Many-bodyab initiostudy of antiferromagnetic{Cr7M}molecular rings

Antiferromagnetic molecular rings are widely studied both for fundamental quantum-mechanical issues and for technological applications, particularly in the field of quantum information processing. Here we present a detailed first-principles study of two families---purple and green---of ${{\mathrm{Cr}}_{7}M}$ antiferromagnetic rings, where $M$ is a divalent transition metal ion ($M={\mathrm{Ni}}^{2+}, {\mathrm{Mn}}^{2+}$, and ${\mathrm{Zn}}^{2+}$). We employ a recently developed flexible and efficient scheme to build ab initio system-specific Hubbard models. From such many-body models we systematically derive the low-energy effective spin Hamiltonian for the rings. Our approach allows us to calculate isotropic as well as anisotropic terms of the spin Hamiltonian, without any a priori assumption on its form. For each compound we calculate magnetic exchange couplings, zero-field splitting tensors, and gyromagnetic tensors, finding good agreement with experimental results.