  Transition metal dichalcogenides are rich in their structural phases, e.g.
1T-TaS2 and 1T-TaSe2 form charge density wave (CDW) under low temperature with
interesting and exotic properties. Here, we present a systematic study of
different structures in two-dimensional TaX2 (X=S, Se, Te) using density
functional theory calculations with consideration of van der Waals interaction.
All the normal phases present metal characteristics with various ground state
and magnetic properties. The lattice reconstruction of CDW drastically affects
the electronic and structural characteristics of 1T-TaS2 and 1T-TaSe2, leading
to a transition from metal to insulator and an emergence of magnetic moment
within periodic atomic clusters called the Star of David. The evaluated
Heisenberg couplings indicate the weak ferromagnetic coupling between the
clusters in monolayer. Furthermore, in bilayer commensurate CDW cases, we find
intriguing phenomenon of the varying magnetic properties with different
stacking orders. The magnetic moment in each layer disappears when two layers
are coupled, but may sustain in certain stackings of interlayer
antiferromagnetic configurations.
