Hamiltonian of the V-15 spin system from first-principles density-functional calculations

We report first-principles all-electron density-functional-based studies of the electronic structure, magnetic ordering, and anisotropy for the V-15 m olecular magnet. From these calculations, we determine a Heisenberg Hamilto nian with five antiferromagnetic and one ferromagnetic exchange couplings. We perform direct diagonalization to determine the temperature dependence o f the susceptibility. This Hamiltonian reproduces the experimentally observ ed spin S = 1/2 ground state and low-lying S = 3/2 excited state. A small a nisotropy term is necessary to account for the temperature independent Dart of the magnetization curve.