  We present a microscopic magnetic model for the spin-liquid candidate
volborthite Cu3V2O7(OH)2(H2O)2. The essentials of this DFT-based model are (i)
the orbital ordering of Cu(1) 3d 3z2-r2 and Cu(2) 3d 3x2-y2, (ii) three
relevant couplings J_ic, J_1 and J_2, (iii) the ferromagnetic nature of J_1 and
(iv) frustration governed by the next-nearest-neighbor exchange interaction
J_2. Our model implies magnetism of frustrated coupled chains in contrast to
the previously proposed anisotropic kagome model. Exact diagonalization studies
reveal agreement with experiments.
