\begin{table}%t6
%\centering
\par
\caption{Some relevant energy levels and ionization potentials (in eV) from our MCDF calculations and from \citet{nist} for Be-like ions. The energy of each term is obtained as the mean of the energy levels weighted statistically.\label{tab6}}
\begin{tabular}{lllll}
\hline\hline
& \multicolumn{2}{c}{N$^{3+}$} & \multicolumn{2}{c}{O$^{4+}$} \\
Level & MCDF & NIST & MCDF & NIST \\
\hline
2s$^{2}$ $^{1}$S & 0 & 0 & 0 & 0 \\ 
2s2p$^{2}$ $^3$P & 8.43 & 8.35 & 10.27 & 10.19 \\
$E_{\rm IP}$ (2s $^{2}$S) & 77.13 & 77.47 & 113.54 & 113.90 \\
$E_{\rm IP}$ (2s2p $^{2}$P) && 87.47 & 125.65 & 125.88 \\
	\hline
	\end{tabular}
\end{table}