Parametric approach to variational two-electron reduced-density-matrix theory
- Department of Chemistry and James Franck Institute, University of Chicago, Chicago, Illinois 60637 (United States)
Two general variational paradigms for computing ground-state energies and properties of molecular quantum systems are (i) the parametrization of the N-particle wave function, as in truncated configuration interaction, which yields an upper bound on the energy in a given basis set and (ii) the constraint of the two-electron reduced-density matrix (2-RDM) by necessary N-representability conditions (without using the wave function) which yields a lower bound on the energy in a given basis set [D. A. Mazziotti, Phys. Rev. Lett. 93, 213001 (2004)]. In this paper we synthesize these two directions in a class of techniques which we call parametric variational 2-RDM methods. The 2-RDM in these methods is parametrized to be size consistent while approximately satisfying the N-representability conditions. We extend an energy functional of Kollmar [C. Kollmar, J. Chem. Phys. 125, 084108 (2006)], which modifies configuration interaction with double excitations to be size consistent, by including not only double but also single excitations explicitly. Using the 2-RDM parametrization, we calculate ground-state energies at both equilibrium and nonequilibrium geometries in correlation-consistent polarized valance double-zeta (cc-pVDZ) basis sets. Energies as well as properties from the parametric variational 2-RDM method, particularly at nonequilibrium geometries, are better in accuracy than those obtained from coupled cluster with single and double excitations. The present work shows clearly that, except in the dissociation of N{sub 2}, the deviation of the 2-RDM from the well-known N-representability conditions, such as the D, Q, and G conditions, is negligible. Furthermore, calculations with helium atoms demonstrate the size consistency of the method. The computational results on N representability and size consistency are especially important because they legitimatize the selected parametrization of the 2-RDM.
- OSTI ID:
- 21020707
- Journal Information:
- Physical Review. A, Vol. 76, Issue 4; Other Information: DOI: 10.1103/PhysRevA.76.042501; (c) 2007 The American Physical Society; Country of input: International Atomic Energy Agency (IAEA); ISSN 1050-2947
- Country of Publication:
- United States
- Language:
- English
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