A scheme for the calculation of molecular properties within the framework of unitary coupled-cluster (UCC) theory in both the electronic ground and excited states is presented. The scheme is based on an expectation-value ansatz, similar to the equation-of-motion coupled-cluster method or the intermediate state representation (ISR) approach of the algebraic–diagrammatic construction (ADC) scheme. Due to the UCC ansatz, the resulting equations cannot be given by closed-form expressions but need to be approximated. Explicit expressions for the expectation value of a general one-particle operator correct through second order in perturbation theory have been derived and coded for the electronic ground state as well as for excited states of predominant single-excitation character. The resulting equations are shown to be equivalent to those of the second-order ADC/ISR procedure. As first computational tests, the second-order UCC method (UCC2) and the one employing third-order amplitudes (also eigenvectors) together with the second-order density matrix, denoted as UCC3(2), are applied to the calculation of dipole moments for a series of small closed- and open-shell systems as well as 4-cyanoindole and 2,3-benzofuran and compared to full configuration interaction or experimental results. For the aromatic organic molecules, the UCC2 method is shown to be sufficient for the ground-state dipole moment, whereas the UCC3(2) scheme is superior for excited-state dipole moments.
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28 August 2020
Research Article|
August 26 2020
Unitary coupled cluster ground- and excited-state molecular properties
Manuel Hodecker
;
Manuel Hodecker
a)
Interdisciplinary Center for Scientific Computing, Heidelberg University
, Im Neuenheimer Feld 205, D-69120 Heidelberg, Germany
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Andreas Dreuw
Andreas Dreuw
b)
Interdisciplinary Center for Scientific Computing, Heidelberg University
, Im Neuenheimer Feld 205, D-69120 Heidelberg, Germany
b)Author to whom correspondence should be addressed: [email protected]
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a)
Current address: Department of Theoretical Chemistry and Biology, KTH Royal Institute of Technology, Malvinas Väg 10, S-114 28 Stockholm, Sweden. Electronic mail: [email protected]
b)Author to whom correspondence should be addressed: [email protected]
J. Chem. Phys. 153, 084112 (2020)
Article history
Received:
June 22 2020
Accepted:
August 12 2020
Citation
Manuel Hodecker, Andreas Dreuw; Unitary coupled cluster ground- and excited-state molecular properties. J. Chem. Phys. 28 August 2020; 153 (8): 084112. https://doi.org/10.1063/5.0019055
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