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@ARTICLE{Horn:172529,
      author       = {Horn, Shawn and Plasser, Felix and Mueller, Thomas and
                      Libisch, Florian and Burgdörfer, Joachim and Lischka, Hans},
      title        = {{A} comparison of singlet and triplet states for one- and
                      two-dimensional graphene nanoribbons using multireference
                      theory},
      journal      = {Theoretical chemistry accounts},
      volume       = {133},
      number       = {8},
      issn         = {1432-2234},
      address      = {Berlin},
      publisher    = {Springer},
      reportid     = {FZJ-2014-05994},
      pages        = {1511},
      year         = {2014},
      abstract     = {This study examines the radical nature and spin symmetry of
                      the ground state of the quasi-linear acene and
                      two-dimensional periacene series. For this purpose,
                      high-level ab initio calculations have been performed using
                      the multireference averaged quadratic coupled cluster theory
                      and the COLUMBUS program package. A reference space
                      consisting of restricted and complete active spaces is taken
                      for the pi-conjugated space, correlating 16 electrons with
                      16 orbitals with the most pronounced open-shell character
                      for the acenes and a complete active-space reference
                      approach with eight electrons in eight orbitals for the
                      periacenes. This reference space is used to construct the
                      total configuration space by means of single and double
                      excitations. By comparison with more extended calculations,
                      it is shown that a focus on the pi space with a 6-31G basis
                      set is sufficient to describe the major features of the
                      electronic character of these compounds. The present
                      findings suggest that the ground state is a singlet for the
                      smaller members of these series, but that for the larger
                      ones, singlet and triplet states are quasi-degenerate. Both
                      the acenes and periacenes exhibit significant polyradical
                      character beyond the traditional diradical.},
      cin          = {JSC},
      ddc          = {540},
      cid          = {I:(DE-Juel1)JSC-20090406},
      pnm          = {411 - Computational Science and Mathematical Methods
                      (POF2-411)},
      pid          = {G:(DE-HGF)POF2-411},
      typ          = {PUB:(DE-HGF)16},
      UT           = {WOS:000337640400001},
      doi          = {10.1007/s00214-014-1511-8},
      url          = {https://juser.fz-juelich.de/record/172529},
}