Currently, I am trying to investigate the diffusivity of 3D fluid through MSD. I learn from the example in DIFFUSE folder, and I still don't understand what is "factor of 4 in 2 variables is for 2d",so it means that if we have 3D system, we should have a different factor number.
variable twopoint equal c_msd/4/(step*dt+1.0e-6)
variable fitslope equal slope(f_9)/4/(10*dt)
Is anyone could explain more about the factor number?
Thank you in advance