\begin{table}%T1
\caption{\label{t1}Computational and experimental ionization potentials (${\it IP}$s) of the four considered PAHs and of Si, with computational values obtained at the B3LYP/D95++** level of theory.}
%\centerline
 {
\begin{tabular}{ccccc}
\hline \hline  \\[-3mm]
 & ${\it IP}$ (eV) & & $\Delta {\it IP}$ (eV)$^{c}$\\
 & Calc.$^a$ & Exp.$^{b}$ &Calc. &Exp.\\
\hline
%\noalign{\smallskip}
naphthalene & 7.82/7.91 & 8.14 & --0.30/--0.21 & --0.01\\
C$_{10}$H$_8$\\
pyrene & 7.10/7.16 & 7.43 & --1.02/--0.96 & --0.72\\
C$_{16}$H$_{10}$\\
coronene & 7.02/7.09 & 7.29 & --1.10/--1.03 & --0.86\\
C$_{24}$H$_{12}$\\
ovalene & 6.40/6.43 & 6.71 & --1.72/--1.69 & --1.44\\
C$_{32}$H$_{14}$\\
Si & 8.12 & 8.15\\
\hline
\end{tabular}}
\medskip
$^a$ Adiabatic/vertical ${\it IP}$.
$^{b}$ Experimental values are extracted from the NIST database.
$^{c}$ $\Delta {\it IP}$ (eV) = ${\it IP}$(PAH) -- ${\it IP}$(Si).
\end{table}