In a simulation, my model has a deformation field, based on my certain boundary condition, but i want its mass center stay in its initial position, so i use history output ,set the whole model as a set, and output its mass center displacement, and it can works, i get the mass center displacement, and then i minus the displacement in my boundary condition, in this case, the mass center displacement should be zero, because i minused it, but it is not zero, still some remain displacement, and then I repeat such, the remain displacement has the same ratio of the minused displacement, because the geometric progression has a convergence（ratio <1), I finally minus the convergence displacement, the final mass center displacement is close to zero, why it happens, and which mass center displacement is correct