\begin{table}%t4
%\centering
\par
\caption{\label{tab_energy_ar}The level energies (Ryd) of Ar$^{8+}$ from different calculations along with the compilation of NIST v3 and CHIANTI~v6.}
\begin{tabular}{cccccccc}
\hline\hline\noalign{\smallskip}
ID  &   Level~specification  & NIST/CHIANTI\tablefootmark{a}  & AS &  FAC &
CHIANTI\tablefootmark{b} & MCHF\tablefootmark{c} \\
\hline\noalign{\smallskip}
1  &  ${\rm     2s^2 2p^6 ~ ^1S_0}$ &              &  0.00000  &            &          &            \\
   2  &  ${\rm  2s^2 2p^5 3s ~ ^3P_2}$ &  18.4672     & 18.4701   & 18.4879    & 18.4267  &  18.4745    \\
   3  &  ${\rm  2s^2 2p^5 3s ~ ^3P_1}$ &  18.5271     & 18.5316   & 18.5510    & 18.4823  &  18.5346    \\
   4  &  ${\rm  2s^2 2p^5 3s ~ ^3P_0}$ &  18.6307     & 18.6318   & 18.6492    & 18.5898  &  18.6377    \\
   5  &  ${\rm  2s^2 2p^5 3s ~ ^1P_1}$ &  18.6967     & 18.7031   & 18.7249    & 18.6463  &  18.7045    \\
   6  &  ${\rm  2s^2 2p^5 3p ~ ^3S_1}$ &  19.5859     & 19.5907   & 19.6006    & 19.5898  &  19.6043    \\
   7  &  ${\rm  2s^2 2p^5 3p ~ ^3D_3}$ &  19.7739     & 19.7862   & 19.8001    & 19.7404  &  19.7945    \\
   8  &  ${\rm  2s^2 2p^5 3p ~ ^3D_2}$ &  19.7826     & 19.7955   & 19.8113    & 19.7475  &  19.8025    \\
   9  &  ${\rm  2s^2 2p^5 3p ~ ^3D_1}$ &  19.8049     & 19.8493   & 19.8647    & 19.7960  &  19.8549    \\
  10  &  ${\rm  2s^2 2p^5 3p ~ ^3P_2}$ &  19.8855     & 19.8968   & 19.9149    & 19.8358  &  19.9050    \\
  11  &  ${\rm  2s^2 2p^5 3p ~ ^1P_1}$ &  19.9489     & 19.9644   & 19.9775    & 19.9104  &  19.9965    \\
  12  &  ${\rm  2s^2 2p^5 3p ~ ^3P_0}$ &  19.9751     & 19.9910   & 20.0055    & 19.9271  &  20.0314    \\
  13  &  ${\rm  2s^2 2p^5 3p ~ ^1D_2}$ &  20.0026     & 20.0131   & 20.0305    & 19.9587  &  19.9685    \\
  14  &  ${\rm  2s^2 2p^5 3p ~ ^3P_1}$ &  20.0104     & 20.0245   & 20.0413    & 19.9645  &  20.0225    \\
  15  &  ${\rm  2s^2 2p^5 3p ~ ^1S_0}$ &  20.7851     & 20.7670   & 20.8484    & 20.6289  &  20.6624    \\
  16  &  ${\rm  2s^2 2p^5 3d ~ ^3P_0}$ &  21.4090     & 21.4282   & 21.4273    & 21.3865  &  21.4359    \\
  17  &  ${\rm  2s^2 2p^5 3d ~ ^3P_1}$ &  21.4276     & 21.4478   & 21.4451    & 21.4038  &  21.4539    \\
  18  &  ${\rm  2s^2 2p^5 3d ~ ^3P_2}$ &  21.4650     & 21.4876   & 21.4831    & 21.4403  &  21.4914    \\
  19  &  ${\rm  2s^2 2p^5 3d ~ ^3F_4}$ &  21.4945     & 21.5229   & 21.5169    & 21.4640  &  21.5230    \\
  20  &  ${\rm  2s^2 2p^5 3d ~ ^3F_3}$ &  21.5215     & 21.5488   & 21.5475    & 21.4885  &  21.5496    \\
  21  &  ${\rm  2s^2 2p^5 3d ~ ^3F_2}$ &  21.5794     & 21.5962   & 21.5951    & 21.5314  &  21.5962    \\
  22  &  ${\rm  2s^2 2p^5 3d ~ ^1F_3}$ &  21.6027     & 21.6352   & 21.6325    & 21.5614  &  21.6305    \\
  23  &  ${\rm  2s^2 2p^5 3d ~ ^3D_1}$ &  21.6963     & 21.7272   & 21.7266    & 21.6480  &  21.7240    \\
  24  &  ${\rm  2s^2 2p^5 3d ~ ^1D_2}$ &  21.7091     & 21.7380   & 21.7358    & 21.6690  &  21.7594    \\
  25  &  ${\rm  2s^2 2p^5 3d ~ ^3D_3}$ &  21.7312     & 21.7616   & 21.7594    & 21.6918  &  21.7622    \\
  26  &  ${\rm  2s^2 2p^5 3d ~ ^3D_2}$ &  21.7334     & 21.7652   & 21.7632    & 21.6909  &  21.7367    \\
  27  &  ${\rm  2s^2 2p^5 3d ~ ^1P_1}$ &  21.9709     & 22.0317   & 22.0372    & 21.9433  &  22.0010    \\
  28  &  ${\rm   2s 2p^6 3s  ~ ^3S_1}$ &              & 24.2031   & 24.2183    & 24.1764  &  24.6381    \\
  29  &  ${\rm   2s 2p^6 3s  ~ ^1S_0}$ &              & 24.4742   & 24.4911    & 24.4184  &  24.7995    \\
  30  &  ${\rm  2s^2 2p^5 4s ~ ^3P_2}$ &  24.6131     & 24.6443   & 24.6311    & 24.5969  &  24.8207    \\
  31  &  ${\rm  2s^2 2p^5 4s ~ ^1P_1}$ &  24.6402     & 24.6718   & 24.6630    & 24.6187  &  24.6671    \\
  32  &  ${\rm  2s^2 2p^5 4s ~ ^3P_0}$ &  24.7794     & 24.8031   & 24.7905    & 24.7609  &             \\
  33  &  ${\rm  2s^2 2p^5 4s ~ ^3P_1}$ &  24.7936     & 24.8230   & 24.8145    & 24.7746  &             \\
  34  &  ${\rm  2s^2 2p^5 4p ~ ^3S_1}$ &  25.1071     & 25.1267   & 25.1214    & 25.0596  &             \\
  35  &  ${\rm  2s^2 2p^5 4p ~ ^3D_3}$ &  25.1591     & 25.1444   & 25.1408    & 25.0981  &             \\
  36  &  ${\rm  2s^2 2p^5 4p ~ ^3D_2}$ &  25.1472     & 25.1500   & 25.1479    & 25.1014  &             \\
  37  &  ${\rm  2s^2 2p^5 4p ~ ^1P_1}$ &  25.1598     & 25.1727   & 25.1722    & 25.1203  &             \\
  38  &  ${\rm  2s^2 2p^5 4p ~ ^3P_2}$ &  25.1817     & 25.1843   & 25.1849    & 25.1307  &             \\
  39  &  ${\rm  2s^2 2p^5 4p ~ ^3P_0}$ &  25.2565     & 25.2714   & 25.2739    & 25.2097  &             \\
  40  &  ${\rm  2s^2 2p^5 4p ~ ^3D_1}$ &  25.3016     & 25.3050   & 25.3021    & 25.2590  &             \\
  41  &  ${\rm  2s^2 2p^5 4p ~ ^1D_2}$ &  25.3225     & 25.3263   & 25.3258    & 25.2787  &             \\
  42  &  ${\rm  2s^2 2p^5 4p ~ ^3P_1}$ &  25.3253     & 25.3285   & 25.3284    & 25.2750  &             \\
  43  &  ${\rm   2s 2p^6 3p  ~ ^3P_0}$ &              & 25.5061   & 25.5425    & 25.4653  &             \\
  44  &  ${\rm   2s 2p^6 3p  ~ ^3P_1}$ &  25.3515     & 25.5140   & 25.5497    & 25.4727  &             \\
  45  &  ${\rm   2s 2p^6 3p  ~ ^3P_2}$ &              & 25.5355   & 25.5696    & 25.4927  &             \\
  46  &  ${\rm  2s^2 2p^5 4p ~ ^1S_0}$ &  25.5795     & 25.5741   & 25.6269    & 25.4414  &             \\
  47  &  ${\rm   2s 2p^6 3p  ~ ^1P_1}$ &  25.4298     & 25.6165   & 25.6467    & 25.5656  &             \\
  48  &  ${\rm  2s^2 2p^5 4d ~ ^3P_0}$ &  25.7158     & 25.7514   & 25.7320    & 25.6922  &             \\
  49  &  ${\rm  2s^2 2p^5 4d ~ ^3P_1}$ &  25.7260     & 25.7622   & 25.7427    & 25.7023  &             \\
  50  &  ${\rm  2s^2 2p^5 4d ~ ^3F_4}$ &  25.7417     & 25.7728   & 25.7537    & 25.7187  &             \\
  51  &  ${\rm  2s^2 2p^5 4d ~ ^3P_2}$ &  25.7448     & 25.7801   & 25.7607    & 25.7205  &             \\
  52  &  ${\rm  2s^2 2p^5 4d ~ ^3F_3}$ &  25.7552     & 25.7857   & 25.7692    & 25.7305  &             \\
  53  &  ${\rm  2s^2 2p^5 4d ~ ^1D_2}$ &  25.7753     & 25.8057   & 25.7905    & 25.7478  &             \\
  54  &  ${\rm  2s^2 2p^5 4d ~ ^3D_3}$ &  25.7859     & 25.8163   & 25.8009    & 25.7560  &             \\
  55  &  ${\rm  2s^2 2p^5 4d ~ ^3D_1}$ &  25.8443     & 25.8824   & 25.8706    & 25.8135  &             \\
  56  &  ${\rm  2s^2 2p^5 4d ~ ^3F_2}$ &  25.9173     & 25.9460   & 25.9300    & 25.8927  &             \\
  57  &  ${\rm  2s^2 2p^5 4d ~ ^3D_2}$ &  25.9255     & 25.9546   & 25.9383    & 25.8973  &             \\
  58  &  ${\rm  2s^2 2p^5 4d ~ ^1F_3}$ &  25.9312     & 25.9595   & 25.9439    & 25.9037  &             \\
  59  &  ${\rm  2s^2 2p^5 4f ~ ^3D_1}$ &  25.9443     & 25.9702   & 25.9638    & 25.9250  &             \\
  60  &  ${\rm  2s^2 2p^5 4f ~ ^3D_2}$ &  25.9487     & 25.9754   & 25.9693    & 25.9276  &             \\
  \hline
\end{tabular}
\tablefoot{\tablefoottext{a}{Sources of NIST v3 are from the unpublished work of %Shirai et~al. (
\citet{SSM99}, and references therein, while
that of CHIANTI~v6 is from the work of %Lepson et~al. (
\citet{LBB03}.} \tablefoottext{b}{Theoretical energies from
%Zhang et~al. (
\citet{ZSC87}.} \tablefoottext{c}{MCHF data available from the
website: \protect\url{http://atoms.vuse.vanderbilt.edu/}}}
\end{table}