  Magnetic properties of a tetrahedral "Ni_4" molecule are discussed in terms
of the Heisenberg model, with magnetic anisotropy terms included, and on the
basis of first-principle calculations within the density functional theory. It
is shown that the isotropic Heisenberg model does not provide an adequate
description of magnetization at low temperatures; an inclusion of single-site
anisotropy terms does not help to improve the situation either. We suppose that
the magnetostriction of the molecule and hence the dependence of interatomic
coupling parameters on the magnetization might be important for an adequate
description of magnetic properties. The first-principle calculations confirm
the system's general preference for antiferromagnetic coupling, as well as the
failure of the isotropic Heisenberg model. A conjugated-gradient search for the
relaxed structure of the "Ni_4" molecule in ferromagnetic and antiferromagnetic
configurations did not show any clear tendency to diversification of
interatomic distances. These calculations however have not yet included the
spin-orbit coupling, which can be essential for analyzing the effects of
magnetostriction.
