title: Is it possible to determine rigorous magnetic Hamiltonians in spin s = 1 systems from density functional theory calculations?
abstract: The variational energies of broken-symmetry single determinants are frequently used (especially in the Kohn-Sham density functional theory) to determine the magnetic coupling between open-shell metal ions in molecular complexes or periodic lattices. Most applications extract the information from the solutions of m(s)(max) and m(s)(min) eigenvalues of S(z) magnetic spin momentum, assuming that a mapping of these energies on the energies of an Ising Hamiltonian is grounded. This approach is unable to predict the possible importance of deviations from the simplest form of the Heisenberg Hamiltonians. For systems involving s=1 magnetic centers, it cannot provide an estimate of neither the biquadratic exchange integral nor the three-body operator interaction that has recently been proven to be of the same order of magnitude [Phys. Rev. B 70, 132412 (2007)]. The present work shows that one may use other broken-symmetry solutions of intermediate values of m(s) to evaluate the amplitude of these additional terms. The here-derived equations rely on the assumption that an extended Hubbard-type Hamiltonian rules the interactions between the magnetic electrons. Numerical illustrations on a model problem of two O(2) molecules and a fragment of the La(2)NiO(4) lattice are reported. The results obtained using a variable percentage of Fock exchange in the BLYP functional are compared to those provided by elaborate wave function calculations. The relevant percentage of Fock exchange is system dependent but a mean value of 30% leads to acceptable amplitudes of the effective exchange interaction.
authors.affiliation: Laboratoire de Chimie et Physique Quantiques, IRSAMC/UMR5626 CNRS, Universite de Toulouse, 118 route de Narbonne, F-31062 Toulouse Cedex 4, France.
authors.forename: Pierre
authors.initials: P
authors.lastname: Labèguerie
authors.forename: Corentin
authors.initials: C
authors.lastname: Boilleau
authors.forename: Roland
authors.initials: R
authors.lastname: Bastardis
authors.forename: Nicolas
authors.initials: N
authors.lastname: Suaud
authors.forename: Nathalie
authors.initials: N
authors.lastname: Guihéry
authors.forename: Jean-Paul
authors.initials: JP
authors.lastname: Malrieu