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Coupled cluster

About: Coupled cluster is a research topic. Over the lifetime, 6280 publications have been published within this topic receiving 301055 citations.


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TL;DR: The spectroscopy of complexes with N>1 provides valuable information on the shape of the potential energy surface in regions that are not accessed by the N=1 He-OCS complex, but that are important for understanding the molecular spectroscopic constants in larger complexes and in droplets.
Abstract: We present a new vibrationally averaged He-OCS potential energy surface that is obtained from a combination of Moller-Plesset perturbation theory for the helium-molecule interaction and coupled cluster theory for the intramolecular vibrational potential. Employing this potential in quantum Monte Carlo calculations for He(N)-OCS complexes shows a blueshift of the OCS vibration for small N that is followed by a transition to a redshift for larger N. The size dependence of the vibrational shift is in good agreement with recent experimental measurements. We then make a comparative study of the effective rotational spectroscopic constants B(eff) and D(eff) calculated for small N values with this vibrationally averaged potential, with the corresponding values obtained from three previous He-OCS potentials. We find that the vibrationally averaged potential provides the most accurate description of the spectroscopic constants over the size range N=1-8 for which experimental data are available. We rationalize this improved description in terms of the detailed differences in the secondary minimum and saddle point regions of the underlying He-OCS interaction potential, in addition to the behavior at the lowest potential minimum. This analysis indicates that the spectroscopy of complexes with N>1 provides valuable information on the shape of the potential energy surface in regions that are not accessed by the N=1 He-OCS complex, but that are important for understanding the molecular spectroscopy in larger complexes and in droplets.

70 citations

Journal ArticleDOI
TL;DR: In this paper, the irreducible tensor operators of the unitary group provide a natural operator basis for the exponential Ansatz which preserves the spin symmetry of the reference state, requires a minimal number of independent cluster amplitudes for each substitution order, and guarantees the invariance of the correlation energy under unitary transformations of core, open-shell, and virtual orbitals.
Abstract: We show that the irreducible tensor operators of the unitary group provide a natural operator basis for the exponential Ansatz which preserves the spin symmetry of the reference state, requires a minimal number of independent cluster amplitudes for each substitution order, and guarantees the invariance of the correlation energy under unitary transformations of core, open-shell, and virtual orbitals. When acting on the closed-shell reference state with nc doubly occupied and nv unoccupied (virtual) orbitals, the irreducible tensor operators of the group U(nc) ⊗ U(nV) generate all Gelfand-Tsetlin (GT) states corresponding to appropriate irreducible representation of U(nc + nv). The tensor operators generating the M-tuply excited states are easily constructed by symmetrizing products of M unitary group generators with the Wigner operators of the symmetric group SM. This provides an alternative to the Nagel-Moshinsky construction of the GT basis. Since the corresponding cluster amplitudes, which are also U(nc) ⊗ U(ns) tensors, can be shown to be connected, the irreducible tensor operators of U(nc) ⊗ U(nv) represent a convenient basis for a spin-adapted full coupled cluster calculation for closed-shell systems. For a high-spin reference determinant with n, singly occupied open-shell orbitals, the corresponding representation of U(n), n=nc + nv + ns is not simply reducible under the group U(nc) ⊗ U(ns) ⊗ U(nv). The multiplicity problem is resolved using the group chain U(n) ⊃ U(nc + nv) ⊗ U(ns) ⊃ U(nc) ⊗U(ns)⊗ U(nv) ⊗ U(nv). The labeling of the resulting configuration-state functions (which, in general, are not GT states when nc > 1) by the irreducible representations of the intermediate group U(nc + nv) ⊗U(ns) turns out to be equivalent to the classification based on the order of interaction with the reference state. The irreducible tensor operators defined by the above chain and corresponding to single, double, and triple substitutions from the first-, second-, and third-order interacting spaces are explicitly constructed from the U(n) generators. The connectedness of the corresponding cluster amplitudes and, consequently, the size extensivity of the resulting spin-adapted open-shell coupled cluster theory are proved using group theoretical arguments. The perturbation expansion of the resulting coupled cluster equations leads to an explicitly connected form of the spin-restricted open-shell many-body perturbation theory. Approximation schemes leading to manageable computational procedures are proposed and their relation to perturbation theory is discussed. © 1995 John Wiley & Sons, Inc.

69 citations

Journal ArticleDOI
TL;DR: These CBS limits can now be used as benchmarks to calibrate more approximate calculations using smaller basis sets, and the sequence of basis sets provides data on convergence patterns for each component of the correlation energy.
Abstract: We extrapolate to the coupled cluster single and double excitation and the perturbative triples (CCSD(T))/complete basis set (CBS) limit with a sequence of optimized n-tuple-ζ augmented polarization augmented (nZaPa) basis sets (n = 4, 5, 6, and 7) for 115 species representing the first two rows of the Periodic Table. The species include the entire set of atoms, positive and negative atomic ions, homonuclear diatomic molecules, and hydrides. The benchmark set also includes the rare gas dimers, polar molecules such as oxides and fluorides, and a few transition states for chemical reactions. The CCSD correlation energies agree with available CCSD-F12b/3C(FIX) values to within ±0.18 mEh root-mean-square (rms) deviation. The (T) components agree to within ±0.10 mEh and the total CCSD(T) correlation energies to within ±0.26 mEh or 0.1% rms deviation, which is probably the better measure, since the largest deviation is 0.43 mEh or 0.13%. These CBS limits can now be used as benchmarks to calibrate more approximate calculations using smaller basis sets. The sequence of basis sets provides data on convergence patterns for each component of the correlation energy.

69 citations

Journal ArticleDOI
TL;DR: The geometry and harmonic frequencies of linear and cyclic structures of C 5, C 7, C 9, C 11, C 13, and C 15 have been calculated using a hybrid density functional method (B3LYP).

69 citations

Journal ArticleDOI
TL;DR: A numerical treatment suitable for the computational investigation of physisorption of molecular hydrogen on carbon nanostructures has not been sufficiently discussed and results used as a reference are actually a product of poorly solved interactions and contaminated estimates with errors which would be of the order of 60%.
Abstract: A numerical treatment suitable for the computational investigation of physisorption of molecular hydrogen on carbon nanostructures has not been sufficiently discussed. In this paper it is shown that results used as a reference are actually a product of poorly solved interactions and contaminated estimates with errors which would be of the order of 60%. Moreover, using ab initio molecular orbital theory, under the rigid monomer supermolecular approach, the physisorption energy of molecular hydrogen on graphene was reinvestigated. The graphene surface was modeled as a coronenelike (C(24)H(12)) graphene sheet. The basis set superposition error was corrected by means of the counterpoise method. The H(2)-H(2) and H(2)-benzene interactions were examined, under systematic combinations of basis sets and correlation methods, including the aug-cc-pVQZ basis set and the coupled cluster correlation method with single, double, and noniterative triple excitations, searching for a numerical treatment with a reasonable trade-off between efficiency and accuracy. Asymmetrical modeling strategies, using diffusion augmented basis sets with preference for the adsorbate, were found to be effective. Also local modeling strategies, using more complete basis sets for the nearest atoms to the adsorbate than for the rest of the substrate, were considered. The aug-cc-pVTZ basis set for the adsorbate and for the nearest atoms to the adsorbate and the cc-pVTZ basis set for the rest of the cluster-modeled graphene, at the second-order Moller-Plesset perturbation theory correlation level, was selected as reference treatment. It was found that the physisorption energy of molecular hydrogen on graphene would be of the order of 0.06 eV, which would be 25% less than what has been previously published, though it would be sufficient to permit the storage of hydrogen physisorbed on carbon. To our knowledge this would be the most realistic theoretical estimate of the mentioned energy to date.

69 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
2023163
2022351
2021267
2020344
2019253
2018244