\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{longtable}

\begin{document}
\begin{tabular}{ |p{3cm}||p{3cm}|p{3cm}|p{3cm}|  }
 \hline
 \multicolumn{4}{|c|}{Energy} \\
 \hline
      State & BSE@qsGW  & dBSE@qsGW&\\
 \hline
  
 $^3$P $(1s^22s^12p^1)$& 2.376    &  2.335  &\\
$^1$P $(1s^22s^12p^1)$&6.241& 6.317&  \\
 $^3$P $(1s^22p^2)$   &7.668 &7.689&  \\
$^1$D $(1s^22p^2)$ & 9.417    & 9.470&\\
 \hline
\end{tabular}


\end{document}