Currently, I am building the model for nanocomposite with particles included. One ides is that making a small cavity in the poxy matrix at first, and then to enlarge it by repulsive potential with the center of the original cavity to a certain sphere with a desired diameter R (litter larger than the diameter of the particle). Finally, insert the particle into the cavity and combine them with some runnings. In LAMMPS, the command of add force would only add some uniform-directed force, like gravity. I wonder whether there is some simple method or command to fulfill the above task.