Approximate account of connected triply and quadruply excited clusters in single reference coupled cluster theory via cluster analysis of projected unrestricted coupled cluster with single and double wave function

Toboła, Robert

doi:10.1016/j.cplett.2014.09.012

Cited by 4 publications

(42 citation statements)

References 47 publications

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“…Different types of wave functions have been exploited as the external source, such as projected unrestricted Hartree-Fock (PUHF), 44,45 valence bond (VB), 46,47 complete active space self-consistent field (CASSCF) or CAS-CI (CAS configuration interaction), 48-53 multireference configuration interaction singles and doubles (MRCISD), [54][55][56][57][58][59][60][61][62][63][64][65] and projected UHFbased CCSD (PUCCSD). 66 Once the higher-order excitation operators (triple and higher) within the active space are obtained, the corresponding amplitudes are then fixed and fed into the traditional CC equations, from which the amplitudes of singles and doubles are determined, and then the correlation energy can be obtained.…”

Section: Introductionmentioning

confidence: 99%

The externally corrected coupled cluster approach with four- and five-body clusters from the CASSCF wave function

2015

The Journal of Chemical Physics

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An externally corrected CCSDt (coupled cluster with singles, doubles, and active triples) approach employing four- and five-body clusters from the complete active space self-consistent field (CASSCF) wave function (denoted as ecCCSDt-CASSCF) is presented. The quadruple and quintuple excitation amplitudes within the active space are extracted from the CASSCF wave function and then fed into the CCSDt-like equations, which can be solved in an iterative way as the standard CCSDt equations. With a size-extensive CASSCF reference function, the ecCCSDt-CASSCF method is size-extensive. When the CASSCF wave function is readily available, the computational cost of the ecCCSDt-CASSCF method scales as the popular CCSD method (if the number of active orbitals is small compared to the total number of orbitals). The ecCCSDt-CASSCF approach has been applied to investigate the potential energy surface for the simultaneous dissociation of two O-H bonds in H2O, the equilibrium distances and spectroscopic constants of 4 diatomic molecules (F2(+), O2(+), Be2, and NiC), and the reaction barriers for the automerization reaction of cyclobutadiene and the Cl + O3 → ClO + O2 reaction. In most cases, the ecCCSDt-CASSCF approach can provide better results than the CASPT2 (second order perturbation theory with a CASSCF reference function) and CCSDT methods.

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Section: Introductionmentioning

confidence: 99%

The externally corrected coupled cluster approach with four- and five-body clusters from the CASSCF wave function

2015

The Journal of Chemical Physics

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“…In addition to the cognitive benefit that is undoubtedly the cluster structure of the projected UHF‐based CCSD (PUCCSD) wave function, current research also provides some practical implications. Namely, cluster amplitudes originating from the cluster analysis can be used to calculate the correlation energy that for obvious reasons will be free from the contamination effects.…”

Section: Introductionmentioning

confidence: 99%

“…Namely, cluster amplitudes originating from the cluster analysis can be used to calculate the correlation energy that for obvious reasons will be free from the contamination effects. Thus, in the case of the

T_{1}^{PUCCSD}

and

T_{2}^{PUCCSD}

cluster contributions, the CCSD(PUCCSD) correlation energy can be calculated using a simple formula based on the standard CCSD theory in which cluster amplitudes

t_{1}^{PUCCSD}

and

t_{2}^{PUCCSD}

are needed only for this. Higher cluster contributions require additional intermediate calculations to obtain needed the amplitudes t 1 and t 2 that can be obtained by solving the externally corrected CC ( ec CC) equations .…”

Section: Introductionmentioning

confidence: 99%

“…Contrary to the

T_{1}^{PUCCSD}

and

T_{2}^{PUCCSD}

cluster contributions, the appearance of the

T_{3}^{PUCCSD}

and

T_{4}^{PUCCSD}

cluster components in the projected UHF‐based CCSD wave function is strictly conditioned by the broken symmetry effects, and this is particularly interesting. Therefore, our attention will be mainly focused on these contributions.…”

Section: Introductionmentioning

confidence: 99%

“…Therefore, our attention will be mainly focused on these contributions. The second reason is that the cluster amplitudes

t_{3}^{PUCCSD}

and

t_{4}^{PUCCSD}

used in the externally corrected UCCSD ec CCSD approach offer much better energies than those derived from the simple CCSD(PUCCSD) calculations.…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Employing broken symmetry effects from unrestricted coupled cluster wave function to determine dynamic and non‐dynamic electron correlation during triple bond breaking in the N₂ molecule

Toboła

2018

Int J of Quantum Chemistry

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The cluster structure of the singlet full symmetric component of the unrestricted Hartree‐Fock (UHF)‐based CCSD wave function describing the triple bond breaking in the nitrogen molecule has been subjected to a detailed analysis for the possibility of determining the dynamic and near‐degeneracy electron correlation. The results obtained show that the spin and symmetry contaminations are not responsible only for the appearance of the artificial hump in the potential energy curve (PEC) generated by the UHF‐based CCSD calculations. A theoretical analysis of this issue indicates that the UHF‐based CCSD wave function and its singlet full symmetric component have a multi‐reference structure. This form of the wave function allows to explain the mechanism for creating cluster contributions in the projected UHF‐based CCSD wave function, which also provides the opportunity to explain the cause of the hump in the nitrogen PEC generated by the UCCSDecCCSD method.

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Externally and internally corrected coupled cluster approaches: an overview

Paldus

2016

J Math Chem

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Approximate account of connected triply and quadruply excited clusters in single reference coupled cluster theory via cluster analysis of projected unrestricted coupled cluster with single and double wave function

Cited by 4 publications

References 47 publications

The externally corrected coupled cluster approach with four- and five-body clusters from the CASSCF wave function

The externally corrected coupled cluster approach with four- and five-body clusters from the CASSCF wave function

Employing broken symmetry effects from unrestricted coupled cluster wave function to determine dynamic and non‐dynamic electron correlation during triple bond breaking in the N₂ molecule

Externally and internally corrected coupled cluster approaches: an overview

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Approximate account of connected triply and quadruply excited clusters in single reference coupled cluster theory via cluster analysis of projected unrestricted coupled cluster with single and double wave function

Cited by 4 publications

References 47 publications

The externally corrected coupled cluster approach with four- and five-body clusters from the CASSCF wave function

The externally corrected coupled cluster approach with four- and five-body clusters from the CASSCF wave function

Employing broken symmetry effects from unrestricted coupled cluster wave function to determine dynamic and non‐dynamic electron correlation during triple bond breaking in the N2 molecule

Externally and internally corrected coupled cluster approaches: an overview

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Employing broken symmetry effects from unrestricted coupled cluster wave function to determine dynamic and non‐dynamic electron correlation during triple bond breaking in the N₂ molecule