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Eliminating spin contamination in auxiliary-field quantum Monte Carlo: Realistic potential energy curve of F2

J. Chem. Phys. 128, 114309 (2008); doi:10.1063/1.2838983

Published 19 March 2008

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Wirawan Purwanto, W. A. Al-Saidi, Henry Krakauer, and Shiwei Zhang
Department of Physics, College of William and Mary, Williamsburg, Virginia 23187-8795, USA
The use of an approximate reference state wave function |Phir> in electronic many-body methods can break the spin symmetry of Born–Oppenheimer spin-independent Hamiltonians. This can result in significant errors, especially when bonds are stretched or broken. A simple spin-projection method is introduced for auxiliary-field quantum Monte Carlo (AFQMC) calculations, which yields spin-contamination-free results, even with a spin-contaminated |Phir>. The method is applied to the difficult F2 molecule, which is unbound within unrestricted Hartree–Fock (UHF). With a UHF |Phir>, spin contamination causes large systematic errors and long equilibration times in AFQMC in the intermediate, bond-breaking region. The spin-projection method eliminates these problems and delivers an accurate potential energy curve from equilibrium to the dissociation limit using the UHF |Phir>. Realistic potential energy curves are obtained with a cc-pVQZ basis. The calculated spectroscopic constants are in excellent agreement with experiment. ©2008 American Institute of Physics
History: Received 10 December 2007; accepted 8 January 2008; published 19 March 2008
Permalink: http://link.aip.org/link/?JCPSA6/128/114309/1
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KEYWORDS and PACS

Keywords
PACS
  • 31.50.-x
    Potential energy surfaces (atoms and molecules)
  • 31.15.xr
    Self-consistent-field methods in atomic and molecular physics
  • 33.15.Fm
    Molecular bond strengths, dissociation energies
  • 33.15.Mt
    Molecular rotation, vibration, and vibration-rotation constants
  • YEAR: 2008

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PUBLICATION DATA

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