The density functional theory (DFT) calculations were carried out based on the Vienna ab initio simulation package (VASP) . The projected augmented wave (PAW) pseudopotentials with 500 eV plane-wave cutoff energy were considered for the electron-ion interactions . The exchange-correlation functional was treated by using the generalized gradient approximation (GGA) using the Perdew-Burke-Ernzerhof (PBE) method . The energy convergence criterion and the force convergence criteria were set as 10 -5 eV and 10 -2 eV/Å, respectively. The Γ-centered Monkhorst-Pack k-meshes were used for the Brillouin zone integration of the unit cells. We employed PHONOPY code to assist our phonon dispersion calculations. We performed ab initio molecular dynamic (MD) simulations at 400 K temperature to see the stability of Ti 2 CSTA 2 (A = Li, Mg) MXenes. 4 × 4 × 1 supercell structures of Ti 2 CST monolayers were considered for both phonon and ab initio MD computations. The climbing image nudged elastic band (CI-NEB) method was used to calculate the diffusion barrier using VASP transition state tools . , we label them as FCC, HCP, HF1 and HF2. In FCC structures, O/Se and S atoms align with Ti atoms, while those surface atoms align with C atoms in HCP structures. O/Se atoms are placed right above Ti atoms and S atoms reside right under C atoms in HF1 structures. On the contrary, O/Se atoms locate on top of C atoms and S atoms locate below Ti atoms in HF2 structures. FCC, HF1 and HF2 systems have P3m1 (156) space-group symmetry, whereas HCP has C1m1 . The relative total energy for each structure is mentioned in Table . It is clear that both monolayers have FCC ground state structures while HCP exhibits the largest energy. This reflects that the preferable sites of both S and O/Se atoms are located right above the Ti atoms. The top and bottom views of the minimum energy structure are shown in Fig. . According to previous studies, Ti 2 CO 2 and Ti 2 CS 2 MXenes also have FCC as the minimum energy structure .