\begin{table}%t2
\caption {Equilibrium geometry (in $\AA$), dipole moment ($\mu$, in Debyes),
 rotational constants ($B_{\rm e}$, $D_{\rm e}$ and $B_{0}$ in MHz), harmonic 
wavenumbers ($\omega_{i}$, in cm\textsuperscript{--1}), and intensities ($I$, 
in Debyes\textsuperscript{2}/ $\AA^{2}$ amu) of $\textit{l}$
-CSiCC. \label{tab2}}
%\centerline
 {\begin{tabular}{c c c c c c c}
\hline \hline
& & RCCSD(T)  & UCCSD(T) & CASSCF & CASSCF & CASSCF \\
& & cc-pVT$Z^a$  & cc-pVT$Z^a$  cc-pVD$Z^b$ & cc-pVT$Z^a$ & cc-pVQ$Z^a$ \\
\hline
$R_1$ & (C-SiCC str) &
    1.8330  &
    1.8320  &
    1.8096  &
    1.7931  &
    1.7886  
\\
$R_2$ & (CSi-CC str) &
    1.6898  &
    1.6911  &
    1.6897  &
    1.6676  &
    1.6640  
\\
$R_3$ & (CSiC-C str) &
    1.2886  &
    1.2891  &
    1.2868  &
    1.2814  &
    1.2789  
\\
$\mu$ && &
&
&
    1.7700  &
    1.8326  
\\
$B_{\rm e}$ &&
    3101.54 &
    3099.96 &
    3129.07 &
    3191.06 &
    3205.37 
\\
$B_0$ &&&
&
    3162.50 & 
\\
$D_{\rm e}$~$\times$~1$0^{-6}$&&&
&
    353.484 &
&
\\
$\omega_1 (\sigma)$ & (C-C-C stretching) &
    1890   &
    1886   &
    1961.2 &
    1940   &
    1941
    ($I=900$) 
\\
$\omega_2 (\sigma)$ & (Si-C stretching) &
    915  &
    909  &
    934.8 &
    976   &
    979
    ($I=32$) 
\\
$\omega_3 (\sigma)$ & (C-C-C stretching) &
    619   &
    619   &
    604.5 &
    655  &
    660
    ($I=2$) 
\\
$\omega_4 (\pi)$ & (trans-bending) &
 i118  &
 i127  &
  211.5 &
  166 &
  164 ($I=47$)
\\
$\omega_5 (\pi)$ & (cis-bending) &
     28  &
     27  &
    91.6 &
    91   &
    86 ($I=0$)
\\
\hline
\\
\end{tabular}}
\smallskip
$^a$ This work. Values obtained using standard approaches implemented in MOLPRO. Refs. 
\citep{51,52};
$^b$ This work. Values derived from our 6D PES;
$^c$ Ref. \citep{10}.
\end{table}