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  • 1
    Digitale Medien
    Digitale Medien
    Structures of the aluminum oxides studied by ab initio methods with natural bond orbital analysis (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 3420-3430 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: We present a comprehensive theoretical analysis for the low-lying isomeric structures, energetics, and vibrational properties of dinuclear aluminum oxides Al2On (n=1–4) to aid interpretation of experimental spectroscopic data for these species. We also carried out natural population and natural bond orbital (NBO) analysis of the correlated and uncorrelated ab initio wave functions in order to elucidate the general bonding principles governing these species. We find that the equilibrium structures generally exhibit high ionic character (viz., effective Al3+ and O2− ionic units), but with significant modifications due to covalency, which is generally enhanced by electron correlation. Although certain previous experimental assignments are confirmed by our studies, in other cases the theoretical results strongly contradict suggested assignments. For a significant number of the reported experimental lines, we currently have no good theoretical candidate species.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.462978
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  • 2
    Digitale Medien
    Digitale Medien
    Natural bond orbitals in multiconfigurational expansions: Local treatment of electron correlation in molecules (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 1095-1108 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: We describe a localized treatment of electron correlation in terms of complete active-space wave functions derived from natural bond orbitals, closely related to the localized transferable units of molecular structure. We find that in many cases the procedure leads to solutions which retain the essentially localized character of their "parent'' orbitals. Such localized solutions lead to an extremely selective treatment of electron correlation in molecules, allowing one to readily extend the concepts of electron correlation in diatomic molecules ("left-right,'' "in-out,'' "angular,'' etc.) to the general polyatomic case using a familiar chemical language. We illustrate the procedure with applications to various diatomic (Li2, CO), triatomic (H2O, symmetric and unsymmetric dissociation), and polyatomic (formaldehyde ground and excited states; malonaldehyde) species, in order to demonstrate the important advantages of this procedure with respect to numerical behavior (improved convergence, reduced dimensionality, easy implementation with standard program packages) and conceptual utility (selectivity, transferability, description in a language common to all chemists).
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.463289
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  • 3
    Digitale Medien
    Digitale Medien
    Boron oxides: Ab initio studies with natural bond orbital analysis (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 1329-1335 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: We employ ab initio theory and natural bond orbital (NBO) analysis to describe the structure, energetics, vibrational properties, and bonding in small boron oxides, BmOn, supplementing recent studies on isovalent aluminum oxide clusters, Al2On, in order to extend the overview of bonding tendencies in group IIIA metal oxides. The comparison of analogous boron and aluminum species reveals many surprising differences, such as the V-shaped (M=B) vs linear (M=Al) geometry of M2O3, the altered tendency toward cyclic structures (higher for M2O2 with M=Al, for M3O3 with M=B), and the diminished role of electron correlation in the boron congeners. Correlation effects are examined by a recently introduced selective, localized multiconfigurational approach. Differences in bonding patterns are traced to basic hybridization and electronegativity shifts (reduced ionic character of B–O vs Al–O bond). Detailed comparisons with recent experimental BmOn data (including isotopomer ir shifts) provide additional support for theoretical assignments in corresponding aluminum species, where significant disagreements between theory and experiment persist.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.464299
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  • 4
    Digitale Medien
    Digitale Medien
    Many-body potentials and dynamics based on diatomics-in-molecules: Vibrational frequency shifts in ArnHF (n=1–12,62) clusters (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 5510-5516 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The conjecture that limited basis diatomics-in-molecules type potentials may serve as an accurate representation of many-body interactions is explored through molecular dynamics simulations of ArnHF (n=1–12,62). The important ingredient in the constructed potentials is the inclusion of ionic configurations of HF. Once the admixture between ionic and covalent configurations is calibrated by reference to an ab initio surface of the ArHF dimer, a single three-body potential energy surface is defined, and used in subsequent simulations of larger clusters. The vibrational frequencies of HF, which are computed from velocity–velocity autocorrelation functions, quantitatively reproduce the cluster size dependent redshifts. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.471790
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  • 5
    Digitale Medien
    Digitale Medien
    Hydrogen bonding described through diatomics-in-ionic-systems: The HF dimer (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 4413-4425 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: With the proper inclusion of ion-pair configurations, the diatomics-in-molecules formalism can be used to accurately describe hydrogen bonding. This is demonstrated for the well characterized prototype, the HF dimer, the structure and entire potential energy surface of which is reproduced within its known accuracy: At the stationary points (potential minimum and saddle points) energies and bond lengths are reproduced with an accuracy of ∼1%, and the soft hydrogen bond angles are determined to within ∼5%. This is accomplished through a minimal basis Hamiltonian—19-dimensional matrix to describe the planar complex—constructed with analytic fits to accurately known or determined pair potentials. The construct includes the H+F− ion-pair states of the HF monomer units. The three-body nature of the inductive ion-pair interactions with neutrals is preserved, in the spirit of diatomic-in-ionic-systems. Based on ab initio estimates, in the limited range of interest, a Gaussian function describes the mixing between ionic and neutral states. The amplitude of this function is the only adjustable parameter in the model. The ionicity anisotropy and nonadditivity of interactions, responsible for the structure of the HF dimer, result naturally from mixing between ionic and neutral surfaces. © 1998 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.475881
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  • 6
    Digitale Medien
    Digitale Medien
    Diatomics-in-molecules description of the Rg–Hal2 rare gas–halogen van der Waals complexes with applications to He–Cl2 (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 4575-4588 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The diatomics-in-molecules (DIM) technique is applied for a description of the low-lying states of the Rg–Hal2 van der Waals complexes correlating with the lowest states of constituent atoms Rg(1S)+Hal(2Pj)+Hal(2Pj). The important feature of this approach is the construction of polyatomic basis functions as products of the Hal2 diatomic eigenstates classified within the Hund "c" scheme and the atomic rare gas wave function. Necessary transformations to the other basis set representations are described, and finally all the matrix elements are expressed in terms of nonrelativistic adiabatic energies of Hal2 and Rg Hal fragments and spin-orbit splitting constant of the halogen atom. Our main concern is to test the DIM-based approximations of different levels taking the He–Cl2 system as an example. Namely, we have compared the results obtained within a hierarchy of approaches: (1) the simplest pairwise potential scheme as a far extreme of the DIM model, (2) the same as (1) but with the different components (Σ and Π) for He–Cl interaction, (3) the accurate DIM technique without spin-orbit terms, and (4) the highest level which takes into account all these contributions. The results have been compared to the other DIM like models as well. The shapes of two-dimensional potential surfaces for the ground (X) and excited (B) states of HeCl2, binding energies De with respect to He+Cl2, stretching and bending vibrational frequencies of the complex, binding energies D0, and spectral shifts for the B←X transition are discussed. © 1997 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.473499
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  • 7
    Digitale Medien
    Digitale Medien
    Importance of correlation in LiBO (1985)
    Springer
    Theoretical chemistry accounts 67 (1985), S. 287-292 
    Details
    ISSN: 1432-2234
    Schlagwort(e): Electronic correlation ; LiBO
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Notizen: Abstract The ground state energies of linear isomers LiBO and LiOB have been computed using the SCF, CASSCF and CI approximations. The changes in relative energies of both isomers due to various approximations are attributed to a different description of partial occupancies of the virtual π-orbitals in the multiconfigurational approaches. The ordering scheme is developed for both virtual canonical SCF and secondary CASSCF orbitals for use in the restricted CI calculations. An extrapolation procedure is proposed for better estimates of the correlation energy. Under the proper treatment of correlation, the energy of LiBO is lower than the energy of LiOB.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00529300
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  • 8
    Digitale Medien
    Digitale Medien
    Study of the geometric structure of the molecules of H2O, Li2O, and LiOH by the method of diatomic fragments in molecules (1979)
    Springer
    Journal of structural chemistry 19 (1979), S. 665-671 
    Details
    ISSN: 1573-8779
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Notizen: Conclusion The results of the present work indicate that the DFM method can be used successfully to determine the geometric parameters of the ground states of triatomic molecules. The additional possibilities provided by the method are associated with the variation of the parameters of the theory which are not single-valued, in particular the parameters of the mixing of states of the same symmetry of the diatomic fragments. It has also been shown that the correct choice of the model of the molecule is of considerable importance.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00752827
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  • 9
    Digitale Medien
    Digitale Medien
    A study of the potential surface of the groud state of HCN by the method of diatomic fragments in molecules (1981)
    Springer
    Journal of structural chemistry 22 (1981), S. 147-152 
    Details
    ISSN: 1573-8779
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00745181
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  • 10
    Digitale Medien
    Digitale Medien
    Calculations of the potential energy surfaces of molecular systems (1983)
    Springer
    Journal of structural chemistry 24 (1983), S. 319-320 
    Details
    ISSN: 1573-8779
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00747400
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