Our rst-principles calculations are performed in the framework of spin-polarized density functional theory as implemented in the Vienna Ab initio Simulation Package (VASP) based on a plane-wave basis set . The projector augmented wave (PAW) potentials are used for describing the core electrons. The generalized gradient approximation (GGA) of Perdew, Burke, and Ernzernhof (PBE) is employed for treating the exchange and correlation. The valence electron congurations considered for Ti, V, O, F, Cl, and Br are 3d 3 4s 1 (4 electrons), 3d 4 4s 1 (5 electrons), 2s 2 2p 4 (6 electrons), 2s 2 2p 5 (7 electrons), 3s 2 3p 5 (7 electrons), 4s 2 4p 5 (7 electrons), respectively. A cuto energy of 500 eV is used for the planewave expansion in all calculations. All structures are fully relaxed until the Hellmann-Feynman forces on all the atoms are less than 10 -3 eV/Å. The lattice parameters a and b are relaxed, keeping the c xed as required for 2D materials, and internal coordinates of the 2D structures are fully relaxed to achieve the lowest energy conguration using conjugate gradient algorithm. To prevent the interaction between the periodic images in the calculations, a vacuum layer with a thickness of approximately 25 Å is added along the z-direction (perpendicular to the monolayer) in the supercell. Note that previous reports[2628] employed about 15-20 Å vacuum layer, and also considered van der Waals interaction between the layers . However, we have not considered van der Waals interaction as we simulate an isolated monolayer. The convergence for the total energy is set as 10 -7 eV. For 1 × 1 × 1 unit cell, the Brillouin zone integration is sampled using a regular 12 × 12 × 1 MonkhorstPack k-point grid, for geometry optimizations, while a denser grid of 18 × 18 × 1 is used for density functional perturbation theory (DFPT) calculations. To study magnetic ordering,