I want to calculate elastic constant C12 for a 2D material by applying strain in the range (-2 to +2 %). The cystal is having tetragonal symmetry. So kindly tell me how to prepare the input.
You can use DFT codes (e.g. SIESTA, Quantum ESPRESSO, ATK,...) in which strain can be induced by applying stress in different directions. Then, you should relax your structure based on these circumstances.