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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: PCCD is generalized to a singlet-paired coupled cluster model (CCD0) intermediate between coupled cluster doubles and pCCD, yielding a method that possesses the invariances of the former and much of the stability of the latter, and retains the full structure of coupled cluster theory.
Abstract: While restricted single-reference coupled cluster theory truncated to singles and doubles (CCSD) provides very accurate results for weakly correlated systems, it usually fails in the presence of static or strong correlation. This failure is generally attributed to the qualitative breakdown of the reference, and can accordingly be corrected by using a multideterminant reference, including higher-body cluster operators in the ansatz, or allowing symmetry breaking in the reference. None of these solutions are ideal; multireference coupled cluster is not black box, including higher-body cluster operators is computationally demanding, and allowing symmetry breaking leads to the loss of good quantum numbers. It has long been recognized that quasidegeneracies can instead be treated by modifying the coupled cluster ansatz. The recently introduced pair coupled cluster doubles (pCCD) approach is one such example which avoids catastrophic failures and accurately models strong correlations in a symmetry-adapted frame...

126 citations

Journal ArticleDOI
TL;DR: In this paper, the general formulas which are convenient for cluster analysis of a configuration interaction wave function are presented, and the results clearly indicate that Sinanoǧlu's statement concerning the relative unimportance of the linked parts of the tetraexcited state coefficients in the ci expansion, is justified for these delocalized systems.
Abstract: The general formulas which are convenient for cluster analysis of a configuration interaction wave function are presented. These formulas are then used for cluster analysis of the “complete” configuration interaction wave functions of the π-electronic models of benzene and butadiene obtained with a semiempirical method of the Pariser–Parr–Pople type using three different parameterizations. For butadiene the calculations are carried out with Huckel, Hartree–Fock, and Brueckner molecular orbitals. The results clearly indicate that Sinanoǧlu's statement [1, 2], concerning the relative unimportance of the linked parts of the tetraexcited state coefficients in the ci expansion, is justified for these delocalized systems.

126 citations

Journal ArticleDOI
TL;DR: New correlation consistent basis sets based on both pseudopotential (PP) and all-electron Douglas-Kroll-Hess (DKH) Hamiltonians have been developed from double- to quadruple-zeta quality for the actinide atoms thorium and uranium, showing systematic convergence towards the complete basis set limit.
Abstract: New correlation consistent basis sets based on both pseudopotential (PP) and all-electron Douglas-Kroll-Hess (DKH) Hamiltonians have been developed from double- to quadruple-zeta quality for the actinide atoms thorium and uranium. Sets for valence electron correlation (5f6s6p6d), cc − pV nZ − PP and cc − pV nZ − DK3, as well as outer-core correlation (valence + 5s5p5d), cc − pwCV nZ − PP and cc − pwCV nZ − DK3, are reported (n = D, T, Q). The -PP sets are constructed in conjunction with small-core, 60-electron PPs, while the -DK3 sets utilized the 3rd-order Douglas-Kroll-Hess scalar relativistic Hamiltonian. Both series of basis sets show systematic convergence towards the complete basis set limit, both at the Hartree-Fock and correlated levels of theory, making them amenable to standard basis set extrapolation techniques. To assess the utility of the new basis sets, extensive coupled cluster composite thermochemistry calculations of ThFn (n = 2 − 4), ThO2, and UFn (n = 4 − 6) have been carried out. After accurately accounting for valence and outer-core correlation, spin-orbit coupling, and even Lamb shift effects, the final 298 K atomization enthalpies of ThF4, ThF3, ThF2, and ThO2 are all within their experimental uncertainties. Bond dissociation energies of ThF4 and ThF3, as well as UF6 and UF5, were similarly accurate. The derived enthalpies of formation for these species also showed a very satisfactory agreement with experiment, demonstrating that the new basis sets allow for the use of accurate composite schemes just as in molecular systems composed only of lighter atoms. The differences between the PP and DK3 approaches were found to increase with the change in formal oxidation state on the actinide atom, approaching 5-6 kcal/mol for the atomization enthalpies of ThF4 and ThO2. The DKH3 atomization energy of ThO2 was calculated to be smaller than the DKH2 value by ∼1 kcal/mol.

126 citations

Journal ArticleDOI
TL;DR: In this article, the performance of Gaussian-2 theory is investigated when higher level theoretical methods are included for correlation effects, geometries, and zero point energies, including Brueckner doubles and coupled cluster methods rather than quadratic configuration interaction.
Abstract: The performance of Gaussian‐2 theory is investigated when higher level theoretical methods are included for correlation effects, geometries, and zero‐point energies A higher level of correlation treatment is examined using Brueckner doubles [BD(T)] and coupled cluster [CCSD(T)] methods rather than quadratic configuration interaction [QCISD(T)] The use of geometries optimized at the QCISD level rather than the second‐order Mo/ller–Plesset level (MP2) and the use of scaled MP2 zero‐point energies rather than scaled Hartree–Fock (HF) zero‐point energies have also been examined The set of 125 energies used for validation of G2 theory [J Chem Phys 94, 7221 (1991)] is used to test out these variations of G2 theory Inclusion of higher levels of correlation treatment has little effect except in the cases of multiply‐bonded systems In these cases better agreement is obtained in some cases and poorer agreement in others so that there is no improvement in overall performance The use of QCISD geometries yields significantly better agreement with experiment for several cases including the ionization potentials of CS and O2, electron affinity of CN, and dissociation energies of N2, O2, CN, and SO2 This leads to a slightly better agreement with experiment overall The MP2 zero‐point energies gives no overall improvement These methods may be useful for specific systems

126 citations

Journal ArticleDOI
TL;DR: In this article, the authors introduced an excited state theory for the optimized orbital coupled cluster doubles (OO-CCD) and valence optimized orbital Coupled Clustering Doubles (VOO)-CCD models, and derived the equations for transition energies using a similarity transformed Hamiltonian.
Abstract: We introduce an excited state theory for the optimized orbital coupled cluster doubles (OO-CCD) and valence optimized orbital coupled cluster doubles (VOO-CCD) models. The equations for transition energies are derived using a similarity transformed Hamiltonian. The effects of orbital relaxation are discussed. We present results for several single-reference molecules (H2O, CH2O, C2H4O, C2H4, BeO), as well as for molecules with significant nondynamical correlation in the ground state (CH+, BH, A 1A1 CH2), and for rectangular O4+. We find that: (i) OO-CCD excitation energies are very close to CCSD excitation energies; (ii) similarly to the complete active space SCF (CASSCF) model, the effects of orbital relaxation are very important for VOO-CCD excited states such that the excitation energies calculated by VOO-CCD and CASSCF with orbitals optimized for the ground state are very close to each other and unsatisfactory; (iii) the VOO-CCD model with an approximate treatment of orbital relaxation describes singly...

126 citations


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