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Open-shell molecular electronic states from the parametric two-electron reduced-density-matrix method

J. Chem. Phys. 130, 164109 (2009); doi:10.1063/1.3116789

Published 23 April 2009

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A. Eugene DePrince, III and David A. Mazziotti
Department of Chemistry and The James Franck Institute, The University of Chicago, Chicago, Illinois 60637, USA
The parametric variational two-electron reduced-density-matrix (2-RDM) method, developed from an analysis of positivity (N-representability) constraints on the 2-RDM, is extended to treat both closed- and open-shell molecules in singlet, doublet, and triplet spin states. The parametric 2-RDM method can be viewed as using N-representability conditions to modify the 2-RDM from a configuration interaction singles-doubles wave function to make the energy size extensive while keeping the 2-RDM approximately N-representable [J. Kollmar, Chem. Phys. 125, 084108 (2006); A. E. DePrince and D. A. Mazziotti, Phys. Rev. A 76, 049903 (2007)]. Vertical excitation energies between triplet and singlet states are computed in a polarized valence triple-zeta basis set. In comparison to traditional single-reference wave function methods, the parametric 2-RDM method recovers a larger percentage of the multireference correlation in the singlet excited states, which improves the accuracy of the vertical excitation energies. Furthermore, we show that molecular geometry optimization within the parametric 2-RDM method can be efficiently performed through a Hellmann–Feynman-like relation for the energy gradient with respect to nuclear coordinates. Both the open-shell extension and the energy-gradient relation are applied to computing relative energies and barrier heights for the isomerization reaction HCN+<-->HNC+. The computed 2-RDMs very nearly satisfy well known, necessary N-representability conditions. ©2009 American Institute of Physics
History: Received 15 January 2009; accepted 19 March 2009; published 23 April 2009
Permalink: http://link.aip.org/link/?JCPSA6/130/164109/1
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KEYWORDS and PACS

Keywords
PACS
  • 31.15.am
    Ab initio relativistic configuration interaction (CI) and many-body perturbation calculations (atoms and molecules)
  • 33.15.Hp
    Molecular barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
  • 33.15.Bh
    General molecular conformation and symmetry; stereochemistry
  • YEAR: 2009

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PUBLICATION DATA

ISSN:
0021-9606 (print)   1089-7690 (online)
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