\begin{table}%t2
%\centering
\par
\caption{\label{additional}Additional information of PES properties compared with experimental values \citep{nist}.}
\small \begin{tabular}{lll}
\hline \hline
Quantity & Theo. & Exp. \\
 \hline
rotational constant & $5.2872$ & 5.241 \\
bond length (H-H) & $1.77866$ $\AA$ & 1.7751 $\AA$ \\
bond length (H-C) & $1.08920$ $\AA$ & 1.0870 $\AA$\\
\hline
\end{tabular} 
\end{table}