What is the minimum distance of Rubidium and Iod ions in a centered cube? I have a problem to visualize the following assignment: 

What is the minimum distance between $\ce{Rb^+}$ and $\ce{I^-}$ ions if radius of  $\ce{Rb^+}$ is ${1.49\cdot10^{-10}}~\mathrm{m}$ and radius of $\ce{I^-}$ is ${2.17\cdot10^{-10}}~\mathrm{m}$ if they are arranged in centered cube?
The distance between $\ce{Rb^{+}}$ and $\ce{I^{-}}$ in the fcc lattice would be $(d(\ce{Rb^{+}})/2) + (d(\ce{I^{-}})/2) = 3.76 \times 10^{-10} \mathrm{m}$