A local framework for calculating coupled cluster singles and doubles excitation energies (LoFEx-CCSD) |
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Authors: | Pablo Baudin Dmytro Bykov Dmitry Liakh Patrick Ettenhuber Kasper Kristensen |
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Affiliation: | 1. qLEAP Center for Theoretical Chemistry, Department of Chemistry, Aarhus University, Aarhus C, Denmarkpablo.baudin@chem.au.dk;3. qLEAP Center for Theoretical Chemistry, Department of Chemistry, Aarhus University, Aarhus C, Denmark;4. Scientific Computing Group, National Center for Computational Sciences, Oak Ridge National Laboratory, Oak Ridge, TN, USA |
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Abstract: | ABSTRACTThe recently developed Local Framework for calculating Excitation energies (LoFEx) is extended to the coupled cluster singles and doubles (CCSD) model. In the new scheme, a standard CCSD excitation energy calculation is carried out within a reduced excitation orbital space (XOS), which is composed of localised molecular orbitals and natural transition orbitals determined from time-dependent Hartree–Fock theory. The presented algorithm uses a series of reduced second-order approximate coupled cluster singles and doubles (CC2) calculations to optimise the XOS in a black-box manner. This ensures that the requested CCSD excitation energies have been determined to a predefined accuracy compared to a conventional CCSD calculation. We present numerical LoFEx-CCSD results for a set of medium-sized organic molecules, which illustrate the black-box nature of the approach and the computational savings obtained for transitions that are local compared to the size of the molecule. In fact, for such local transitions, the LoFEx-CCSD scheme can be applied to molecular systems where a conventional CCSD implementation is intractable. |
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Keywords: | Excitation energies large molecules coupled cluster theory CCSD local correlation |
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