, per site. Hubbard and Pariser–Parr–Pople (PPP) models illustrate ionic problems, with zero, one, or two electrons in each ϕp, while isotropic Heisenberg models illustrate spin problems, with only purely covalent VB diagrams. The difficulty of nonorthogonal VB diagrams is by‐passed by exploiting the finite dimensionality of the complete basis and working with unsymmetric sparse matrices. We introduce efficient bit manipulations for generating, storing, and handling VB diagrams as integers and describe a new coordinate relaxation method for the ground and lowest excited states of unsymmetric sparse matrices. Antiferromagnetic spin‐½ Heisenberg rings and chains of N ⩽ 20 spins, or 2N spin functions, are solved in C2 symmetry as illustrative examples. The lowest S = 1 and 0 excitations are related to domain walls, or spin solitons, and studied for alternations corresponding to polyacetylene. VB diagrams with arbitrary S and nonneighbor interactions are constructed for both spin and ionic problems, thus extending diagrammatic VB theory to other topologies.
Calculation of periodic molecular systems with perturbed periodicity within the PCILO framework. II. Model systems
Authors: Lochmann R. , Weller Th.
Published: December 2004

The energetic behavior of one‐dimensional (HF)n and (H2O)n chains with added or inserted H2O or HF was studied by means of the PCILOPSI method presented in Part I. The results were compared with the ones by the original PCILO method. The stabilization energies are in good qualitative agreement; energy differences due to impurity effects even quantitatively agree. The results show that the PCILOPSI method is well suited for the study of perturbed periodic molecular systems.
A formulation of the quantum‐mechanical many‐body problem in terms of hyperspherical coordinates
Authors: Avery John , Zhen‐Yi Wen
Published: December 2004

Hyperspherical coordinates are used to construct a matrix representation of a general N‐particle Hamiltonian in the case where the interaction is electrostatic. The Yukawa interaction can be treated similarly, as is shown in an appendix. The basis functions used to construct the matrix representation of H are mononomials inthe 3N coordinates of the particles, multiplied by functions of the hyperradius. Methods for transforming from this representation to a symmetry‐adapted representation are also discussed.
Unitary group approach to the many‐electron problem. III. Matrix elements of spin‐dependent Hamiltonians
Authors: Gould M. D. , Chandler G. S.
Published: December 2004

This is the final paper in a series of three directed toward the evaluation of spin‐dependent Hamiltonians. In this paper we derive the reduced 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 a direct evaluation of the matrix elements of spin‐dependent Hamiltonians in the spin‐orbit basis. An alternative (indirect) method, which employs the use of U(2n) ↓ U(n) × U(2) subduction coefficients, is also discussed.
Unitary group approach to crystal field theory: Projection operator approach
Authors: Kent R. D. , Schlesinger M.
Published: December 2004

A treatment for a strong crystal field of cubic symmetry is given using the unitary group approach. Projection operator techniques are utilized to achieve results equivalent to those given recently by Wen Zhenyi.
Geometrical derivatives of dipole moments and polarizabilities
Authors: Simons Jack , Jørgensen Poul
Published: December 2004

Molecular 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

The 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

Ab 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

A 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

Ab 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

A 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

In 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

A 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

Calculations 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

Most 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.
Theoretical studies of reaction mechanism in chemistry
Authors: Havlas Zdeněk , Zahradník Rudolf
Published: December 2004

After defining reaction mechanism, reaction path, reaction coordinate, reaction profile, and classical trajectories, dynamic and static approaches suitable (or promising) for analysis of reaction mechanism are critically discussed.
Gas phase unimolecular 1,1‐hydrogen elimination: Reaction mechanism and isotope effect
Authors: Barone Vincenzo , Bianchi Nadia , Lelj Francesco , Russo Nino
Published: December 2004

The 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

In 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

A 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

Some 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

The 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

Charge 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

In 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

After 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

In 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

The 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

It 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

The 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

Enzymes 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

A 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.
Chairman'S remarks, session on systems containing heavy atoms: Paradoxes and problems of large‐molecule and heavy‐molecule calculations
Authors: Gáspár R.
Published: December 2004

Two paradoxes are shown to exist: one in conection with spin and one in connection with computation on large (biological) molecules. Some possible mathematical and physical methods are listed and discussed that may help in the reduction of the problems.
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

The 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

LCGTO‐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

Three‐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).
Accurate local‐space treatment of hydrogen bonding in large systems
Authors: Kirtman Bernard , Demelo Celso P.
Published: December 2004

Tests of the local‐space approximation for electronic structure are carried out on a variety of hydrogenbonded systems. Regardless of size they are all found to be well localized in the sense of this approximation. For a local space defined by the valence atomic orbitals on XH ··· Y, one obtains a bond energy within 6% of the “exact” full‐space value. The potential energy surface and the electronic charge distribution are also accurately determined. In contrast with cluster models the local‐space approximation satisfactorily accounts for long‐range electrostatic and polarization effects.