You can  use  gausian  09 or hyper chemical  programs to achieve  your  job 

First, research the literature so you won't double the existing papers. See e.g. http://www.sciencedirect.com/science/article/pii/S0166128099000329

Since your system is very small, you can go to very accurate methods (depending on available machines). Complete basis set calculations has been done 17 years ago (see the paper above) so now you should be able to do basically anything, including CCSDT or perhaps even G1-4.

You should definitely try both singlet an triplet, even if just to see if your methods of choice can reproduce experimental energy difference.