I'm looking into the move_x and move_f functions. Do I understand the rvec structure properly: Atom i is described by rvec (x[i],v[i],f[i]), and the cartesian components are accessible with the following statement, printf("atom i=%d is at X=(%f, %f, %f)\n", i, x[i][0], x[i][0], x[i][0]); regards, NT Moore