1. Create a supercell with one crystalline axis parallel to the direction along which you want to apply the strain

2. Manually, vary the lattice parameter along that particualr direction and relax the ionic positions for such a uniaxially strained setup.

3. From the converged outputs, you can estimate the stress tensors and further calculate the desired properties (band structure, dos, optical excitations etc.).

Mohammad Saeed Bahramy

What the strain I need to apply is biaxial strain not uniaxial strain, biaxial strain have two axes in the crystal face like ［105］［113］and cannot do like what you said

Well, then you can (probably) do it as follows:

First, create a supercell with a crystal face containing those two axes. Then project these two directions onto the corresponding two supercell lattice parameters within this crystalline plane (let's call them v_x and v_y). The relative magnitude (v_x/v_y) of the vector components decomposed this way gives you a measure by which you can biaxially strain the aforementioned lattice parameters. In other words, The amount of compression along x-direction must be propotional to the ratio v_x/v_y and vice versa for the y-direction.