  Correlated phenomena such as magnetism and Mott phase are a very
controversial issue in two-dimensional transition metal dichalcogenides
(TMDCs). With the aim of finding the value of correlation strength and
understanding the origin of ferromagnetic order in TMDCs, we first identify
relevant low-energy degrees of freedom on both octahedral T and trigonal
prismatic H lattices in MX2 (M=Ti, V, Cr, Mn, Fe, Co, Ni; X=S, Se, Te) and then
determine the strength of the effective Coulomb interactions between localized
d electrons from the first principles using the constrained random-phase
approximation. The on-site Coulomb interaction (Hubbard U) values lie in the
range 1.4-3.7 eV (1.1-3.6 eV) and depend on the ground-state electronic
structure, d-electron number, and correlated subspace. For most of the TMDCs we
obtain 1 < U/W_b < 2 (the bandwidth W_b), which turn out to be larger than the
corresponding values in elementary transition metals. On the basis of the
calculated U and exchange J interaction, we have checked the condition to be
fulfilled for the formation of the ferromagnetic order by Stoner criterion. The
results indicate that experimentally observed MnX2 (X=S, Se) and VX2 (X=S, Se)
have an intrinsic ferromagnetic behavior in pristine form, although V-based
materials are close vicinity to the critical point separating ferromagnetic
from paramagnetic phase.
