Dear Materials Studio Users,

I am simulating a friction process in Materials Studio by using the Perl script (The model can be checked below). I use Translate command to move the atoms in horizontal direction and externalforce command to apply an external normal loading to the model. The whole process is under NVT ensemble. The command I use to output the net force in both sliding direction and vertical direction is (Sliding in X direction and normal loading in Z direction):

my \\\$forcex = \\\$Group->NetForce->X;

my \\\$forcez = \\\$Group->NetForce->Z;

However, in the result, I found that the net force in Z direction is fluctuating, and the magnitude is far from the value I gave in the script. The screenshot of part of the output is shown below. The external force in vertical direction I gave is 1500 kcal/mol/A. You can see it's very different from the values of nForce in the screenshot (fForce is the net force along sliding direction, nForce is the net force along the vertical direction). Does anyone know the reason of it? What is the correct way of calculating the normal loading in friction process? Any suggestion is appreciated.