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Title: Ab initio coupled-cluster and multi-reference configuration interaction studies of the low-lying electronic states of 1,2,3,4-cyclobutanetetraone

In this paper, the four, closely spaced, lowest energy electronic states of the challenging, D4h-symmetric, 1,2,3,4-cyclobutanetetraone (C4O4) molecule have been investigated using high-level ab initio methods. The calculated states include the closed-shell singlet 8π(1A1g) state, the singlet 10π(1A1g) state, in which the π-type lowest unoccupied molecular orbital (LUMO) of the 8π(1A1g) reference is doubly occupied and the σ-type highest occupied molecular orbital (HOMO) is empty, and the open-shell singlet and triplet states, designated as 9π(1B2u) and 9π(3B2u), respectively, originating from single occupancy of the HOMO and LUMO. Our focus is on single-reference coupled-cluster (CC) approaches capable of handling electronic near-degeneracies in diradicals, especially the completely renormalised CR-CC(2,3) and active-space CCSDt methods, along with their CCSD and EOMCCSD counterparts. The internally contracted multi-reference configuration interaction calculations with a quasi-degenerate Davidson correction are performed as well. Our computations demonstrate that the state ordering is 9π(3B2u) < 8π(1A1g) < 9π(1B2u) < 10π(1A1g) and that the 8π(1A1g) - 9π(3B2u) gap is in the 7–11 kJ/mol range, in reasonable agreement with the negative ion photoelectron spectroscopy measurements, which give 6.27 ± 0.5 kJ/mol. Finally, in addition to the theory level used, geometry relaxation and basis set play a significant role in determining the state orderingmore » and energy spacings. In particular, it is unsafe to use lower level, non-CC geometries and smaller basis sets.« less
 [1] ;  [1] ;  [1] ;  [2] ;  [3]
  1. Michigan State Univ., East Lansing, MI (United States). Dept. of Chemistry
  2. Univ. of North Texas, Denton, TX (United States). Dept. of Chemistry. Center for Advanced Scientific Computing and Modeling
  3. Michigan State Univ., East Lansing, MI (United States). Dept. of Chemistry. Dept. of Physics and Astronomy
Publication Date:
OSTI Identifier:
Grant/Contract Number:
FG02-01ER15228; CHE-0910527; B0027
Accepted Manuscript
Journal Name:
Molecular Physics
Additional Journal Information:
Journal Volume: 114; Journal Issue: 5; Journal ID: ISSN 0026-8976
Taylor & Francis
Research Org:
Michigan State Univ., East Lansing, MI (United States); Univ. of North Texas, Denton, TX (United States)
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22); National Science Foundation (NSF) (United States); Robert A. Welch Foundation (United States)
Country of Publication:
United States
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY 1,2,3,4-cyclobutanetetraone; coupled-cluster theory; multi-reference configuration interaction; singlet–triplet gap; electronic excitations