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  • 1
    Electronic Resource
    Electronic Resource
    Energy- versus amplitude-corrected coupled-cluster approaches. III. Accurate computation of spectroscopic data exemplified on the HF molecule (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 117 (2002), S. 1941-1955 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The so-called energy-corrected coupled-cluster method with singles and doubles (CCSD), which is closely related to the recently proposed method of moments of Kowalski and Piecuch, accounts for the nondynamic correlation effects, which are missing in the standard single-reference CCSD, by projecting onto the modest-size multireference (MR) configuration-interaction SD (CISD) wave function when evaluating the energy, rather than onto the single-determinantal CCSD reference. The ability of this approach to generate reliable potential energy curves (PECs) or surfaces over a wide range of geometries is examined using a realistic ab initio model of the HF molecule. The PECs obtained with the correlation-consistent polarized valence double (triple, quadruple) zeta cc-pVXZ (T,Q) basis sets are used, together with the PEC extrapolated to the complete basis set limit, to compute the rotational and vibrational levels, which are then compared with the experimentally determined values as well as with the directly measured vibrational frequencies. A comparison is also made with the earlier results [X. Li, J. Mol. Struct.: THEOCHEM 547, 69 (2001)] that were obtained with the amplitude-corrected CCSD method, namely the so-called reduced MR CCSD that exploits the same MR CISD wave function, as well as with recently published renormalized CCSD(T) results [P. Piecuch et al., J. Chem. Phys. 115, 5796 (2001)]. It is shown that both the amplitude- and energy-corrected CCSD PECs may be used to generate highly accurate and reliable spectroscopic data once the basis set limit is achieved. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1488597
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  • 2
    Electronic Resource
    Electronic Resource
    Energy versus amplitude corrected coupled-cluster approaches. I (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 5759-5773 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In the spirit of recently proposed renormalized CCSD(T) and CCSD(TQ) methods [K. Kowalski and P. Piecuch, J. Chem. Phys. 113, 5644 (2000), and references therein], we explore the additive, noniterative energy corrections for both the standard and reduced multireference (RMR) CCSD approaches. Our study is based on a simple asymmetric energy expression of the standard single reference (SR) coupled cluster theory and casts a new light on the RMR CCSD method and its relationship with the corresponding MR CISD and SR reference CCSD methods, thus interrelating the amplitude and energy corrected schemes. These developments are illustrated on two exactly solvable model systems, namely, on the DZP models of the H4 system and of the HF molecule. We find that appropriately projected CCSD provides an almost identical energies as does the RMR CCSD method. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1398088
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  • 3
    Electronic Resource
    Electronic Resource
    Energy versus amplitude corrected coupled-cluster approaches. II. Breaking the triple bond (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 5774-5783 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We examine the effectiveness of various energy corrections to the standard CCSD and to the reduced multireference (RMR) CCSD methods. These corrections are based on the asymmetric energy formula, but instead of projecting onto the reference configuration, as in the standard CCSD method, we employ for this purpose either the MR CISD wave function that is based on a suitable model space of the kind used in RMR CCSD, or simply the zero-order wave function in that model space. Both full complete-active-space and severely-truncated model spaces are employed. The method is applied to the prototypical case of the triple-bond dissociation, namely, to the exactly solvable double-zeta model of the N2 molecule. It is shown that in this way we can eliminate the breakdown of the standard CCSD method in the region of highly stretched geometries and obtain reliable potential energy curves. The comparison with the recently proposed renormalized CCSD(T) and variational CCD methods is also briefly addressed. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1398089
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  • 4
    Electronic Resource
    Electronic Resource
    Reduced multireference coupled cluster method: Ro-vibrational spectra of N2 (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 9966-9977 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The reduced multireference (RMR) coupled cluster method with singles and doubles (CCSD), relying on four- and eight-dimensional reference spaces, is employed to generate the potential energy curve for the nitrogen molecule, using both the correlation-consistent polarized valence-triple-zeta (cc-pVTZ) and atomic natural orbital basis sets. The resulting potentials are then used to compute the ro-vibrational term values, which are compared with the available experimentally determined values. Moreover, a direct comparison with the measured spectral line frequencies, for both the fundamental rotational Raman band and the vibrationally excited Q branches, is also carried out. Comparisons with the single reference CCSD, as well as with the corresponding multireference configuration interaction results that serve as the external source for RMR CCSD, are also made. An excellent performance of RMR CCSD, and its systematic improvement with the increasing dimension of the reference space employed, is demonstrated for the cc-pVTZ basis set. The fundamental Raman band frequencies are computed with an error of less than 1 cm−1, while for the vibrationally excited bands a systematic deviation of about 7 cm−1 is found. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1323260
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  • 5
    Electronic Resource
    Electronic Resource
    Unitary group based open-shell coupled cluster approach and triplet and open-shell singlet stabilities of Hartree–Fock references (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 6536-6547 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The performance of recently introduced state-selective coupled-cluster method, based on the unitary group approach to the many-electron correlation problem and enabling a properly spin-adapted treatment of open-shell states, is examined for open-shell singlet and triplet states having unstable restricted Hartree–Fock solutions that serve as a reference. The study employs an ab initio model at the double-zeta-plus-polarization level of approximation describing the dissociation of the methylene radical in its lowest triplet and open-shell singlet states into the CH and H fragments in their ground states. It is shown that the spin restricted Hartree–Fock equations have different solutions in the bonding and nonbonding regions, which coexist in a small transition region, where the corresponding potential energy curves intersect, and which is determined by the vanishing of the lowest root of the stability problem for each solution. These instabilities are then responsible for a slight discontinuity in the potential energy curves in the transition region when the correlation effects are accounted for. It is shown that the overall performance of the unitary group based coupled cluster method gives very satisfactory results at a moderate cost. Finally, the possible pitfalls of the spin orbital coupled cluster method employing the unrestricted Hartree–Fock reference are illustrated for the triplet ground state dissociation model of methylene. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470380
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  • 6
    Electronic Resource
    Electronic Resource
    Spin-adapted open-shell state-selective coupled cluster approach and doublet stability of its Hartree–Fock reference (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 2013-2023 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The performance of the unitary group based state-selective coupled cluster approach, in both linear and quadratic approximations, is examined for the case of doublet ground states of the first two alkaline earth hydrides in the whole range of internuclear separation. It is shown that the doublet instability of the restricted open-shell Hartree–Fock wave functions is responsible for the singular behavior of the linear coupled cluster potential energy curves, as well as for slight discontinuity in these curves when the bilinear terms are included. The effect of using different types of orbitals is investigated and the results are compared with full or very precise limited configuration interaction results as well as with the coupled cluster results employing the unrestricted Hartree–Fock reference, which is free of the instability problems in open shell systems. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468766
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  • 7
    Electronic Resource
    Electronic Resource
    Comparison of the open-shell state-universal and state-selective coupled-cluster theories: H4 and H8 models (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 1024-1034 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The spin nonadapted or partially spin adapted spin orbital multireference (MR) state-universal (SU) or Hilbert space coupled cluster (CC) method is analyzed from the viewpoint of possible spin symmetry breaking or spin contamination and contrasted with the recently introduced state specific (SS), automatically spin adapted, spin-free unitary group based CC method. It is shown that the partial spin-adaptation of the two-determinantal MR SUCC approach can only be effective if the complementary singlet and triplet states have a similar structure in the sense that their weights at corresponding singlet and triplet configurations, having the same orbital occupancies, are identical or nearly identical. The performance of both SS and SU CC methods, and of corresponding CI method (all truncated at the double excitation level) for the lowest lying open-shell singlet and triplet states of the H4 and H8 model systems was examined, showing the superiority of the unitary group based CC approach in all studied aspects. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469812
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  • 8
    Electronic Resource
    Electronic Resource
    Unitary group based state-selective coupled-cluster method: Comparison of the first order interacting space and the full single and double excitation space approximations (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 8897-8905 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A unitary group based state selective coupled-cluster method, truncated at the first order interacting space and at the full single and double excitation space levels, has been applied to examine a complete dissociation (atomization) of the OH, NH2, CH3, and CN radicals. These molecules were chosen to model various bond breaking processes, including the dissociation of one, two, and three single bonds as well as of a multiple (triple) bond. In all cases the coupled-cluster expansion employing the full single and double excitation space represents an improvement over the interacting space approximation. Although this improvement is more significant for geometries involving stretched bonds than for the equilibrium ones, it is found to be rather small in all cases that were examined. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468943
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  • 9
    Electronic Resource
    Electronic Resource
    Automation of the implementation of spin-adapted open-shell coupled-cluster theories relying on the unitary group formalism (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 8812-8826 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new implementation of the orthogonally spin-adapted open-shell (OS) coupled-cluster (CC) formalism that is based on the unitary group approach to many-electron correlation problem is described. Although the emphasis is on the so-called state specific single-reference but multiconfigurational OS CC approach, the developed algorithms as well as the actual codes are also amenable to multireference CC applications of the state-universal type. A special attention is given to simple OS doublets and OS singlet and triplet cases, the former being applicable to the ground states of radicals and the latter to the excited states of closed shell systems. The encoding of the underlying formalism is fully automated and is based on a convenient decomposition of the Hamiltonian into the effective zero-, one-, and two-orbital contributions as well as on the general strategy that focuses on the excitation operator driven evaluation of individual absolute, linear, quadratic, etc., coupled cluster coefficients, rather than on the standard molecular (spin) orbital driven algorithms. In this way unnecessary duplications are avoided and efficient codes are developed both for the general formula generation and final executable modules. A thorough testing of this procedure on a number of model cases is described and several illustrative applications at the ab initio level are provided.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468074
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  • 10
    Electronic Resource
    Electronic Resource
    Simultaneous handling of dynamical and nondynamical correlation via reduced multireference coupled cluster method: Geometry and harmonic force field of ozone (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 2844-2852 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The geometry and vibrational frequencies of the ozone molecule are studied using the reduced multireference coupled cluster method with singles and doubles (RMR CCSD) relying on a two-electron/two-orbital (2,2) active space. The role played by the choice of the reference space is analyzed in detail by considering both the spin adapted, two-configuration reference space and the complete three-configuration reference space based on the (2,2) active space. It is shown that while the two-reference RMR CCSD approach, using the C2v symmetry-adapted (2,2) active space, provides a better description of the geometry and harmonic frequencies of symmetric vibrational modes than the standard single-reference (SR) CCSD method, the three-reference approach is essential to achieve an accurate description of the harmonic frequency of the asymmetric stretching mode. A unique behavior of ozone in this regard is highlighted by a comparison with other symmetric triatomics. The comparison with results obtained by other methods, including the SR configuration interaction with singles and doubles (CISD), the standard SR CCSD, as well as two-reference and large reference space MR CISD methods, clearly demonstrates the superiority of the RMR CCSD approach. In fact, the performance of a small reference space RMR CCSD method is similar to that of a large reference space MR CISD, thus enabling a compact description of both dynamical and nondynamical correlation effects in states having a multireference character. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477926
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