brownian motion command

hello dear users
i am simulating the cubic with nanofluids including water and cu…cu is a sphere region (bottom image) that is build by entering code in lammps(bottom code).it is fix until the end of run but i want it to have a brownian motion and moves away,does anybody know how can cu atoms have brownian motion?i mean entering code in lammps…
i have found this command .is it true and how should i edit it?(fix pull metal spring tether 50.0 0.0 0.0 0.0 25.0)
i appreciate any helps
thank you very much

lattice fcc 3.615 #Cu lattice constant
region Cu sphere 27 27 27 9 units box #x=27 ,y=27,z=27 ,r=9
create_atoms 3 region Cu
pair_coeff 1 3 lj/cut 1.2105 1.5925 #H-Cu epsilon sigma
pair_coeff 2 3 lj/cut 0 .0095# 1.0385 #O-Cu epsilon
pair_coeff 3 3 eam cu.eam
pair_coeff 1 4 lj/cut 1.2105 1.5925 #H-Cu epsilon sigma
pair_coeff 2 4 lj/cut 0 .0095# 1.0385
pair_coeff 4 4 eam cu.eam

vb.JPG