  In this paper, we investigated structures and charge density waves, including
its origin of CDW, by using first-principles calculations. Firstly, we
performed structure searches of NbSe_2 under different pressures to explore the
possible structures of the CDW phase in NbSe_2. The stability of the resulted
structure is proved by extensive studies of energy calculations and phonon
spectra. Based on the detailed analysis on electronic properties (both in real
space and K space) and band structures during CDW transitions, we believe the
structure distortion and the displacement of atoms are the origin of CDW, which
also cause the change of the Fermi surfaces and electronic density of states
(DOS). In the end, the results of band structure and DOS were used to explain
the reason why CDW and superconducting orders compete with each other at low
temperature.
