LoFEx — A local framework for calculating excitation energies: Illustrations using RI-CC2 linear response theory

Baudin, Pablo; Kristensen, Kasper

doi:10.1063/1.4953360

Cited by 43 publications

(54 citation statements)

References 40 publications

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“…The parameters used in the following investigation have been set to the same default values as in Ref. 39, i.e., the LoFEx excitation energy threshold was set to τ ω = 0.02 eV and the number of orbitals added to the XOS in each LoFEx iteration corresponds to ten times the average number of orbitals per atom. For the spectrum-LoFEx strategy we have chosen τ f = 0.001.…”

Section: Resultsmentioning

confidence: 99%

“…In a previous publication 39 we have introduced the LoFEx algorithm as a framework to calculate CC2 excitation energies of large molecules. In this section, we summarize the LoFEx procedure and extend it to the computation of CC2 oscillator strengths.…”

Section: Excitation Energies and Oscillator Strengths Within Lofexmentioning

confidence: 99%

“…We have shown in Ref. 39 (where τ ω was denoted τ XOS ), that this procedure can result in significant speed-ups compared to standard CC2 implementations without loss of accuracy.…”

Section: A Excitation Energiesmentioning

confidence: 99%

“…1 and Ref. 39) and the oscillator strength is only calculated once in the optimized XOS (XOS (n−1) ).…”

Section: B Oscillator Strengthsmentioning

confidence: 99%

“…[33][34][35] In this context, we can also mention the reduced virtual space 36 and ONIOM strategies. 37,38 In a recent publication, 39 we have introduced a new strategy for the calculation of CC excitation energies at a reduced computational cost, in which we focused on the second-order approximated CC singles and doubles (CC2) model. In our local framework for calculating excitation energies (LoFEx), the locality of correlation effects is used to generate a state-specific mixed orbital space composed of the dominant pair of natural transition orbitals (NTOs), obtained from time-dependent Hartree-Fock (TDHF) theory, and localized molecular orbitals (LMOs).…”

Section: Introductionmentioning

confidence: 99%

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CC2 oscillator strengths within the local framework for calculating excitation energies (LoFEx)

Baudin

Kjærgaard

Kristensen

2017

The Journal of Chemical Physics

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In a recent work [P. Baudin and K. Kristensen, J. Chem. Phys. 144, 224106 (2016)], we introduced a local framework for calculating excitation energies (LoFEx), based on second-order approximated coupled cluster (CC2) linear-response theory. LoFEx is a black-box method in which a reduced excitation orbital space (XOS) is optimized to provide coupled cluster (CC) excitation energies at a reduced computational cost. In this article, we present an extension of the LoFEx algorithm to the calculation of CC2 oscillator strengths. Two different strategies are suggested, in which the size of the XOS is determined based on the excitation energy or the oscillator strength of the targeted transitions. The two strategies are applied to a set of medium-sized organic molecules in order to assess both the accuracy and the computational cost of the methods. The results show that CC2 excitation energies and oscillator strengths can be calculated at a reduced computational cost, provided that the targeted transitions are local compared to the size of the molecule. To illustrate the potential of LoFEx for large molecules, both strategies have been successfully applied to the lowest transition of the bivalirudin molecule (4255 basis functions) and compared with time-dependent density functional theory.

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Section: Resultsmentioning

confidence: 99%

Section: Excitation Energies and Oscillator Strengths Within Lofexmentioning

confidence: 99%

“…We have shown in Ref. 39 (where τ ω was denoted τ XOS ), that this procedure can result in significant speed-ups compared to standard CC2 implementations without loss of accuracy.…”

Section: A Excitation Energiesmentioning

confidence: 99%

“…1 and Ref. 39) and the oscillator strength is only calculated once in the optimized XOS (XOS (n−1) ).…”

Section: B Oscillator Strengthsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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CC2 oscillator strengths within the local framework for calculating excitation energies (LoFEx)

Baudin

Kjærgaard

Kristensen

2017

The Journal of Chemical Physics

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Single‐reference coupled cluster methods for computing excitation energies in large molecules: The efficiency and accuracy of approximations

Izsák

2019

WIREs Comput Mol Sci

100

110

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While methodological developments in the last decade made it possible to compute coupled cluster (CC) energies including excitations up to a perturbative triples correction for molecules containing several hundred atoms, a similar breakthrough has not yet been reported for excited state computations. Accurate CC methods for excited states are still expensive, although some promising candidates for an efficient and accurate excited state CC method have emerged recently. This review examines the various approximation schemes with particular emphasis on their performance for excitation energies and summarizes the best state-of-the-art results which may pave the way for a robust excited state method applicable to molecules of hundreds of atoms.Among these, special attention will be given to exploiting the techniques of similarity transformation, perturbative approximations as well as integral decomposition, local and embedding techniques within the equation of motion CC framework. This article is categorized under: Electronic Structure Theory > Ab Initio Electronic Structure Methods Structure and Mechanism > Molecular Structures K E Y W O R D S accuracy, coupled cluster, efficiency, excited states, single reference | INTRODUCTIONIn their lucid review written a few years ago, 1 Sneskov and Christiansen discussed the various coupled cluster (CC) methods available for excited states. Using the systematic machinery of response theory, they described a hierarchy of methods, established a link between response theory and equation of motion (EOM) methods, and finished by discussing a number of approaches that might be used to reduce the computational cost. The present review aims at providing an overview of efficient methods available primarily for large molecules, and hence the emphasis will be laid more upon recent methodological advances which reduce the cost and on the extent this affects the accuracy of these methods. For ground state calculations, choosing a reference method for benchmarks is quite easy, since the CC singles doubles and perturbative triples, or CCSD(T), ansatz 2 is not only widely regarded as the gold standard of quantum chemistry, but it has also recently become possible to compute CCSD(T) energies for molecules containing several hundred atoms. 3 For excited states, progress has been

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Multilevel CC2 and CCSD in Reduced Orbital Spaces: Electronic Excitations in Large Molecular Systems

Folkestad

Kjønstad

Goletto

et al. 2021

J. Chem. Theory Comput.

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We present efficient implementations of the multilevel CC2 (MLCC2) and multilevel CCSD (MLCCSD) models. As the system size increases, MLCC2 and MLCCSD exhibit the scaling of the lower-level coupled cluster model. To treat large systems, we combine MLCC2 and MLCCSD with a reduced-space approach in which the multilevel coupled cluster calculation is performed in a significantly truncated molecular orbital basis. The truncation scheme is based on the selection of an active region of the molecular system and the subsequent construction of localized Hartree–Fock orbitals. These orbitals are used in the multilevel coupled cluster calculation. The electron repulsion integrals are Cholesky decomposed using a screening protocol that guarantees accuracy in the truncated molecular orbital basis and reduces computational cost. The Cholesky factors are constructed directly in the truncated basis, ensuring low storage requirements. Systems for which Hartree–Fock is too expensive can be treated by using a multilevel Hartree–Fock reference. With the reduced-space approach, we can handle systems with more than a thousand atoms. This is demonstrated for paranitroaniline in aqueous solution.

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LoFEx — A local framework for calculating excitation energies: Illustrations using RI-CC2 linear response theory

Cited by 43 publications

References 40 publications

CC2 oscillator strengths within the local framework for calculating excitation energies (LoFEx)

CC2 oscillator strengths within the local framework for calculating excitation energies (LoFEx)

Single‐reference coupled cluster methods for computing excitation energies in large molecules: The efficiency and accuracy of approximations

Multilevel CC2 and CCSD in Reduced Orbital Spaces: Electronic Excitations in Large Molecular Systems

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