Is this older discussion helpful?

https://www.researchgate.net/post/How_VASP_calculate_elastic_constants

Dear Jürgen Weippert

Thank you for the suggestion. But the discussion is about elastic constant calculation. I want to apply strain to my material, such as 2%, 4% strain, etc.

I have read in some articles that we need to change the lattice parameters. But I couldn't figure out how to do it. I hope someone will have an answer for this.

OK, that should be easy. If you want to apply this sort of strain, you need to stretch the unit cell along the desired axis, i.e. lines 3-5 here:

https://www.vasp.at/wiki/index.php/POSCAR

Then you perform a single-point calculation, so no geometry optimization.

Dear Jürgen Weippert

Thank you for the suggestion, sir. Will try the selective dynamics method.

But still, how can we specify that we have applied a certain amount of strain; like 2% or 4%?

All you need to do is a stretch along an axis, e.g. with the cell from the example:

0.0 0.5 0.5

0.5 0.0 0.5

0.5 0.5 0.0

becomes

0.0 0.51 0.5

0.5 0.0 0.5

0.5 0.51 0.0

Then you have strained the elementary cell by 2% along the y/b/whatever axis.

If that is not the definition of strain you are using, you need to say what you want to do, I was a bit surprised to see "strains" in % anyway, to me the usual unit of strain is an energy unit, so eV or kJ/mol.