A comparison of different density functional theory (DFT) and molecular orbital (MO) methods for calculating molecular and energetic properties of low-coordinated phosphorus compounds is reported. While DFT methods include both Becke-Lee-Yang-Parr (BLYP and B3LYP) nonlocal functionals, MO methods involve second-order perturbation theory (MP2), quadratic configuration interaction [QCISD(T)], and coupled-cluster theory [CCSD(T)], in conjunction with the 6-31G(d,p), 6-311++G(3df,2p), and 6-311++G(3df,3pd) basis sets. Properties examined include geometrical parameters of the different CH3P equilibrium structures (phosphaethene, phosphinocarbene, methylphosphinidene, and a phosphacarbyne) and relevant transition structures for isomerisations and rearrangements in both the lowest-lying singlet and tripler states, vibrational wave numbers, relative energies, barrier heights, and singlet-triplet energy gaps. In addition, the heat of formation, ionization energy, and proton affinity of phosphaethene are also evaluated. Overall, the B3LYP method, when employed with a large basis set, yields energetic results comparable to the CCSD(T) results. Nevertheless, both DFT methods fail to predict the behavior of the addition/elimination reactions of the hydrogen atom in the triplet state.