  Using quantum chemistry calculations we shed fresh light on the electronic
structure and magnetic properties of RuCl3, a proposed realization of the
honeycomb Kitaev spin model. It is found that the nearest-neighbor Kitaev
exchange K is weaker than in 5d5 Ir oxides but still larger than other
effective spin couplings. The electronic-structure computations also indicate a
ferromagnetic K in the halide, which is supported by a detailed analysis of the
field-dependent magnetization. From exact-diagonalization calculations for
extended Kitaev-Heisenberg Hamiltonians we additionally find that a transition
from zigzag order to a spin-liquid ground state can be induced in RuCl3 with
external magnetic field.
