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  • 1
    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.
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  • 2
    ISSN: 0040-5744
    Keywords: Key words: Formaldehyde ; Electronic spectrum ; Perturbation theory
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary.  The electronically excited states of formaldehyde are examined by means of multiconfigurational second-order perturbation (CASPT2) theory with extended ANO-type basis sets. The calculations comprised five valence excited states plus all singlet 3s, 3p, and 3d members of the Rydberg series converging on the first ionization. The computed vertical excitation energies were found to be within 0.2 eV of the available experimental energies. Full geometry optimization has been performed for five valence excited states. Assuming a planar geometry, the “0–0” transition for the valence 1A1(π→π*) state is calculated to appear near 7.9 eV, close to the (n y →3p) region. This state is, however, not planar and the true adiabatic energy is 7.4 eV, which is 2.3 eV below the corresponding vertical transition.
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  • 3
    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.
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  • 4
    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.
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  • 5
    ISSN: 0040-5744
    Keywords: Key words: 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 configurations d n , d n-1 s,  and d 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.
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  • 6
    ISSN: 0040-5744
    Keywords: Key words: Electronic spectrum ; Methylene cyclopropene
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract.  The vertical electronic spectrum of methylenecyclopropene, the prototype of the nonalternant hydrocarbons known as fulvenes, has been studied using multiconfigurational second-order perturbation theory. The calculations comprise three valence states and the 3s, 3p,  and 3d members of the Rydberg series converging to the first π ionization limit. Vertical excitation energies to three valence states are found at 4.13, 6.12, and 6.82 eV. The second of them corresponds to an excitation from the highest occupied σ orbital to a π* orbital, while the other two are π→π* excitations. The third transition gives rise to the most intense feature in the electronic spectrum. The results are rationalized within the scheme of two interacting double bonds. Comparisons are made between this and the previous theoretical calculations of the electronic spectra of related systems and also between the available experimental data of methylenecyclopropene in solution.
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  • 7
    ISSN: 1520-5126
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
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  • 8
    ISSN: 1520-5126
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
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  • 9
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The ground (N) state and the 1B1u(V) excited state of planar ethylene have been studied at the CPF and MR-SDCI levels of theory, using an extended CGTO basis set of the ANO type. The investigation especially addresses the near-degeneracy problem in the ground state and the coupling between the diffuse character of the π* orbital and the amount of correlation included in the wave function of the V state. The MR-SDCI results yield a vertical excitation energy in the range 7.8-8.0 eV, whereas the CPF result is 7.9 eV. The best MR-SDCI result for 〈1π*‖z2‖1π*〉 is 7.8, whereas CPF calculations based on MR-SDCI INOS give the value 6.7. It is clear from the results that these numbers have not converged and that more extended calculations than was possible in the present work would yield an even more compact wave function.
    Additional Material: 9 Ill.
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  • 10
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Multiconfigurational second-order perturbation theory is tested for the calculation of molecular structure and binding energies. The scheme is based on the Complete Active Space (CAS) SCF method, which gives a proper description of the major features in the electronic structure, independent of its complexity, accounts for all near degeneracy effects, and includes full orbital relaxation. Remaining dynamic electron correlation effects are in a subsequent step added using second-order perturbation theory with the CASSCF wave function as the reference state (CASPT2). The approach is applied to the calculation of equilibrium geometry and atomization energies for 27 benchmark molecules containing first-row atoms (the “G1” test). Large atomic natural orbital (ANO)-type basis sets are applied (5s4p3d2f for Li—F and 3s2p1d for H). It is shown that the CASSCF/CASPT2 approach is able to predict the equilibrium geometry with an accuracy better than 0.01 Å for bond distances and 0°-2° for bond angles. Calculated atomization energies are underestimated with between 3 and 6 kcal/mol times the number of extra electron pairs formed. The error in the heat of reaction for a number of isogyric reaction (no difference in number of pairs) varies between -2.5 and +1.0 kcal/mol. The same type of accuracy is obtained in calculations for excited states. The molecules B2, C2, FO, FOO, and FOOF have also been studied. Results for the first three molecules are in accordance with those of the benchmark molecules. The FO bond distance in FOO is predicted to be 0.02 Å longer than experiment. The heat of formation for FOO is computed to be 2.9 kcal/mol with an uncertainty of ±3 kcal/mol. Preliminary results for FOOF (obtained with a smaller basis set) indicate that the approach yields a somewhat too long FO bond distance (1.64 Å compared to 1.58 Å experimentally). © 1993 John Wiley & Sons, Inc.
    Additional Material: 4 Tab.
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