\begin{table}%t4
%\begin{center}
\par
\small \caption{Chemical content of the benchmark calculations.}\label{chem1}
\begin{tabular}{l} 
\hline \hline
 Chemical species in the models\\
\hline
 H, H$^+$, H$_2$, H$_2^+$, H$_3^+$\\
 O, O$^+$, OH$^+$, OH, O$_2$, O$_2^+$, H$_2$O, H$_2$O$^+$, H$_3$O$^+$\\
 C, C$^+$, CH, CH$^+$, CH$_2$, CH$_2^+$, CH$_3$,\\
 CH$_3^+$, CH$_4$, CH$_4^+$, CH$_5^+$, CO, CO$^+$,HCO$^+$\\
He, He$^+$, e$^-$\\ \hline
\end{tabular}
%\end{center}
\end{table}