The electronic structure has been carried out with the explicitly correlated, F12, coupled cluster method, CCSD(T)-F12b , with the F12 adapted triple zeta basis set , cc-pVTZ-F12 (CCSD(T)-F12b/VTZ-F12) using molpro2010 code . As noted in several studies, this level of computation provides a good compromise between quality and computational efficiency . Cl + isotopologues) .