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 _CCSD_, CCSD with perturbative triple corrections _CCSD_T__, and CCSD with partial iterative triple excitations _CCSDT-3 and CC3_. Physical properties computed for the global minimum _X˜ 1_+HCP_ include harmonic vibrational frequencies with the cc-pV5Z CCSD_T_ 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 D0_HCP_X˜ 1_+_→H_2S_+CP_X 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-CC3_. Four stationary points were located on the lowest-lying excited state potential energy surface, 1_− →1A_, with excitation energies Te of 101.4 kcal mol−1_1A_ HCP_, 104.6 kcal mol−1_1_− HCP_, 122.3 kcal mol−1_1A_ HPC_, and 171.6 kcal mol−1_1_− HPC_ at the cc-pVQZ EOM-CCSDT-3 level of theory. The physical properties of the 1A_ state with a predicted bond angle of 129.5° compare well with the experimentally reported first singlet state _A˜ 1A__. The excitation energy predicted for this excitation is T0=99.4 kcal mol−1_34 800 cm−1 , 4.31 eV_, in essentially perfect agreement with the experimental value of T0=99.3 kcal mol−1_34 746 cm−1 ,4.308 eV_. For the second lowest-lying excited singlet surface, 1_→1A_, four stationary points were found with Te values of 111.2 kcal mol−1 _21A_ HCP_, 112.4 kcal mol−1 _1_ HPC_, 125.6 kcal mol−1_2 1A_ HCP_, and 177.8 kcal mol−1_1_ HPC_. The predicted CP bond length and frequencies of the 2 1A_ state with a bond angle of 89.8° _1.707 Å, 666 and 979 cm−1_ compare reasonably well with those for the experimentally reported C ˜ 1A_ state _1.69 Å, 615 and 969 cm−1_. However, the excitation energy and bond angle do not agree well: theoretical values of 108.7 kcal mol−1 and 89.8° versus experimental values of 115.1 kcal mol−1 and 113°.