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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 5451-5454 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The multiconfigurational spin tensor electron propagator approximation to the electron propagator is applied for the first time to the calculation of electron affinities. The electron affinities of Li, Na, and K are calculated as the ionization potentials of the negative ions. On average these electron affinities differ from experiment by 0.003 eV, giving the best theoretical values reported to data.
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 6336-6337 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 2572-2581 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The multiconfigurational spin-tensor electron propagator method (MCSTEP) gives accurate ionization potentials (IPs) and electron affinities (EAs) for both closed-shell and open-shell molecules, including for highly correlated systems. Both principal and lower-lying shakeup IPs can be accurately obtained and straightforwardly characterized using MCSTEP. To further test this new technique, we have applied the MCSTEP approach to the open-shell, polyatomic radical NH2. We report and characterize vertical IPs 0–20 eV, including several ionizations that have not, as yet, been observed experimentally. IPs to both singlet and triplet states of NH+2 are accurately calculated using the same MCSCF reference state. We predict the presence of previously undetected, observable vertical PES IPs to states of 3A2, 1A1, and 1A2 symmetries at 16.86, 18.00, and 18.26 eV, respectively. Also, we calculate adiabatic IPs by a modified application of the procedure. By examining the two lowest adiabatic IPs with both MCSTEP and Δmultireference singles+doubles CI at geometries established by Pople's research group and with the basis set used by that group augmented by hydrogen d polarization functions we conclude that their MP4 procedures overestimate the IPs with their best basis set by ∼0.2–0.3 eV.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 2884-2893 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present and develop the multiconfigurational particle–particle propagator (MCP2P) method for directly determining vertical double ionization potentials (DIPs) and double electron affinities. The MCP2P method gives highly accurate DIPs. In the first calculations with this method (obtained using a moderate-sized MCP2P complete active space) the average absolute deviation from Δfull configuration interaction within the same basis set for the DIPs to the lowest (in energy) five 1,3S Be2+ states is 0.04 eV.
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 1533-1539 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Accurate values of the electronic transition moment function for the first positive (A 3∑+u−B 3Πg) and infrared afterglow (B 3Πg −B'3∑−u) systems of N2 were obtained through multiconfigurational linear response (also known as the multiconfigurational time-dependent Hartree–Fock, MCLR/MCTDHF). MCLR/MCTDHF furnished both excitation energies and transition moments. MCLR/MCTDHF calculations on the first positive system demonstrate the importance of configurations involving the 2σg orbital. To obtain accurate values, configurations were chosen with a complete active space of all valence orbitals (2σg, 2σu, 1πu, 3σg, 1πg, 3σu). This is the first MCLR/MCTDHF calculation for molecules as large as first row diatomics in which a complete active space including the entire valence shell is employed. The results are in substantial agreement with accurate theoretical calculations by others and, hence, disagree in their overall behavior with the previously published empirical values. One of our conclusions is that the experimental values for the first positive system far from the equilibrium region are in error.
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 7223-7228 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Semiempirical molecular dynamics is used to study the collision of C60 and C+60. Particles are propagated classically using forces calculated from the modified neglect of differential overlap (MNDO) Hamiltonian. By assigning different collision energies (Ec) and impact parameters (b) to the Buckminster fullerenes (buckyballs), we simulated six collision events: four head on collisions with impact parameter b=0 and collisions energies of 100, 150, 200, and 400 eV, and two collisions with b=1.5 Rb (Rb=buckyball radius) and Ec=100 and 400 eV. The head on collisions show that at 100 eV the two buckyballs scatter off one another and at 200 and 400 eV they fuse, while at 150 eV they either scatter or form a metastable dimer depending on how the simulation is prepared. This barrier is consistent with recent experiments. In addition, we observe tetrahedral bonding in the C+120 structure formed in the 200 eV, b=0 collision, while at 400 eV, b=0 we see large rings and chains of carbon atoms. The off center collisions also display interesting structural features. In the 100 eV b=1.5 Rb collision, the buckyballs graze one another, distorting their cage structure and scattering at an angle relative to their incident velocities. The buckyballs in the 400 eV, b=1.5 Rb collision also scatter, but in contrast to the 100 eV, b=1.5 Rb collision, the individual buckyballs are severely distorted, forming what we refer to as an "open mouth'' structure.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4832-4841 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The ground and first and second 1A‘ potential surfaces of methyl mercaptan (CH3SH) are calculated as a function of the C–S and S–H bond coordinates using the ab initio effective valence shell Hamiltonian (Hν) method. The computations for this highly nontrivial system provide the first serious tests for choosing restricted valence spaces and for computing global potential energy surfaces with the Hν methods. The quasidegeneracy constraints on the Hν method suggest choosing a valence space which consists of the two (3a‘ and 10a') highest energy occupied orbitals in the ground state and the three lowest a' unoccupied orbitals. The global potential surfaces are computed with a modest basis, but larger basis set computations at selected geometries test convergence for vertical excitation energies, ionization potentials, and C–S and S–H bond energies. The calculations are compared to both experiment and other calculations for this system. The computations assist in the interpretation of CH3SH photodissociation dynamics observed by Butler and co-workers.
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  • 8
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using a 〈9s9p5d〉 contracted GTO basis we have calculated low-lying excitation energies of singlet and triplet symmetry for the Be atom using Δfull CI, ΔCI(1s) with double occupancy in the 1s orbital, multiconfiguration linear response (MCLR), and ΔMCSCF approaches. The Δfull CI results agree very closely with the experimental excitation energies except for higher excitations where obvious basis set defects occur. The MCLR calculations shows that with an adequately chosen MCSCF reference state the MCLR calculation is capable of mimicking the Δfull CI results. The MCLR results are closer to the Δfull CI results than the ΔCI(1s). The ΔMCSCF excitation energies show that this approach can only be used with extreme care to determine excitation energies.
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  • 9
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 98 (1994), S. 3467-3471 
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 1304-1316 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Low-lying π-electron vertical excitation energies of trans-butadiene are calculated using the effective valence shell Hamiltonian method. The results are compared with previous experimental and theoretical analyses of this molecule's congested electronic spectra. The computations employ a large basis set (126 functions) which includes both diffuse functions on the carbon atoms and polarization functions on all atoms. Good agreement is obtained with the experimentally well known vertical excitation energies to the 1 3Bu, 1 3Ag, and 1 1Bu states where deviations from experiment are only 0.01, 0.01, and 0.22 eV, respectively. We confirm the experimental assignment of a valence like 1Ag state around 7.4 eV (calculated at 7.49 eV). Likewise, a member of a symmetry allowed 3p Rydberg series (of Au or Bu symmetry) in the electron impact spectrum with origin at 7.07 eV is assigned as the 2 1Bu state (with calculated vertical excitation energy of 7.00 eV). Most experiments place the 2 1Ag state above the 1 1Bu state; however, a resonance Raman assignment places it below. Our calculated excitation to the 2 1Ag state is 0.05 eV above the 2 1Bu state, about 0.5 eV lower than previous ab initio determinations. The computed vertical excitation energies are in good agreement with the interpretation of experimental electronic spectra, are in much better agreement with experiment than previously published ab initio calculations, provide the first definitive assignment of the 2 1Bu state at 7.08 eV, and conclusively assign the 3 1Ag state at 7.4 eV. The accuracy of the large basis effective valence shell Hamiltonian is, in part, due to retention of both valence and Rydberg orbitals in the valence space, a feature which has a bearing on intruder state problems and on current semiempirical π-electron theories.
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