\begin{table}%t1
\caption{Comparison between available far-IR gas-phase laboratory 
measurements \citep[][ first column]{zha96,pir04,pir06}, our DFT 
results, both unscaled (second column) and scaled with an empirical factor 
of $\sim$0.958 (third column) and the previously published DFT results
\citep[][ last column]{lan96,mar96}.
\label{empirical_factor}}
%\centerline
 {\begin{tabular}{cccc}
\hline 
\hline
\noalign{\smallskip}
\multicolumn{4}{c}{Band positions (cm$^{-1}$)} \\
\noalign{\smallskip}
Experimental & Unscaled DFT & Scaled DFT  & Previous DFT \\
\noalign{\smallskip}
\hline
\noalign{\smallskip}
\multicolumn{4}{c}{Naphthalene (C$_{10}$H$_{8}$)}\\
\noalign{\smallskip}
167.0$^{a}$, 166$^{c}$ & 178.2 & 170.8 & 171.8$^{d}$, 171$^{e}$ \\
360.6$^{a}$, 359$^{c}$ & 376.3 & 360.5 & 357.7$^{d}$, 359$^{e}$ \\
\noalign{\smallskip}
\hline
\noalign{\smallskip}
\multicolumn{4}{c}{Anthracene (C$_{14}$H$_{10}$)}\\
\noalign{\smallskip}
87$^{c}$ & 94.7 & 90.7 & 91.0$^{d}$, 90$^{e}$ \\
227$^{c}$ & 239.0 & 228.9 & 232.3$^{d}$, 232$^{e}$ \\
\noalign{\smallskip}
\hline
\noalign{\smallskip}
\multicolumn{4}{c}{Phenanthrene (C$_{14}$H$_{10}$)}\\
\noalign{\smallskip}
97$^{c}$ & 104.3 & 99.9 & 99$^{e}$ \\
221$^{c}$ & 235.8 & 225.9 & 226.8$^{d}$, 226$^{e}$ \\
\noalign{\smallskip}
\hline
\noalign{\smallskip}
\multicolumn{4}{c}{Pyrene (C$_{16}$H$_{10}$)}\\
\noalign{\smallskip}
95.0$^{a}$, 95$^{c}$ & 102.7 & 98.4 & 98.6$^{d}$ \\
214.2$^{a}$, 207$^{c}$ & 218.4 & 209.2 & 210.0$^{d}$ \\
350.0$^{a}$, 348$^{c}$ & 368.6 & 353.1 & 353.5$^{d}$ \\
\noalign{\smallskip}
\hline
\noalign{\smallskip}
\multicolumn{4}{c}{Chrysene (C$_{18}$H$_{12}$)}\\
\noalign{\smallskip}
173$^{c}$ & 192.8 & 184.7 & -- \\
231.5$^{a}$, 233$^{c}$ & 242.7 & 232.5 & 233.0$^{d}$ \\
\noalign{\smallskip}
\hline
\noalign{\smallskip}
\multicolumn{4}{c}{Perylene (C$_{20}$H$_{12}$)}\\
\noalign{\smallskip}
 -- & 98.9 & 94.8 & 98.6$^{d}$ \\
 --    & 182.7 & 175.0 & 175.9$^{d}$\\
464.9$^{b}$ & 479.7 & 459.6 & 462.0$^{d}$ \\
\hline
\end{tabular}}
\par
\smallskip$^{a}$ Gas-phase emission measurements \citep{zha96};\\ 
$^{b}$ Gas-phase emission measurements \citep{pir04};\\
$^{c}$ Gas-phase emission measurements \citep{pir06};\\
$^{d}$ B3LYP/4\textendash31G calculation \citep{lan96};\\
$^{e}$ B3LYP/cc-pVDZ calculation \citep{mar96}.
\end{table}