The relative energies of the (X) over tilde, (a) over tilde, and A states of CHBr and CHI and their atomization and dissociation energies in the complete basis limit were determined by extrapolating (R/U)CCSD(T) and Davidson corrected MRCI energies calculated with the aug-cc-pVxZ, = T,Q,5) basis sets, which were corrected for core valence correlation, spin orbit coupling, and zero point energies The all-electron calculations on the bromine containing molecules were explicitly corrected for scalar relativity, while in the iodo systems they are implicit in the ECP28MDF pseudopotential of iodine The geometries and vibrational frequencies were calculated at the CASPT2/cc-pVTZ level of theory The computed singlet triplet splittings (5.7 and 3.7 kcal mol(-1) for CHBr and CHI respectively) are in close agreement with the recent experimental values, while the predicted (A) over tilde <- (X) over tilde (X) over tilde excitation energies are. within similar to 1 kcal mol(-1) of experiment. The barriers to linearity and dissociation on the (A) over tilde surface were also characterized For CHI and Cl, the predicted heats of formation at 298 K are 134 5 +/- 1 0 and 103 9 +/- 1.0 kcal mol(-1), respectively. The spin orbit splitting in iodomethylidyne (Cl) is computed to be 746 cm(-1), although that value may be an underestimate by similar to 20%