  We use density functional theory to study the structural, magnetic and
electronic structure of the organo-metallic quantum magnet
$\mathrm{NiCl_2-4SC(NH_2)_2}$ (DTN). Recent work has demonstrated the quasi-1D
nature of the molecular crystal and its quantum phase transitions at low
temperatures. This includes a magneto-electric coupling and, when doped with
Br, the presence of an exotic Bose-glass state. We systematically show that, by
using the generalized gradient approximation (GGA) with inclusion of a van der
Waals term to account for weak inter-molecular forces and by introducing a
Hubbard $U$ term to the total energy, our calculations reproduce the magnetic
anisotropy, the inter-molecular exchange coupling strength and the
magneto-electric effect in DTN, which were observed in previous experiments.
Further analysis into the electronic structure gives insight into the
underlying magnetic interactions, including what mechanisms may be causing the
ME effect. Using this computationally efficient model, we predict what effect
applying an electric field might have on the magnetic properties of this
quantum magnet.
