  The understanding of the halo current properties during disruptions is key to
design and operate large scale tokamaks in view of the large thermal and
electromagnetic loads that they entail. For the first time, we present a fully
self-consistent model for halo current simulations including neutral particles
and sheath boundary conditions. The model is used to simulate Vertical
Displacement Events (VDEs) occurring in the COMPASS tokamak. Recent COMPASS
experiments have shown that the parallel halo current density at the
plasma-wall interface is limited by the ion saturation current during
VDE-induced disruptions. We show that usual MHD boundary conditions can lead to
the violation of this physical limit and we implement this current density
limitation through a boundary condition for the electrostatic potential. Sheath
boundary conditions for the density, the heat flux, the parallel velocity and a
realistic parameter choice (e.g. Spitzer $\eta$ and Spitzer-H\"arm
$\chi_\parallel$ values) extend present VDE simulations beyond the state of the
art. Experimental measurements of the current density, temperature and heat
flux profiles at the COMPASS divertor are compared with the results obtained
from axisymmetric simulations. Since the ion saturation current density
($J_{sat}$) is shown to be essential to determine the halo current profile,
parametric scans are performed to study its dependence on different quantities
such as the plasma resistivity and the particle and heat diffusion
coefficients. In this respect, the plasma resistivity in the halo region
broadens significantly the $J_{sat}$ profile, increasing the halo width at a
similar total halo current.
