Several ab initio methods are applied to the study of two linear isome
rs of methylidyne phosphine, HCP and HPC. Single-reference correlation
methods ranging from second-order Moller-Plesset theory to coupled cl
uster singles and doubles theory with noniterative inclusion of connec
ted triple excitations were applied in a variety of basis sets of incr
easing size. Ln addition, complete active space self-consistent field
wave functions, multireference singles and doubles configuration inter
action, and averaged coupled pair functional theory were also applied
to HPC. For HCP comparison of the single-reference based results is ma
de with experimental data and previous theory to assess the accuracy o
f the methods employed. The same single-reference-based methods are th
en applied to linear HPC in order to assess whether it is a stable min
imum or a transition state (imaginary frequencies in two dimensions).
It is found that linear HPC is not a minimum on the groundstate potent
ial surface. However, relatively high levels of correlation must be us
ed when single-reference-based methods are employed to arrive at this
conclusion. The multireference-based methods are then applied to HPC,
and they too predict that it is not a minimum on the ground electronic
potential surface. The qualitative nature of linear HPC is examined u
sing CASSCF and CCSD wave functions.