摘要:

AbstractConceptual, logical and dynamical restrictions on the criteria characterizing the mutual bonding of particles are examined. In accord with these restrictions, observables consisting of fixed linear combinations of the particles' position and conjugate momentum are used to construct appropriate “boundedness” criteria. As a result, comparable localization of relative position and relative momentum are obtained for mutually bound particles; neither excessively compact nor excessively diffuse localization can result for bonding to oc

ISSN:0021-2148    

DOI:10.1002/ijch.198000002

出版商:WILEY‐VCH Verlag    

年代:1980

数据来源: WILEY

摘要:

AbstractIn this paper we review and exemplify a new and rigorous approach to the problem of molecular structure and its morphogenesis: the theory of quantum topology. The basis for this approach is provided by the topology of the total charge density in a given molecular system. The essential observation is that the only local maxima of a ground state distribution occur at the positions of the nuclei. The nuclei are therefore identified as point attractors of the gradient vector field of the charge density. The associated basins partition the molecular system into atomic fragments. Each atom is a stable structural unit defined as the union of an attractor and its basin. The common boundary of two neighbouring atomic fragments, the interatomic surface, contains a particular critical point, which generates a pair of gradient paths linking the two neighbouring attractors. The union of this pair of gradient paths and their endpoints is called a bond path. The network of bond paths defines a molecular graph of the system.Having defined a unique molecular graph for any molecular geometry, the total configuration space is partitioned into a finite number of regions. Each region is associated with a particular structure defined as an equivalence class of molecular graphs. A chemical reaction in which chemical bonds are broken and/or formed is therefore a trajectory in configuration space which must cross one of the boundaries between two neighbouring structural regions. These boundaries form the catastrophe set of the system which, like a phase diagram in thermodynamics, denotes the points of “balance” between neighbouring structures. A general analysis of the structural changes in an ABC type system is given in detail together with specific examples of all possible structural elements in a molecular system.The properties of the topologically defined atoms and their temporal changes are identified within a general formulation of subspace quantum mechanics. It is shown that the quantum mechanical partitioning of a system into subsystems coincides with the topological partitioning: both are defined by the same set of “zero flux” surfaces. Consequently the total energy, or any other property, is partitioned into additive atomic contributions.We show that, in general, a definite structure can be assigned to a given molecular system. Quantum mechanically this structure is associated with an open neighbourhood of the most probable nuclear geometry. Finally we generalize the notion of molecular structure to non‐isolated molecules and, in contrast to recent work by Woolley, we conclude that molecular structure exists in spite of intermolecular interactions and not as a resul

ISSN:0021-2148    

DOI:10.1002/ijch.198000003

出版商:WILEY‐VCH Verlag    

年代:1980

数据来源: WILEY

摘要:

AbstractA discussion of the quantum mechanics of molecules with the emphasis on quantum mechanical principles is presented in the expectation that a better understanding of the assumptions tacitly embodied in conventional quantum chemistry will follow. The discussion centres mainly on the position of the chemical structure hypothesis in a quantum mechanical theory based on quantum states. Various different kinds of molecular states are identified according to characteristic properties of their wavefunctions, ranging from the completely delocalized (fully symmetrized) eigenstates to the “molecular structure states” of rigid molecules. I suggest that the adiabatic separation of electrons and nuclei can be avoided by working with the generator coordinate method (GCM) which promises to be a powerful non‐adiabatic formalism for molecular quantum t

ISSN:0021-2148    

DOI:10.1002/ijch.198000004

出版商:WILEY‐VCH Verlag    

年代:1980

数据来源: WILEY

摘要:

AbstractThe idea that a single molecule has a shape—that is, a longlived nearly fixed (relative) arrangement of nuclei in space—has been shown by Woolley to be a classical idea imposed on the quantum mechanical picture of matter. In this report the argument that the Born–Oppenheimer approximation is semi‐classical is reviewed, and some pioneering computations free of the Born–Oppenheimer approximation are discussed. The problem of reconciling the results of these computations with the powerful and successful molecular structure theory is described, and a mode of computation is proposed which strikes a compromise between the sometimes inadequate Born–Oppenheimer Hamiltonian and the complicated full molecular

ISSN:0021-2148    

DOI:10.1002/ijch.198000005

出版商:WILEY‐VCH Verlag    

年代:1980

数据来源: WILEY

摘要:

AbstractThe classical concept of molecular structure (namely, a set of atoms with a well‐defined geometrical arrangement in space) originated well before the advent of quantum theory, and, contrary to widespread views, these two conceptual schemes (classical molecular structure and quantum theory) are not easy to reconcile. We begin with a preliminary discussion of molecular structure in terms of space and time correlations in the framework of quantum theory. Then we summarize the salient features of some critical views raised‐during recent years (essentially by Woolley) concerning the concept of molecular structure. The essential claim of this viewpoint is that “molecular structure” is not an intrinsic attribute of an isolated molecule, and appears rather as an effect of the environment (other molecules? vacuum electromagnetic field, according to quantum electrodynamics?). Afterwards, we discuss these issues in detail: we show that it is necessary to distinguish between a “quantum” or “potential” structure (which may be deduced in a straightforward way from the quantum treatment of the molecule), and the “classical” molecular structure (according to which the molecule is considered as a set of rotating and vibrating material points, corresponding to the atoms); we briefly discuss some aspects of the Born–Oppenheimer approximation. From our discussion, we deduce that the “classical structure” concept is indeed non‐trivial from a purely quantum‐mechanical point of view; in actual fact, it is related to the problem of the “classical limit” of quantum mechanics, and it appears that, at the present time, none of these classical concepts and behaviours is convincingly derivable from a strict quantum mechanical basis. Finally, we consider these same problems from the point of view of stochastic electrodynamics (SED), which is one of the stochastic models for microphysics proposed so far as a possible alternative or extension of usual quantum theory. In the framework of such a theory, the “classical limit” and, in particular, the “classical molecular structure” no longer poses a problem, but aside from these satisfactory qualitative aspects, several quantitative failures of SED prevent us from accepting it, at least in its present state, as a genuine physical alternative to quantum theory. We therefore conclude that “classical molecular structure”, and classical concepts in general, remain an open problem in

ISSN:0021-2148    

DOI:10.1002/ijch.198000006

出版商:WILEY‐VCH Verlag    

年代:1980

数据来源: WILEY

摘要:

AbstractFor purposes of interpretation and understanding, the valence bond orbital model of electronic structure provides a number of advantages over the more common molecular orbital model. This derives primarily from the unique description of valence bond orbitals in terms of highly localized bond pairs, lone pairs, etc. Optimization of the orbitals by a self‐consistent‐field procedure brings out features which were not evident from the semi‐empirical treatments of earlier workers. Calculations on CH4, NH3, H2O and H2S show that optimized valence bond orbitals are often bent even at the equilibrium nuclear configuration and generally do not follow the nuclei when bond angles are varied. Analysis of a large basis set calculation on H2O shows that the valence bond wavefunction can be fitted well with a minimal basis, making it possible to interpret more clearly the connection between hybridization and interorbital angles. It is shown how the valence bond method provides a rigorous basis for factorization even of difficult second order physical properties, such as polarizability, into separated bond and lone pair contributions. These contributions can be further decomposed into sums of atomic valence state values plus corrections due to bond polarization effects. Finally, the advantages of using the valence bond model as a starting point for the perturbation treatment of electron correlation are pointed out. Calculations on H2and LiH indicate that valence bond theory need only be carried through second order to obtain results comparable in accuracy to a third order treatment based on molecular orb

ISSN:0021-2148    

DOI:10.1002/ijch.198000007

出版商:WILEY‐VCH Verlag    

年代:1980

数据来源: WILEY

摘要:

AbstractScattering methods used to calculate the electronic structure of molecules and solids are reviewed, and it is suggested that scattering theory concepts should be further used and developed for extracting information about the chemical bonding in molecules and solids.

ISSN:0021-2148    

DOI:10.1002/ijch.198000008

出版商:WILEY‐VCH Verlag    

年代:1980

数据来源: WILEY

摘要:

AbstractThe pertinent features of the antisymmetrized geminal power are brought out with a view of describing the chemical bond and the electronic properties of molecules. It is demonstrated that for certain hamiltonians it is consistent with a response function derived from a simple moment expansion characteristic of the random phase approximation. Deficiencies with regard to “size consistency” are pointed out. Simple examples are analyzed in order to familiarize the reader with the nature of the approximati

ISSN:0021-2148    

DOI:10.1002/ijch.198000009

出版商:WILEY‐VCH Verlag    

年代:1980

数据来源: WILEY

摘要:

AbstractIn order to highlight some of the apparent contradictions between semiempirical andab initiotheories of valence, a brief review of some facets of the two approaches is presented. The recently developed effective valence shell Hamiltonian, , is then briefly introduced to bridge the gap. Results of purelyab initioHvcalculations for atoms and simple molecules are mentioned. Theab initioone‐center integrals are quantitatively much different from those of semiempirical theories. However, the exact contains extra terms, three‐ (or higher) electron interactions, which are totally absent in semiempirical theories. This paper shows that the nonclassical, three‐electron terms are incorporated into semiempirical one‐ and two‐electron integrals, and a parametrization scheme illustrating this is presented. As an example, the carbon system is chosen and the one‐center carbon semiempirical integrals are calculated from the results ofab initiocalculations for C and CH. This enables discussions of questions of the degree of transferability of semiempirical one‐center integrals. The one‐center one‐ and two‐electron integrals obtained fromab initiocalculations by including the effects of three‐electron terms are found to be close to the traditional

ISSN:0021-2148    

DOI:10.1002/ijch.198000010

出版商:WILEY‐VCH Verlag    

年代:1980

数据来源: WILEY

摘要:

AbstractStarting from a localized description of molecular wavefunctions, approximate descriptions of chemical groups are obtained. The degree of conservation of these representations is verified over a fairly large number of compounds by using different tests, from which it is seen that the influence of intramolecular interactions is strong enough to necessitate the inclusion of these effects in the description of molecular systems in terms of transferable models of groups. Further analyses demonstrate that intragroup interactions can be approximated, to a good degree of precision, by classical interactions only. The usefulness of a zeroth order representation of a group in terms of nonorthogonal transferable models of localized orbitals, coupled to an evaluation of classical intragroup interactions, is discussed, and a simple computational method for the evaluation of the changes in reaction barriers due to substitution of chemical groups in a given molecular framework is briefly outlined.

ISSN:0021-2148    

DOI:10.1002/ijch.198000011

出版商:WILEY‐VCH Verlag    

年代:1980

数据来源: WILEY