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I am trying to calculate the phonon dispersion curve for a 2D material. Once I have properly optimized the primitive structure until forces are completely negligible, I created a supercell. However, when I computed the forces for this undistorted supercell, there were huge forces.

One of the things I have tried is increasing the grid NGX,Y,Z manually in the input. But, when I check the OUTPUT it seems that VASP is overriding the grid I have selected. Below follows the relevant parts of my output:

max r-space proj IRMAX = 1 max aug-charges IRDMAX= 423666

dimension x,y,z NGX = 180 NGY = 180 NGZ = 200

dimension x,y,z NGXF= 360 NGYF= 360 NGZF= 400

support grid NGXF= 720 NGYF= 720 NGZF= 800


(...)

I would recommend the setting:

dimension x,y,z NGX = 129 NGY = 129 NGZ = 175

(...)

NNGXGis ok and might be reduce to 130

NGY is ok and might be reduce to 130

NGZ is ok and might be reduce to 174

Broyden mixing: mesh for mixing (old mesh)

NGX = 65 NGY = 65 NGZ = 87

(NGX =360 NGY =360 NGZ =400)

gives a total of 367575 points

It seems that VASP is not using the grid I define in the input.
I wonder how to force to get the grid I want and solve the problem with the forces in the supercell cell

Check if there is no mistake in producing the supercell.
For primitive cell and the supercell you should get
similar forces (k-point sampling should be proportionally
decreased).