The singlet ground ͑X 1 ⌺ + ͒ and excited ͑ 1 ⌺ − , 1 ⌬͒ states of HCP and HPC have been systematically investigated using ab initio molecular electronic structure theory. For the ground state, geometries of the two linear stationary points have been optimized and physical properties have been predicted utilizing restricted self-consistent field theory, coupled cluster theory with single and double excitations ͑CCSD͒, CCSD with perturbative triple corrections ͓CCSD͑T͔͒, and CCSD with partial iterative triple excitations ͑CCSDT-3 and CC3͒. Physical properties computed for the global minimum ͑X 1 ⌺ + HCP͒ include harmonic vibrational frequencies with the cc-pV5Z CCSD͑T͒ method of 1 = 3344 cm −1 , 2 = 689 cm −1 , and 3 = 1298 cm −1 . Linear HPC, a stationary point of Hessian index 2, is predicted to lie 75.2 kcal mol −1 above the global minimum HCP. The dissociation energy D 0 ͓HCP͑X 1 ⌺ + ͒ → H͑ 2 S͒ +CP͑X 2 ⌺ + ͔͒ of HCP is predicted to be 119.0 kcal mol −1 , which is very close to the experimental lower limit of 119.1 kcal mol −1 . Eight singlet excited states were examined and their physical properties were determined employing three equation-of-motion coupled cluster methods ͑EOM-CCSD, EOM-CCSDT-3, and EOM-CC3͒.