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  • 1
    Electronic Resource
    Electronic Resource
    Strained delocalized carbenoid ring systems — A theoretical investigation (1996)
    Springer
    Theoretical chemistry accounts 93 (1996), S. 17-33 
    Details
    ISSN: 1432-2234
    Keywords: Ring compounds ; Carbenes ; Quantum chemical methods ; CASSCF ; CASPT2
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract A number of ring compounds containing a divalent carbon center (carbenes) have been studied usingab initio quantum chemical methods. The studied systems include: imidazol-2-ylidene, 4-pyranylidene, 9-xanthylidene, cyclohexa-2,5-dienylidene and 4-oxocyclohexa-2,5-dienylidene. Extended ANO type basis sets were used. Wave functions and energies were obtained with a multiconfigurational approach (CASSCF), where dynamic correlation effects are treated by using second-order perturbation theory (CASPT2). The singlet-triplet splitting has been found to depend linearly on the energy separation between the two carbene orbitals. All systems, where this splitting is larger than about 10 eV have been found to have a singlet ground state, while those with a smaller gap have a triplet ground state. A number of excited states have been characterized. Computed excitation energies are in agreement with experiment in cases where such information is available.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01113855
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  • 2
    Electronic Resource
    Electronic Resource
    Density matrix averaged atomic natural orbital (ANO) basis sets for correlated molecular wave functions (1990)
    Springer
    Theoretical chemistry accounts 77 (1990), S. 291-306 
    Details
    ISSN: 1432-2234
    Keywords: Atomic natural orbitals ; Basis sets ; General contraction
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary Generally contracted basis sets for first row atoms have been constructed using the Atomic Natural Orbital (ANO) approach, with modifications for allowing symmetry breaking and state averaging. The ANOs are constructed by averaging over several atomic states, positive and negative ions, and atoms in an external electric field. The contracted basis sets give virtually identical results as the corresponding uncontracted sets for the atomic properties, which they have been designed to reproduce. The design objective has been to describe the ionization potential, the electron affinity, and the polarizability as accurately as possible. The result is a set of well-balanced basis sets for molecular calculations. The starting primitive sets are 8s4p3d for hydrogen, 9s4p3d for helium, and 14s9p4d3f for the heavier first row atoms.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01120130
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  • 3
    Electronic Resource
    Electronic Resource
    Density matrix averaged atomic natural orbital (ANO) basis sets for correlated molecular wave functions (1995)
    Springer
    Theoretical chemistry accounts 90 (1995), S. 87-114 
    Details
    ISSN: 1432-2234
    Keywords: Atomic natural orbitals ; Basis sets ; General contraction
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary Generally contracted Basis sets for the atoms H-Kr have been constructed using the atomic natural orbital (ANO) approach, with modifications for allowing symmetry breaking and state averaging. The ANO's are constructed by averaging over the most significant electronic states, the ground state of the cation, the ground state of the anion for some atoms and the homonuclear diatomic molecule at equilibrium distance for some atoms. The contracted basis sets yield excellent results for properties of molecules such as bond-strengths and-lengths, vibrational frequencies, and good results for valence spectra, ionization potentials and electron affinities of the atoms, considering the small size of these sets. The basis sets presented in this article constitute a balanced sequence of basis sets suitable for larger systems, where economy in basis set size is of importance.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01113842
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  • 4
    Electronic Resource
    Electronic Resource
    A CASPT2 study of the valence and lowest Rydberg electronic states of benzene and phenol (1995)
    Springer
    Theoretical chemistry accounts 91 (1995), S. 91-108 
    Details
    ISSN: 1432-2234
    Keywords: C6H6 ; C6H6O ; Electronic excitation ; ab initio ; CASPT2 ; Oscillator strength ; Rydberg states
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary The valence excited states and the 3s, 3p, and 3d (united atom) Rydberg states of benzene and phenol have been obtained by the CASPT2 method, which computes a second-order perturbation correction to complete active space self-consistent field (CASSCF) energies. All non-zero dipole oscillator strengths are also computed, at the CASSCF level. For benzene, 16 singlet and 16 triplet states with excitation energies up to ca. 7.86 eV (63 400 cm–1) are obtained. Of these, 12 singlet and three triplet energies are experimentally known well enough to allow meaningful comparison. The average error is around 0.1 eV. The highest of these singlet states (21 E2g) is the highest valence ππ* state predicted by elementary π-electron theory. Its energy is then considerably lower than has been suggested from laser flash experiments, but in perfect agreement with a reinterpretation of that experiment. For phenol, 27 singlet states are obtained, in the range 4.53–7.84 eV (63 300 cm−1). Only the lowest has a well-known experimental energy, which agrees with the computed result within 0.03 eV. The ionization energy is in error by 0.05 eV.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01113865
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  • 5
    Electronic Resource
    Electronic Resource
    Density matrix averaged atomic natural orbital (ANO) basis sets for correlated molecular wave functions (1995)
    Springer
    Theoretical chemistry accounts 92 (1995), S. 149-181 
    Details
    ISSN: 1432-2234
    Keywords: Basis sets ; Atomic natural orbitals ; General contraction
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Generally contracted basis sets for the first row transition metal atoms Sc-Zn have been constructed using the atomic natural orbital (ANO) approach, with modifications for allowing symmetry breaking and state averaging. The ANOs are constructed by averaging over the three electronic configurationsd n ,d n−1 s, andd n−2 s 2 for the neutral atom as well as the ground state for the cation and the ground state atom in an external electric field. The primitive sets are 21s15p10d6f4g. Contraction to 6s5p4d3f2g yields results that are virtually identical to those obtained with the corresponding uncontracted basis sets for the atomic properties, which they have been designed to reproduce. Slightly larger deviations are obtained with the 5s4p3d2f1g for the polarizability, while energetic properties still have only small errors. The design objective has been to describe the ionization potential, the polarizability and the valence spectrum as accurately as possible. The result is a set of well-balanced basis sets for molecular calculations, which can be used together with basis sets of the same quality for the first and second row atoms.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01114922
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  • 6
    Electronic Resource
    Electronic Resource
    Polarized basis sets and the calculation of infrared intensities from nuclear electric shielding tensors (1985)
    Springer
    Theoretical chemistry accounts 68 (1985), S. 431-444 
    Details
    ISSN: 1432-2234
    Keywords: Basis sets ; Perturbation-dependent basis sets ; Polarized basis sets ; Hellmann-Feynman theorem ; Infrared intensities ; Molecular properties ; H2O
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The idea of the basis set polarization which follows from the known dependence of basis set functions on the perturbation strength is applied to the calculation of the dipole moment derivatives with respect to nuclear displacements. The differentiation of the dipole moment function is replaced by the straightforward evaluation of derivatives of the intramolecular electric field with respect to the external electric field strength. The method and its efficiency are illustrated by a series of calculations of the dipole moment derivatives for the water molecule. Already a polarized basis set of 26 CGTO's derived from the minimal CGTO basis set provides fairly reasonable results.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00527668
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  • 7
    Electronic Resource
    Electronic Resource
    A CASPT2 study of the valence and lowest Rydberg electronic states of benzene and phenol (1995)
    Springer
    Theoretica chimica acta 91 (1995), S. 91-108 
    Details
    ISSN: 0040-5744
    Keywords: Key words: C6H6 ; C6H6O ; Electronic excitation ; ab initio ; CASPT2 ; Oscillator strength ; Rydberg states
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary.  The valence excited states and the 3s, 3p, and 3d (united atom) Rydberg states of benzene and phenol have been obtained by the CASPT2 method, which computes a second-order perturbation correction to complete active space self-consistent field (CASSCF) energies. All non-zero dipole oscillator strengths are also computed, at the CASSCF level. For benzene, 16 singlet and 16 triplet states with excitation energies up to ca. 7.86 eV (63 400 cm-1) are obtained. Of these, 12 singlet and three triplet energies are experimentally known well enough to allow meaningful comparison. The average error is around 0.1 eV. The highest of these singlet states (21E2 g) is the highest valence ππ* state predicted by elementary π-electron theory. Its energy is then considerably lower than has been suggested from laser flash experiments, but in perfect agreement with a reinterpretation of that experiment. For phenol, 27 singlet states are obtained, in the range 4.53–7.84 eV (63 300 cm-1). Only the lowest has a well-known experimental energy, which agrees with the computed result within 0.03 eV. The ionization energy is in error by 0.05 eV.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01113865
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  • 8
    Electronic Resource
    Electronic Resource
    Strained delocalized carbenoid ring systems – A theoretical investigation (1996)
    Springer
    Theoretica chimica acta 93 (1996), S. 17-33 
    Details
    ISSN: 0040-5744
    Keywords: Key words: Ring compounds ; Carbenes ; Quantum chemical methods ; CASSCF ; CASPT2
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract.  A number of ring compounds containing a divalent carbon center (carbenes) have been studied using ab initio quantum chemical methods. The studied systems include: imidazol-2-ylidene, 4-pyranylidene, 9-xanthylidene, cyclohexa-2,5-dienylidene and 4-oxocyclohexa-2,5-dienylidene. Extended ANO type basis sets were used. Wave functions and energies were obtained with a multiconfigurational approach (CASSCF), where dynamic correlation effects are treated by using second-order perturbation theory (CASPT2). The singlet–triplet splitting has been found to depend linearly on the energy separation between the two carbene orbitals. All systems, where this splitting is larger than about 10 eV have been found to have a singlet ground state, while those with a smaller gap have a triplet ground state. A number of excited states have been characterized. Computed excitation energies are in agreement with experiment in cases where such information is available.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01113855
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  • 9
    Electronic Resource
    Electronic Resource
    Density matrix averaged atomic natural orbital (ANO) basis sets for correlated molecular wave functions IV. Medium size basis sets for the atoms H—Kr (1995)
    Springer
    Theoretica chimica acta 90 (1995), S. 87-114 
    Details
    ISSN: 0040-5744
    Keywords: Key words: Atomic natural orbitals ; Basis sets ; General contraction
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary.  Generally contracted Basis sets for the atoms H–Kr have been constructed using the atomic natural orbital (ANO) approach, with modifications for allowing symmetry breaking and state averaging. The ANO’s are constructed by averaging over the most significant electronic states, the ground state of the cation, the ground state of the anion for some atoms and the homonuclear diatomic molecule at equilibrium distance for some atoms. The contracted basis sets yield excellent results for properties of molecules such as bond-strengths and -lengths, vibrational frequencies, and good results for valence spectra, ionization potentials and electron affinities of the atoms, considering the small size of these sets. The basis sets presented in this article constitute a balanced sequence of basis sets suitable for larger systems, where economy in basis set size is of importance.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01113842
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