Parametric two-electron reduced-density-matrix method with application to diradical rectangular H4

Andrew M. Sand, David A. Mazziotti

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8 Scopus citations


Parameterization of the two-electron reduced density matrix (2-RDM) has made possible the determination of electronic energies with greater accuracy and reduced computational cost compared to traditional electron-pair theories, including coupled cluster with single and double excitations [D.A. Mazziotti, Phys. Rev. Lett. 101 (2008) 253002]. We apply the method to an H4 model system, a rectangular arrangement of two H2 monomers (P4), which is often used for benchmark calculations of multireference methods. At the square geometry, H4 becomes a diradical. We find that the parametric 2-RDM method obtains occupation numbers of 0.5471 and 0.4489 for the Nth and (N+1)th natural orbitals, respectively, which indicate diradical character. Energies and orbital occupation numbers obtained from the parametric 2-RDM method are found to be more accurate than single-reference wavefunction methods of comparable computational cost. We report energies and natural orbital occupation numbers for several geometries in the rectangular H4 system.

Original languageEnglish (US)
Pages (from-to)44-49
Number of pages6
JournalComputational and Theoretical Chemistry
StatePublished - Jan 1 2013

Bibliographical note

Funding Information:
D.A.M. gratefully acknowledges the NSF, the ARO, the Henry-Camille Dreyfus Foundation, the David-Lucile Packard Foundation, the Keck Foundation, and the Microsoft Corporation for their support.

Copyright 2013 Elsevier B.V., All rights reserved.


  • Electron correlation
  • Parametric method
  • Potential energy curves
  • Reduced density matrices


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