@article{36128327ce2142b29c37159babc50d2f,
title = "Active-space coupled-cluster study of electronic states of Be 3 ",
abstract = "The active-space coupled-cluster (CC) and equation-of-motion (EOM) CC methods with all single and double excitations with triple excitations defined via active orbitals (CCSDt, EOMCCSDt), as implemented with TENSOR CONTRACTION ENGINE, are applied to the challenging Be 3 system, which is characterized by a large number of low-lying excited states dominated by two-electron transitions and significant high-order correlation effects in the ground electronic state. It is demonstrated that the CCSDt and EOMCCSDt methods provide an excellent description of complicated electronic quasidegeneracies present in the Be 3 cluster. Different strategies for defining triple excitations within the CCSDt/EOMCCSDt approach are discussed.",
author = "Karol Kowalski and So Hirata and Marta W{\l}och and Piotr Piecuch and Windus, {Theresa L.}",
note = "Funding Information: Most of the work has been performed using the Molecular Science Computing Facility (MSCF) in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL) at the Pacific Northwest National Laboratory. The Pacific Northwest National Laboratory is operated for the U.S. Department of Energy by the Battelle Memorial Institute under Contract No. DE-AC05-76RL01830. This work has been supported by the U.S. Department of Energy (Grant No. DE-FG02-01ER15228; P.P.) and the National Science Foundation (Grant No. CHE-0450462; S.H.). Some calculations were performed on the computer systems provided by the High Performance Computing Center at Michigan State University. Table I. A comparison of the vertical excitation energies (in eV) of the triangular Be 3 system (the geometry taken from Ref. 45 ) as described by the ANO basis set of Ref. 46 , obtained with various variants of the EOMCCSDt approach, with those resulting from the EOMCCSD, CR-EOMCCSD(T), and FCI calculations. The corresponding ground-state energies, in hartree, are provided too. State FCI a EOMCCSD CR-EOMCCSD(T) b EOMCCSDt(I) c EOMCCSDt(II) c EOMCCSDt(III) c X A 1 ′ 1 − 43.882 330 − 43.864 904 − 43.873 110 − 43.879 340 − 43.875 338 − 43.868 242 1 E ″ 1 1.67 ( S ) 1.718 1.647 1.658 1.678 1.695 1 A 1 ″ 1 1.78 ( D ) 1.877 2.060 2.526 1 E ′ 1 2.04 ( S ) 2.122 1.988 2.031 2.063 2.083 2 E ″ 1 2.61 ( D ) 2.683 2.844 3.297 2 E ′ 1 2.68 ( D ) 9.315 3.032 2.708 2.880 3.296 1 A 2 ′ 1 2.89 ( S ) 2.932 2.790 2.871 2.894 2.904 2 A 1 ′ 1 2.91 ( S ) 3.029 2.896 2.937 2.975 2.996 3 E ″ 1 3.04 ( D ) 4.232 3.546 3.115 3.314 3.875 3 E ′ 1 3.08 ( D ) 4.823 3.338 3.247 3.447 4.075 1 A 2 ″ 1 3.16 ( S , D ) 3.993 3.358 3.302 3.437 3.754 2 A 1 ″ 1 3.31 ( D ) 5.271 3.601 3.439 3.627 4.073 4 E ′ 1 3.48 ( D ) 4.570 3.633 3.541 3.743 4.253 3 A 1 ′ 1 3.64 ( D ) 4.935 4.033 3.846 3.988 4.370 4 E ″ 1 3.90 ( D ) 5.286 3.954 4.018 4.212 4.779 3 A 1 ″ 1 3.99 ( S , D ) 4.244 3.891 4.112 4.208 4.677 2 A 2 ″ 1 4.12 ( D ) 5.995 4.384 4.183 4.374 4.882 a The FCI results from Ref. 45 . The letter S ( D ) at each FCI value indicates the single (double) excitation character of excited state. b The ID∕IB variant of the CR-EOMCCSD(T) approach of Ref. 23 . c The active space consisting of the three highest-energy occupied and nine lowest-energy unoccupied molecular orbitals correlating with the 2 s and 2 p shells of the Be atoms was employed in the CCSDt∕EOMCCSDt calculations. FIG. 1. The FCI, CCSD, CCSD(T), and CCSDt (version I) ground-state potential-energy curves for the linear symmetric dissociation of the linear Be 3 molecule (as described by the ANO basis set of Ref. 46 ) as functions of the distance between the central and each of the lateral Be atoms in the linear Be 3 system. The active space consisting of the three highest-energy occupied and nine lowest-energy unoccupied molecular orbitals was used in the CCSDt(I) calculations. ",
year = "2005",
doi = "10.1063/1.2001656",
language = "English (US)",
volume = "123",
journal = "Journal of Chemical Physics",
issn = "0021-9606",
publisher = "American Institute of Physics",
number = "7",
}