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  • 1
    Electronic Resource
    Electronic Resource
    The electronic spectrum of pyrrole (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 525-537 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The electronic spectrum of pyrrole has been investigated by performing calculations using a hierarchy of coupled-cluster models consisting of CCS, CC2, CCSD, and CC3. Basis-set effects have been investigated by carrying out calculations using correlation-consistent basis sets augmented with functions especially designed for the description of Rydberg states. Oscillator strengths, excited state dipole moments, and second moments of the electronic charge distributions have been used to characterize the electronic transitions and final states. Structures and vibrational frequencies have been calculated for a few selected states, and the importance of distinguishing between vertical and adiabatic transitions for accurate comparison with experiment has been emphasized. The experimental spectrum has been scrutinized in the relevant energy region, and the accuracy of recent calculations [CASPT2, MRMP, ADC(2)] has been critically reexamined. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479332
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  • 2
    Electronic Resource
    Electronic Resource
    Spin-restricted open-shell coupled-cluster theory (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 9028-9038 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Spin-restricted CC theory is suggested as a new approach for the treatment of high-spin open-shell systems in CC theory. Spin constraints are imposed on the wave function in the sense that the projected spin eigenvalue equations are fulfilled within the (truncated) excitation space. These constraints allow a reduction in the number of independent amplitudes, thus decreasing the computational cost when implemented efficiently. The approach ensures that the spin expectation value always corresponds to the exact value, though the wave function is (for truncated CC treatments) not rigorously spin-adapted. For the specific case of high-spin doublets, detailed equations are derived for amplitudes, energy and first derivatives of the energy within the computationally most useful singles and doubles approximation. Numerical examples demonstrate the excellent performance of the spin-restricted CC approach relative to the more traditional spin-orbital based CC treatments, suggesting that it might be an attractive alternative for the treatment of high-spin open-shell systems. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475220
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  • 3
    Electronic Resource
    Electronic Resource
    Many-body methods for excited state potential energy surfaces. II. Analytic second derivatives for excited state energies in the equation-of-motion coupled cluster method (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 8931-8943 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Two distinct theoretical formalisms are developed for evaluating second derivatives of the energy analytically within the equation-of-motion coupled cluster approximation for excited electronic states (EOMEE-CC). In the first approach, both perturbations are treated equivalently. In the alternative formulation, the final operator expression is not symmetric with respect to interchange of the perturbations, and calculation of the second derivative requires that four systems of linear equations be solved for the first-order response of wave function parameters. However, only two systems need to be solved when the symmetric strategy is followed. While the symmetric approach superficially appears to be both more elegant and better suited for practical calculations, analysis shows that the former assertion is open to question and the latter only conditionally true. In particular, the asymmetric formulation is shown to be the preferred choice for all cases in which a large number of perturbations is involved. This is a rather important conclusion that holds not only for the EOMEE-CC method, but also for CC treatments of the electronic ground state and their finite-order many-body perturbation theory approximations. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470083
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  • 4
    Electronic Resource
    Electronic Resource
    Analytic energy derivatives for ionized states described by the equation-of-motion coupled cluster method (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 8938-8944 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The theory for analytic energy derivatives of excited electronic states described by the equation-of-motion coupled cluster (EOM-CC) method has been generalized to treat cases in which reference and final states differ in the number of electrons. While this work specializes to the sector of Fock space that corresponds to ionization of the reference, the approach can be trivially modified for electron attached final states. Unlike traditional coupled cluster methods that are based on single determinant reference functions, several electronic configurations are treated in a balanced way by EOM-CC. Therefore, this quantum chemical approach is appropriate for problems that involve important nondynamic electron correlation effects. Furthermore, a fully spin adapted treatment of doublet electronic states is guaranteed when a spin restricted closed shell reference state is used—a desirable feature that is not easily achieved in standard coupled cluster approaches. The efficient implementation of analytic gradients reported here allows this variant of EOM-CC theory to be routinely applied to multidimensional potential energy surfaces for the first time. Use of the method is illustrated by an investigation of the formyloxyl radical (HCOO), which suffers from notorious symmetry breaking effects.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468022
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  • 5
    Electronic Resource
    Electronic Resource
    Rovibrationally averaged nuclear magnetic shielding tensors calculated at the coupled-cluster level (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 11051-11059 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Nuclear magnetic shielding tensor functions for H2, HF, N2, CO, and F2 are calculated at the coupled-cluster singles and doubles level augmented by a perturbative correction for triple excitations [CCSD(T)]. The shielding constants for the lowest rovibrational states of these diatomics are obtained by solving the rovibrational Schrödinger equation with the finite-element method. For H, C, and F, absolute scales for the nuclear magnetic shielding constants have been obtained by combining computed diamagnetic shieldings with paramagnetic contributions deduced from measured spin-rotation constants and calculated rovibrational corrections. Since the experimental spin-rotation constants for N2 and CO are inaccurate, shielding scales for N and O based on coupled cluster calculations are probably the most accurate available. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472905
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  • 6
    Electronic Resource
    Electronic Resource
    Perturbative treatment of triple excitations in coupled-cluster calculations of nuclear magnetic shielding constants (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 2574-2583 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A theory for the calculation of nuclear magnetic shielding constants at the coupled-cluster singles and doubles level augmented by a perturbative correction for connected triple excitations (CCSD(T)) has been developed and implemented. The approach, which is based on the gauge-including atomic orbital (GIAO) ansatz, is illustrated by several numerical examples. These include a comparison of CCSD(T) and other highly correlated methods with full configuration interaction for the BH molecule, and a systematic comparison with experiment for HF, H2O,NH3, CH4, N2, CO, HCN, and F2. The results demonstrate the importance of triple excitations in establishing quantitative accuracy. Finally, the ability of GIAO-CCSD(T) to make accurate predictions for difficult cases is explored in calculations for formaldehyde (CH2O), diazomethane(CH2NN), and ozone (O3). © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471005
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  • 7
    Electronic Resource
    Electronic Resource
    The first excited singlet state of s-tetrazine: A theoretical analysis of some outstanding questions (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 9859-9869 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The equation-of-motion coupled cluster method for excited electronic states (EOMEE-CC) is applied to study the structure and selected properties of the first excited singlet state of s-tetrazine. Adiabatic S1←S0 excitation energies obtained with large basis sets containing up to 270 functions are uniformly somewhat above the experimental 0–0 value of 2.238 eV, but nevertheless are the most accurate calculations reported to date for this quantity. The equilibrium geometry of S1 predicted in this study is in excellent agreement with another high-level calculation, and moreover is quantitatively consistent with both the intensity of vibrational progressions observed in absorption and measured rotational constants for S1. The EOMEE-CC harmonic force field of S1 is the first to satisfactorily describe the low frequency in-plane b3g bending mode, notably its marked reduction in frequency upon excitation and characteristic positive anharmonicity. Both of these effects are due to a strong second-order Jahn–Teller interaction between S1 and the nominal S2(1Au) state, which is also investigated superficially in this work. Finally, results presented for the static dipole polarizabilies of the S1 state join others in calling for a reinvestigation of the experimentally determined parameters. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471750
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  • 8
    Electronic Resource
    Electronic Resource
    Triple excitation effects in coupled-cluster calculations of indirect spin–spin coupling constants (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 1619-1622 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The effect of triple excitations in coupled-cluster calculations of indirect spin-spin coupling constants is investigated in coupled-cluster singles and doubles (CCSD) calculations augmented by a perturbative treatment of triples [CCSD(T)], in calculations based on the CC3 model as well as in coupled-cluster singles, doubles, and triples (CCSDT) calculations. Though triple excitation effects are in most cases not particularly pronounced, it is demonstrated that among the approximate schemes for handling triples only the CC3 model with no orbital relaxation included (unrelaxed CC3) provides an adequate description. The otherwise successful CCSD(T) aproach appears to either significantly overestimate triple excitation effects or to yield corrections with the wrong sign in comparison to CCSDT. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1386698
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  • 9
    Electronic Resource
    Electronic Resource
    Analytic gradients for the coupled-cluster singles, doubles, and triples (CCSDT) model (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 1773-1782 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The first implementation of analytic gradients for the coupled-cluster singles, doubles, triples (CCSDT) model is described. The relevant theoretical expressions are given in a diagrammatic form together with the corresponding algebraic formulas. The computational requirements of CCSDT gradient calculations are discussed and their applicability demonstrated by performing benchmark calculations for molecular geometries with large correlation-consistent basis sets. A statistical analysis of the data reveals that CCSDT and CCSD(T) in most cases perform equally well. The CCSDT calculations thus provide further evidence for the high accuracy of the CCSD(T) approach. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1429244
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  • 10
    Electronic Resource
    Electronic Resource
    Analytic second derivatives for the full coupled-cluster singles, doubles, and triples model: Nuclear magnetic shielding constants for BH, HF, CO, N2, N2O, and O3 (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 4773-4776 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An implementation of analytic second derivatives for the coupled-cluster singles, doubles, and triples (CCSDT) model is reported and applied to the calculation of nuclear magnetic shielding constants of BH, HF, CO, N2, N2O, and O3. The CCSDT calculations yield further evidence for the high accuracy provided by chemical shift calculations employing the coupled-cluster singles, doubles (CCSD) approach augmented by a perturbative triples correction [CCSD(T)]. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1462612
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