  Due to the excellent physical properties, two dimensional materials have
attracted widespread attention from researchers. In this article, we discuss a
transition metal dichalcogenide, ReSeS monolayer with 1T" phase, with extremely
low symmetry through the first principles calculation. It belongs to the space
group P1 and has Jauns structure. Due to the broken of the central inversion
symmetry. Raman and infrared active modes in the vibrational spectrum no longer
mutually exclusive, which has become a spectral fingerprint of this material.
First principles calculation indicates that the bandgap decreases when the
material has geometric deformation, and we propose two models to explain this
phenomenon and provide the physical pictures. The first model requires
constructing a low symmetry Wannier analytically simplify Hamiltonian to obtain
4 energy levels of mixed orbital, Then the changes of bandgap can be
qualitatively described through the difference between these energy levels; The
second model analyzes the variations of bandgap through the bonding or
antibonding characteristics of the valence and conduction band. We point out
that it is a "saturation effect", where the energy of the valence and
conduction band has difficulties in further changing under the deformation,
resulting in the reduction of the bandgap under both compression and tensile
deformation. Meanwhile, we discuss a metallic phase of ReSeS, called 1T' phase,
to explain the formation of the bandgap. Compared to the 1T" phase, it has
higher symmetry. Semi-filled d-orbital bands in 1T' phases will become unstable
and split into bonding and antibonding bands, leading to the opening of
bandgap. This process is also known as Peierls phase transition. Finally, we
calculated the effective mass of electron and the absorption spectrum. Results
point out the potential of ReSeS in the preparation of electrical or optical
devices.
