Materials Studio - Castep - Geometry optimization?

Dear all

While doing geometry optimization using materials studio - Castep for mixture atoms,

I'm receiving an error as follows

"Error geom_precon_exp_get_r_NN: increased r_cut to twice system extent without finding neighbours for all atoms. This can happen if your system is too small; try setting r_cut manually"

how to rectify it?

Thanks in advance