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@ARTICLE{Das:840330,
      author       = {Das, Anita and Mueller, Thomas and Plasser, Felix and
                      Krisiloff, David B. and Carter, Emily A. and Lischka, Hans},
      title        = {{L}ocal {E}lectron {C}orrelation {T}reatment in {E}xtended
                      {M}ultireference {C}alculations: {E}ffect of
                      {A}cceptor–{D}onor {S}ubstituents on the {B}iradical
                      {C}haracter of the {P}olycyclic {A}romatic {H}ydrocarbon
                      {H}eptazethrene},
      journal      = {Journal of chemical theory and computation},
      volume       = {13},
      number       = {6},
      issn         = {1549-9626},
      address      = {Washington, DC},
      reportid     = {FZJ-2017-07872},
      pages        = {2612 - 2622},
      year         = {2017},
      abstract     = {The implementation of a local correlation (LC) treatment of
                      multireference (MR) configuration interaction approaches
                      within the COLUMBUS program system is reported. The LC
                      treatment is based on the weak pairs approximation of Sæbø
                      and Pulay (Ann. Rev. Phys. Chem. 1993, 44, 213) and a
                      geometrical analysis of Walter et al. (Chem. Phys. Lett.
                      2001, 346, 177). The removal of simultaneous single
                      excitations out of the weak pairs is based on the reference
                      doubly occupied space only, leading to a straightforward
                      program implementation and a conceptual simplicity in terms
                      of well-defined localized orbitals. Reductions of up to an
                      order of magnitude in the configuration space expansion and
                      in computer time for the Davidson diagonalization step are
                      found. The selection of the active and the virtual orbital
                      spaces is not affected by this procedure. This treatment is
                      successfully applied to the singlet biradical heptazethrene
                      and its different acceptor–donor substituents:
                      4,12-dicyanoheptazethrene, 4,12-diaminoheptazethrene, and
                      4-amino-12-cyanoheptazethrene. Simultaneous insertion of
                      pairs of donor and acceptor groups increases the biradical
                      character; for push–pull substitution, this effect is
                      significantly smaller. In addition, results obtained from
                      spin-corrected unrestricted density functional theory
                      calculations are supported by our MR calculations.},
      cin          = {JSC},
      ddc          = {540},
      cid          = {I:(DE-Juel1)JSC-20090406},
      pnm          = {511 - Computational Science and Mathematical Methods
                      (POF3-511)},
      pid          = {G:(DE-HGF)POF3-511},
      typ          = {PUB:(DE-HGF)16},
      pubmed       = {pmid:28441477},
      UT           = {WOS:000403530100023},
      doi          = {10.1021/acs.jctc.7b00156},
      url          = {https://juser.fz-juelich.de/record/840330},
}