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  • 1
    Electronic Resource
    Electronic Resource
    Modern valence bond representations of CASSCF wavefunctions (1996)
    Springer
    Theoretical chemistry accounts 93 (1996), S. 343-366 
    Details
    ISSN: 1432-2234
    Keywords: CASSCF ; Spin-coupled ; VB ; CASVB
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Exact transformations of “N electrons inN orbitals” CASSCF structure spaces are examined that lead to modern valence bond representations, in which the total wavefunction is dominated by covalent structures built from a common product ofnonorthogonal orbitals. The resulting descriptions of the electronic structure may be compared directly with those that arise in the spin-coupled (orfull-GVB) approach. Using singlet methylene, methane and ozone as representative examples, various overlap-based and energy-based criteria are investigated for generating modern VB representations of “N inN” CASSCF wavefunctions, which we denote CASVB.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01129215
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    Paper (German National Licenses)
    Fulltext
  • 2
    Electronic Resource
    Electronic Resource
    Symmetry adaptation and the utilization of point group symmetry in valence bond calculations, including CASVB (1997)
    Springer
    Theoretical chemistry accounts 95 (1997), S. 131-150 
    Details
    ISSN: 1432-2234
    Keywords: CASVB ; CASSCF ; VB ; Symmetry adaptation ; Spin-coupled
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract We present practical approaches for symmetry-adapting valence bond wave functions, with emphasis on the CASVB method. Significant savings in the computational effort become available, both in relation to the application of the Hamiltonian operator and to the reduced number of variational parameters. Results are presented for modern VB representations of CASSCF descriptions of benzene and diborane.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02341697
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    Paper (German National Licenses)
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  • 3
    Electronic Resource
    Electronic Resource
    Modern valence-bond description of chemical reaction mechanisms: the 1,3-dipolar addition of fulminic acid to ethyne (1998)
    Springer
    Theoretical chemistry accounts 100 (1998), S. 222-229 
    Details
    ISSN: 1432-2234
    Keywords: Key words: Spin-coupled approach ; 1 ; 3-Dipolar cycloadditions ; Reaction mechanisms ; Valence-bond theory
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract. The electronic mechanism for the gas-phase 1,3-dipolar cycloaddition of fulminic acid (HCNO) to ethyne is studied through a combination of modern valence-bond theory in its spin-coupled (SC) form and intrinsic reaction coordinate calculations utilizing a complete-active-space self-consistent field wavefunction. It is shown that the concerted reaction follows a “heterolytic” route, during which three orbital pairs corresponding to three distinct bonds in the reactants (an in-plane π bond in ethyne, and a C-N and an N-O in-plane bond in HCNO) shift simultaneously to create the two new bonds closing the isoxazole ring and a nitrogen lone pair. The analysis of the SC wavefunction strongly suggests that the reacting system remains nonaromatic throughout the most important part of the cycloaddition process. This investigation provides the first demonstration of an alternative SC description of a bond rearrangement, achieved through the movement of singlet orbital pairs through space, during which at least one of the orbitals within a pair becomes completely detached from the atomic centre with which it is associated initially and ends up localized about another centre.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s002140050383
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    Paper (German National Licenses)
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  • 4
    Electronic Resource
    Electronic Resource
    A New Approach to Valence Bond Calculations: CASVB (1997)
    Springer
    Molecular engineering 7 (1997), S. 67-85 
    Details
    ISSN: 1572-8951
    Keywords: Valence-bond wavefunctions ; non-unitary orbital transformations ; CASVB
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract We present a new approach to the fully-variational optimization of general types of valence-bond wavefunction. Analogous techniques may be used to generate modern valence-bond representations of CASSCF structure spaces. These various procedures, which we term CASVB, are made possible by a highly efficient algorithm for performing exactly the transformations of full CI spaces that arise from general, non-unitary orbital transformations. As examples, we consider modern valence-bond representations of 'N in N' CASSCF descriptions of methane and diborane, in which the total wavefunction is dominated by covalent structures built from a common product of N nonorthogonal orbitals. These two cases highlight some of the possible differences that may arise between representations of CASSCF wavefunctions and the corresponding fully-variational valence bond results.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1008270112260
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    Paper (German National Licenses)
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  • 5
    Electronic Resource
    Electronic Resource
    Modern valence bond representations of CASSCF wavefunctions (1996)
    Springer
    Theoretica chimica acta 93 (1996), S. 343-366 
    Details
    ISSN: 0040-5744
    Keywords: Key words: CASSCF ; Spin-coupled ; VB ; CASVB
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract.  Exact transformations of “N electrons in N orbitals” CASSCF structure spaces are examined that lead to modern valence bond representations, in which the total wavefunction is dominated by covalent structures built from a common product of nonorthogonal orbitals. The resulting descriptions of the electronic structure may be compared directly with those that arise in the spin-coupled (or full-GVB) approach. Using singlet methylene, methane and ozone as representative examples, various overlap-based and energy-based criteria are investigated for generating modern VB representations of “N in N” CASSCF wavefunctions, which we denote CASVB.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01129215
    Library Location Call Number Volume/Issue/Year Availability
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    Paper (German National Licenses)
    Fulltext
  • 6
    Electronic Resource
    Electronic Resource
    Fully variational optimization of modern VB wave functions using the CASVB strategy (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 439-451 
    Details
    ISSN: 0020-7608
    Keywords: CASVB ; modern VB ; nonorthogonal orbital optimization ; multiconfiguration spin-coupled ; CASSCF ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We outline the CASVB strategy, which may be used either to generate very compact modern valence bond representations of CASSCF wave functions or to optimize general types of modern VB wave function. A simple algorithm is presented for the elimination of redundant or constrained parameters from the appropriate second-order optimization problems. Selected results for three systems - benzene, the allyl radical, and LiH - are used to illustrate various ways in which CASVB procedures can be used to perform fully variational optimizations of nonorthogonal orbitals and structure coefficients in modern VB wave functions.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 439-451, 1997
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
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    Paper (German National Licenses)
  • 7
    Electronic Resource
    Electronic Resource
    Modern valence bond descriptions of molecular excited states: An application of CASVB (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 637-650 
    Details
    ISSN: 0020-7608
    Keywords: excited states ; modern valence bond theory ; CASVB ; spin-coupled wave function ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The valence bond description of excited states is considered in the context of the CASVB approach, with emphasis on the simultaneous optimization of orbitals and structure coefficients for highly general types of wave function, as well as the close connection to the complete active space self-consistent field (CASSCF) method. In addition to fully variational optimization of modern valence bond (VB) wave functions, one may also generate compact representations of CASSCF wave functions, essentially irrespective of their nature. Consequences of generalizing the CASVB methodology to other than the lowest-lying state of a given symmetry are explored for both the overlap- and energy-based optimization criteria. The methodology is illustrated by means of applications to the 21Ag state of trans-1,3-butadiene, the c˜1A1 state of methylene, and the first excited singlet state of benzene.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 637-650, 1998
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
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    Paper (German National Licenses)
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