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  • 1
    Electronic Resource
    Electronic Resource
    Theoretical studies on inorganic ring systems. Tetraphosphabicyclobutane, cyclotriphosphane, and white phosphorus: ring strain and hybridization (1986)
    s.l. : American Chemical Society
    Inorganic chemistry 25 (1986), S. 4382-4385 
    Details
    ISSN: 1520-510X
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/ic00244a020
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  • 2
    Electronic Resource
    Electronic Resource
    CEPA calculations on open-shell molecules. 7. Electronic structure and properties of HNS (1986)
    s.l. : American Chemical Society
    Inorganic chemistry 25 (1986), S. 4221-4228 
    Details
    ISSN: 1520-510X
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/ic00243a032
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  • 3
    Electronic Resource
    Electronic Resource
    Substituent effects on bonding properties in diphosphenes, disilenes and diimines (1984)
    s.l. : American Chemical Society
    Inorganic chemistry 23 (1984), S. 3369-3373 
    Details
    ISSN: 1520-510X
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/ic00189a020
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  • 4
    Electronic Resource
    Electronic Resource
    Ab initio calculations of van der Waals interactions in one- and two-dimensional infinite periodic systems (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 2603-2614 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new CEPA-PNO (coupled electron pair approach with pair natural orbitals) method for the calculation of correlation energies in infinite periodic systems is proposed and applied to one- and two-dimensional He. The method starts from a crystal orbital Hartree-Fock (COHF) wavefunction with the occupied Bloch orbitals transformed into Wannier orbitals. The coupled-cluster equations for the infinite system are simplified by CEPA-type approximations: A CEPA-0 (or linear coupled-cluster) formula is applied for the small intercell contributions to the total correlation energy while CI-SD, ACPF or other CEPA variants are used for the large intracell contributions. The enormous number of single and double excitations into the virtual space is greatly reduced by the use of pair natural orbitals (PNOs), which leads to large savings in the necessary computer time and disk storage. First applications to the van der Waals interaction in the linear chain and the hexagonal plane of He atoms, performed with medium size and large atomic basis sets, show that an accuracy can be reached for the infinite systems which is comparable to the accuracy of the corresponding calculations for small He clusters. Because of the extended use of the translational symmetry of the Wannier orbitals, the calculations for the linear infinite systems are even considerably faster than those for the oligomers He5 and He7. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470521
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  • 5
    Electronic Resource
    Electronic Resource
    Ab initio calculations for the adsorption of small molecules on metal oxide surfaces. I. Cluster calculations for carbon monoxide CO on nickel oxide NiO(100) (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 2583-2592 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Quantum chemical ab initio calculations for the adsorption of CO on the NiO(100) surface have been performed at different levels of accuracy: self-consistent field (SCF), complete active space self-consistent field, and coupled electron pair approximation. Basis sets of double zeta and triple zeta + polarization (TZP) quality have been used. The NiO(100) surface is represented by a cluster containing one Ni2+ ion and the five adjacent O2− ions. The charge neutrality of the cluster and the saturation of the dangling bonds is achieved by adding eight protons, which gives the total composition Ni(H2O)3(OH)2. Alternatively, the Ni2+(O2−)5 unit is embedded in a lattice of point charges which correctly represent the half-infinite ionic crystal. In the most favorable configuration, CO is adsorbed linearly in the on-top position on the Ni2+ ion, with the C atom pointing toward the surface. The binding energies at the SCF level are rather small, only 0.08 eV and 0.03 eV for CO and OC bound to the cluster (TZP basis set, counterpoise correction for the basis set superposition error included). For the two configurations the equilibrium Ni–C and Ni–O distances are 5.40 and 5.50 a0, respectively. Electron correlation does not change these values markedly. Estimating the errors in our calculation we arrive at binding energies of 0.10±0.05 and 0.05±0.05 eV, respectively, for CO and OC on NiO(100). This is in agreement with the experimental estimates. The bonding is attributed predominantly to electrostatic and inductive forces. No genuine "chemical'' bond (overlap, charge transfer) exists between CO and the NiO(100) surface, i.e., CO is only physisorbed to this ionic surface. The harmonic vibration frequencies for the Ni–C stretching and the Ni–C–O bending vibrations are estimated to 52 and 139 cm−1, respectively.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463098
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  • 6
    Electronic Resource
    Electronic Resource
    Ab initio calculation of the lowest singlet and triplet states in CH2, CHF, CF2, and CHCH3 (1974)
    Springer
    Theoretical chemistry accounts 35 (1974), S. 309-327 
    Details
    ISSN: 1432-2234
    Keywords: Small carbene radicals ; Singlet-triplet energy separation ; Pair correlation energies ; Equilibrium bond angles
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The lowest singlet and triplet states of the radicals CH2, CHF, CF2, and CHCH3 have been investigated both in SCF and IEPA approximation (“independent electron pair approach” to account for electron correlation). The SCF calculations yield triplet ground states for CH2, CHF, and CHCH3, and a singlet ground state for CF2. Electron correlation stabilizes the singlet state by about 14 kcal/mole with respect to the triplet for all four radicals leading to a singlet ground state also for CHF. The final triplet-singlet energy separations are 10, 6, −11, −47 kcal/mole for CH2, CHCH3, CHF, CF2, respectively. Values for equilibrium bond angles, ionization potentials and bond energies are also given.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00548481
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  • 7
    Electronic Resource
    Electronic Resource
    Note on open shell restricted SCF calculations for rotation barriers about C-C double bonds: Ethylene and allene (1977)
    Springer
    Theoretical chemistry accounts 45 (1977), S. 89-94 
    Details
    ISSN: 1432-2234
    Keywords: Rotation barriers about C-C double bonds ; Ethylene ; Allene
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract It is shown thatab initio open-shell SCF calculations yield acceptable results for rotation barriers about double bonds in contrast to more conventional closed-shell SCF calculations. Using basis sets of double zeta+polarization quality the SCF values for the rotation barriers of ethylene and allene are obtained to be 48 and 50 kcal/mole, respectively. An IEPA estimate of the influence of electron correlation leads to values of 64 and 52 kcal/mole, respectively, which are in reasonable agreement with the experimental values.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00552543
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  • 8
    Electronic Resource
    Electronic Resource
    Ab initio Study, Including Electron Correlation, of the Electronic Structures, the Dipole Moments, the Static Polarizabilities and of the Harmonic Fields of H2CO, H2CS and H2SiO (1980)
    Springer
    Theoretical chemistry accounts 57 (1980), S. 337-337 
    Details
    ISSN: 1432-2234
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00552746
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  • 9
    Electronic Resource
    Electronic Resource
    A multi-configuration reference CEPA method based on pair natural orbitals (1993)
    Springer
    Theoretical chemistry accounts 87 (1993), S. 129-145 
    Details
    ISSN: 1432-2234
    Keywords: Coupled electron pair approximation (CEPA) ; Multi-reference configuration interaction ; Pair natural orbitals (PNOs)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary A multi-reference CI scheme is proposed which is aiming at a considerable reduction of the generally very large number of configurations of CI expansions in multi-configuration reference cases. This reduction is achieved by combining the idea of internal contraction, the concept of pair natural orbitals (PNO's) and CEPA (coupled electron pair) type approximations for the contributions of higher than double excitations. This latter estimate leads to size consistent results and also permits to employ reference wavefunctions that contain only the dominantly occupied configurations of the considered system. Applications to two test cases, the lowest states (3 P,1 D and1 S) of the carbon atom and the symmetry forbiddenC 2v insertion reaction of Be and H2, show that our method is able to truncate CI expansions to lengths of no more than 103–104 without losing more than 1–2% of the correlation energy. The calculated excitation energies and energy barriers agree with the full CI results in the respective basis within about 1 kcal/mol. Thus the MC-CEPA-PNO method presents a very efficient way to obtain “chemical accuracy” in CI-calculations for molecular systems.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01113534
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  • 10
    Electronic Resource
    Electronic Resource
    The direct determination of Brueckner orbitals with application to the H2 molecule (1972)
    Springer
    Theoretical chemistry accounts 24 (1972), S. 152-169 
    Details
    ISSN: 1432-2234
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Zusammenfassung Es wird eine Iterationsmethode beschrieben, die eine direkte Bestimmung genäherter Bruecknerorbitale für kleine atomare und molekulare Systeme gestattet. Ausgehend von der Hartree-Fock-Determinante bestimmt man zunächst Paar-NOs (PNOs) der unabhängigen Elektronenpaare im HF-Feld der übrigen Elektronen des Systems. Mit Hilfe des verallgemeinerten Brillouin-Theorems erhält man dann ein genähertes Bruecknerorbital für jedes Elektronenpaar. Dies Verfahren muß bis zur Selbstkonsistenz wiederholt werden, die man im allgemeinen nach 4–5 solcher Makroiterationen erreicht. Anwendung auf den Grundzustand des H2-Moleküls zeigt, wie wichtig die Benutzung von Bruecknerorbitalen ist, wenn man gute Erwartungswerte von Einelektronenoperatoren und das richtige asymptotische Verhalten der Potentialkurve bei großen Abständen erhalten will.
    Abstract: Résumé On propose un schéma itératif qui permet le calcul direct des orbitales de Brueckner approchées pour les atomes et les petites molécules. Partant du déterminant Hartree-Fock on calcule d'abord les PNO (pair natural orbitals) des paires indépendantes d'électrons dans le champ effectif des autres électrons dans l'approximation Hartree-Fock. Ensuite le théorème de Brillouin généralisé permet de calculer les orbitales de Brueckner approchées pour chaque paire d'electrons. On recommence le calcul des PNO, la “self-consistance” de cette méthode itérative s'établit en général au bout de 4 ou 5 macroitérations. Les applications à la molécule H2 démontrent que l'utilisation d'orbitales de Brueckner s'impose si l'on s'intéresse à des bonnes valeurs moyennes d'opérateurs monoélectroniques ou au comportement asymptotique correct des courbes de potentiel aux grandes distances.
    Notes: Abstract An iterative procedure is presented which allows for the direct determination of approximate Brueckner orbitals for small atomic and molecular systems. Starting from the Hartree-Fock determinant one first determines pair natural orbitals (PNOs) of independent electron pairs in the HF-field of the remaining electrons. The use of the generalized Brillouin-theorem then leads to an approximate Brueckner orbital for each electron pair. This procedure must be repeated up to self-consistency which is reached generally after 4–5 macroiterations. Applications to the ground state of H2 show how important the use of Brueckner orbitals is to get good expectation values of one-electron operators and the correct asymptotic behaviour of the potential energy curve for large internuclear distances.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00641396
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