\begin{table}%[th]%t2
   %\centering
\par
   \caption{The energy meshes (in unit of $q^2$, residual charge of ion) used for each ion.} \label{tab_mesh}
\par
\begin{tabular}{c|cccc}
\hline\hline 
{\rm Mesh}      & \multicolumn{4}{c}{Atomic number}\\ 
%\hline
  $q^2${\rm Ryd}  & 12--14  & 15--21 &   22--30 &   31--36 \\
\hline 
$1\times10^{-4}$  & $\bullet$  &          &           &           \\
$5\times10^{-5}$  &            & $\bullet$  &           &           \\
$1\times10^{-5}$  &            &          & $\bullet$   &           \\
$5\times10^{-6}$  &            &          &           & $\bullet$   \\
\hline
\end{tabular}
\end{table}