  We use a combined ab-initio calculations and k.p theory based approach to
derive a low-energy effective Hamiltonian for monolayer MoS2 at the K point of
the Brillouin zone. It captures the features which are present in
first-principles calculations but not explained by the theory of Xiao et al.
[Phys Rev Lett 108, 196802 (2012)], namely the trigonal warping of the valence
and conduction bands, the electron-hole symmetry breaking, and the
spin-splitting of the conduction band. We also consider other points in the
Brillouin zone which might be important for transport properties. Our findings
lead to a more quantitative understanding of the properties of this material in
the ballistic limit.
