Young's modulus

Hi all
I want to calculate Young’s modulus of Au by imposing displacements on the fcc system by using displace_atoms command. I want to calculate Young’s modulus through determining the second derivative of strain energy and using finite difference formulation to this end. I have a question:
If I want to impose variable displacments ( i.e U =0.005x or V= 0.005y or W= 0.005z) on atoms, is it reasonable to calculate the strain energy “exactly” after imposing displacment command without any fix command that provides production level becuase I should compute the strain energy exactly at specific strain in finite difference formulation?