  We present here the minimal tight--binding model for a single layer of
transition metal dichalcogenides (TMDCs) MX$_{2}$ (M--metal, X--chalcogen)
which illuminates the physics and captures band nesting, massive Dirac Fermions
and Valley Lande and Zeeman magnetic field effects. TMDCs share the hexagonal
lattice with graphene but their electronic bands require much more complex
atomic orbitals. Using symmetry arguments, a minimal basis consisting of 3
metal d--orbitals and 3 chalcogen dimer p--orbitals is constructed. The
tunneling matrix elements between nearest neighbor metal and chalcogen orbitals
are explicitly derived at $K$, $-K$ and $\Gamma$ points of the Brillouin zone.
The nearest neighbor tunneling matrix elements connect specific metal and
sulfur orbitals yielding an effective $6\times 6$ Hamiltonian giving correct
composition of metal and chalcogen orbitals but not the direct gap at $K$
points. The direct gap at $K$, correct masses and conduction band minima at $Q$
points responsible for band nesting are obtained by inclusion of next neighbor
Mo--Mo tunneling. The parameters of the next nearest neighbor model are
successfully fitted to MX$_2$ (M=Mo, X=S) density functional (DFT) ab--initio
calculations of the highest valence and lowest conduction band dispersion along
$K-\Gamma$ line in the Brillouin zone. The effective two--band massive Dirac
Hamiltonian for MoS$_2$, Lande g--factors and valley Zeeman splitting are
obtained.
