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  • 1
    Electronic Resource
    Electronic Resource
    On the radiative lifetimes of the a 3Σ+ state of NO+ (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 456-465 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The radiative lifetimes of the lowest excited triplet state of NO+ have been evaluated using new integral and CI programs for the spinless and the spin dependent components of the molecular Hamiltonian. The Rayleigh–Schrödinger perturbation theory, compared to direct CI techniques, provides a numerically more stable and rapidly convergent expansion for the spin–orbit terms of the molecular wave functions. Based on this theory, we have performed the analysis of the Rayleigh–Schrödinger components of the singlet–triplet transition moment and found that the most important contribution to the radiative lifetimes is given by the second excited B 1Π state and not by the lowest A 1Π state as previously assumed on energy grounds. The radiative lifetimes computed for the lowest five vibrational states of a 3Σ+, resulting from the difference of two large contributions from the B 1Π and the A 1Π states, are found extremely sensitive to small variations in the computed 〈1Σ+||μ||B 1Πβ〉 transition dipole function and to the 〈a 3Σ+,1,1||HLαSα||A 1Πβ,0, 0〉 spin–orbit interaction. Using matrix elements evaluated at two levels of CI, the radiative lifetimes are estimated 330∼270 and 270∼250 ms, respectively.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465769
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  • 2
    Electronic Resource
    Electronic Resource
    Hartree–Fock operators to improve virtual orbitals and configuration interaction energies (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 5849-5856 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The definitions of Hartree–Fock operators able to provide virtual orbitals more suited for correlation and excited electronic states and for configuration interaction (CI) treatments have been reviewed. From the comparisons, a simple procedure to improve these operators has emerged and discussed. In our approach, the sign of the pair and total electronic densities is changed to make the interelectron potential attractive for excited electrons. The orbitals generated from the modified operators have been compared to canonical HF orbitals by performing large scale CI computations on ground and excited states of the NO+ molecule improving the CI energies and the dipole moments for all states and the convergence properties of CI. Similarly, using truncated orbital subspaces, the ground state MP2 correlation energy becomes closer to the basis limit for this property.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467148
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  • 3
    Electronic Resource
    Electronic Resource
    The S0(1Ag)–S1(1B2u) vibronic transition in benzene: An ab initio study (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 2458-2464 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The four e2g false origin bands and the a1g progression of the S0(1Ag)–S1(1B2u) transition in benzene are simulated ab initio with the recently introduced configuration interaction singles (CIS) with 6-31G orbitals. The ground and excited state CC and CH bond lengths are optimized and compared with the experiment; the CC bond elongation upon excitation is found to be slightly underestimated. The vibrational force fields are calculated at the stationary points of S0 and S1. The 1Ag force field is calculated at the Hartree–Fock level while the 1B2u force field is calculated at the CIS level of theory. The two force fields are scaled to fit the experimental frequencies and the normal mode rotation upon excitation, i.e., the Duschinsky matrix, is obtained. In agreement with previous empirical fitting of the S1(1B2u) vibrational frequencies, the Duschinsky matrix is found to be nearly diagonal with the exception of the b2u modes submatrix which shows a large amount of mixing. The mixing of the b2u modes is larger before scaling but is subsequently reduced after scaling. The normal modes and the optimized geometries are used to calculate the amount of displacement, upon excitation, of the equilibrium position of the totally symmetric modes. This displacement causes the Franck–Condon progression and is slightly underestimated by the calculation. The intensity of the four e2g false origins in the absorption spectrum of S1 is calculated and the Herzberg–Teller intensities of the four bands are found to be very close to the experiment. In particular, the relative intensity of the CCC bend (ν6) and CC stretch (ν8) bands is nicely reproduced. This result is discussed in light of similar calculations at the semiempirical level of theory. We conclude that CIS can be of great value for the unravelling of vibronic spectra of conjugated systems.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466494
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  • 4
    Electronic Resource
    Electronic Resource
    Microscopic Heterogeneity in a Bowlic Columnar Mesophase as Probed with Fluorescence Depolarization Measurements (1995)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 99 (1995), S. 15981-15986 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100043a043
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  • 5
    Electronic Resource
    Electronic Resource
    Rovibrational energy levels and equilibrium geometry of HCP (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 3132-3141 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The ground state potential energy surface for HCP has been investigated theoretically. A large fraction of electron correlation is included by multireference internally contracted configuration interaction from CASSCF reference wave functions using large orbital expansions. The origin of the potential is then shifted and the force constants scaled to reproduce all spectroscopic data available for the four isotopically substituted species. Variational calculations of vibrational and rotational frequencies for transitions up to J = 7 ← 6 have finally been performed, with accuracy which is typically ± 5 cm−1 for vibrations and ± 10 MHz for most rotations. By comparison with the results of the perturbation treatment the importance of the ν1:2ν3 Fermi interaction for vibrational frequencies and effective rotational constants has been determined. From computed and experimental ground state rotational constants, the molecular equilibrium geometry has also been estimated. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472181
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  • 6
    Electronic Resource
    Electronic Resource
    The gas-phase infrared spectra of anthracene-h10 and anthracene-d10 (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 9004-9012 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The IR spectra of anthracene-h10 and -d10 have been recorded for the first time in the gas phase from 450 to 3200 cm−1 with a resolution of 0.2 cm−1, using a multipass cell heated to 100°C. For the assignment of vibrational bands we have evaluated the theoretical spectrum using density functional theory (DFT) and scaled self consistent field force fields. We found that both methods reproduce the sequence of the experimental frequencies to a good accuracy, allowing in most cases consistent and unambiguous assignments. The relative intensities of C14H10 and C14D10 have also been measured and compared to theory. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474033
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  • 7
    Electronic Resource
    Electronic Resource
    Efficient truncation strategies for multi-reference configuration interaction molecular energies and properties (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 7085-7092 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Modified virtual orbitals are proposed for multi-reference configuration interaction (MRCI) treatments and a modified Fock operator is defined for the orbital transformation. The main property of the modified orbitals is to improve the convergence properties of the configuration interaction (CI) expansion, which can be exploited to truncate, partially, the expansion in the external space. Simple tests are presented to show that the orbital transformation may be useful to perform FullCI type of treatments for subsets of orbitals and electrons, and to improve the MRCI second-order corrections and energies. Compared to other well-established techniques for accurate MRCI treatments, it is believed that this method offers advantages for electronic structures with many active orbitals and electrons using large orbital basis sets. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477391
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  • 8
    Electronic Resource
    Electronic Resource
    Dissociation energies and heats of formation of NH and NH+ (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 10265-10272 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Starting from the results of a recent ion–molecule reactive scattering experiment [Tosi et al., J. Chem. Phys. 100, 4300 (1994)], a precise estimate of the upper bound for the heat of formation of NH+ has been derived [ΔHf,0(NH+)≤17.175±0.006 eV], and from this, upper and lower bounds for the heat of formation of NH and for the dissociation energies of NH+ (X 2Π and a 4Σ− states) and NH (X 3Σ−), respectively. In order to verify the degree of accuracy attainable on the theoretical counterpart for these systems, high level ab initio computations, using extended multireference configuration interaction expansion and recently developed core-valence correlation consistent basis sets [Woon et al., J. Chem. Phys. 103, 4572 (1995)], have been also performed, comparing the results with the corresponding values estimated from the experimental data and previous theoretical literature values. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474102
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  • 9
    Electronic Resource
    Electronic Resource
    Energy levels of HCN+ and DCN+ in the vibronically coupled X 2Π and A 2Σ+ states (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 11200-11212 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The X 2Π and A 2Σ+ electronic states of HCN+ have been studied using a previously developed method [Carter et al., Mol. Phys. 98, 1967 (2000)] suitable for triatomic molecules showing three-state (Renner-Teller+vibronic) interactions. Ab initio three-dimensional diabatic potential energy surfaces for the 2Π(1 2A′,1 2 A″) and 2Σ+(2 2A′) states have been computed at the multireference configuration interaction level of theory, using extended Gaussian basis sets. Additional computations were done to determine the barrier to isomerization over the three surfaces and the spin–orbit constant for the 2Π state. Energies, spin–orbit splittings, and rotational constants have then been calculated for all rovibronic levels of Σ and Π symmetry up to 5800 cm−1 for HCN+ and 4800 cm−1 for DCN+. Assignments based on plots of vibrational wave functions are also provided. These computations have been finally used to revise previous interpretations of photoelectron spectra. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1414347
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  • 10
    Electronic Resource
    Electronic Resource
    On the lowest electronic states of the C2F radical (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 467-473 
    Details
    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 adiabatic energy surfaces of the lowest three electronic states [2(2A′ and 2A′)] and 2Σ+[2A′] of the C2F radical were investigated by the Hartree-Fock multiconfiguration self-consistent field (HF - MCSCF) ab initio method using a large set of atomic natural orbitals (ANO) and an extended configuration space, and the results were shown to be in agreement with the predictions of valence theory for this radical. The electronic ground state was found to have a bent equilibrium structure, hence contradicting the Walsh rule which predicts for the isoelectronic molecules a 2 linear state. The three states were found to be nearly degenerate and the potential energy surfaces of the two lowest electronic states exhibit an avoided crossing at an energy ∼2000 cm-1 above the ground-state minimum, lower than the highest vibrational fundamental. The strong adiabatic interaction which is responsible for the ordering of the electronic states and their equilibrium geometry involves not only the bending coordinate as normally found for Renner-Teller pairs of states, but also the C - C stretching coordinate, due to the near degeneracy of the 2Σ+ and the 2 lowest electronic states at linear geometries. © 1996 John Wiley & Sons, Inc.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
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